Choose the Right Starting Hole for an NPT Tap
Revised: August 16, 2026 Editorial method: This reference compares published tooling charts, labels calculated conversions, separates cutting-tap and roll-form data, and identifies conflicts rather than treating repeated third-party recommendations as governing requirements.
An NPT tap drill size chart is a starting-point reference, not a substitute for the instructions supplied with a particular tap.
Its dimensions come from a chart explicitly labeled for NPT/NPTF tapping without a taper reamer. Do not treat it as an official ASME table or a mandatory dimension for every tap, material, machine, and application.
NPT tap drill size chart for conventional cutting taps
Conventional cutting taps—direct tapping without a taper reamer
The following baseline reproduces the pitch and no-taper-reamer recommendations in Allied Machine’s National Pipe Threads tap-drill chart.
| Nominal NPT size | TPI | Drill designation or fraction | Decimal inches | Calculated millimeters | Alternatives? |
|---|---|---|---|---|---|
| 1/16 | 27 | C | 0.2420 | ≈6.15 mm | Yes—see conflict table |
| 1/8 | 27 | Q | 0.3320 | ≈8.43 mm | Yes—see conflict table |
| 1/4 | 18 | 7/16 | 0.4375 | ≈11.11 mm | Broad agreement in cited cutting-tap charts |
| 3/8 | 18 | 9/16 | 0.5625 | ≈14.29 mm | Yes—see conflict table |
| 1/2 | 14 | 45/64 | 0.7031 | ≈17.86 mm | Yes—see conflict table |
| 3/4 | 14 | 29/32 | 0.9063 | ≈23.02 mm | Yes—see conflict table |
| 1 | 11-1/2 | 1-9/64 | 1.1406 | ≈28.97 mm | Yes—see conflict table |
| 1-1/4 | 11-1/2 | 1-31/64 | 1.4844 | ≈37.70 mm | Yes—see conflict table |
| 1-1/2 | 11-1/2 | 1-23/32 | 1.7188 | ≈43.66 mm | Yes—see conflict table |
| 2 | 11-1/2 | 2-7/32 | 2.2188 | ≈56.36 mm | Yes—see conflict table |
| 2-1/2 | 8 | 2-5/8 | 2.6250 | ≈66.68 mm | Yes—see conflict table |
| 3 | 8 | 3-1/4 | 3.2500 | 82.55 mm | Yes—see conflict table |
| 3-1/2 | 8 | 3-3/4 | 3.7500 | 95.25 mm | No conflict identified in supplied charts |
| 4 | 8 | 4-1/4 | 4.2500 | 107.95 mm | No conflict identified in supplied charts |
The millimeter figures are calculated using 25.4 millimeters per inch and rounded for convenient comparison. They are not independently sourced metric specifications. Letter drills are shown with their stated decimal diameters; exact fractional-inch values are carried to four decimal places where useful.
Warning: Published recommendations conflict, particularly from 1/16 through 1-1/2 NPT, with additional disagreements at 2, 2-1/2, and 3 NPT. The tap manufacturer’s data takes precedence for a specific tool. Use this table only when its stated conditions—conventional cutting tap and direct tapping without a taper reamer—match the operation.
The table selects a recommended starting hole for one bounded process. It does not establish how far to run the tap, whether the finished thread is acceptable, what sealing method is required, or whether the part complies with a governing specification.
How to read the chart without confusing nominal size and drill diameter
The nominal NPT size identifies the pipe-thread system and corresponding mating size. It is not the physical diameter of the tap drill and should not be transferred to a drawing as though it were a measured hole dimension.
That distinction explains why a recommended starting hole can look much larger than the nominal size. A 1/4 NPT callout does not mean drilling a 0.250-inch hole. In the conventional cutting-tap baseline, the starting hole for 1/4-18 NPT is 7/16 inch, or 0.4375 inch. The nominal designation identifies the thread family; the drill column identifies the recommended hole preparation.
The TPI column means threads per inch. The cited baseline groups the pitches as follows:
- 1/16 and 1/8 NPT use 27 TPI.
- 1/4 and 3/8 NPT use 18 TPI.
- 1/2 and 3/4 NPT use 14 TPI.
- 1 through 2 NPT use 11-1/2 TPI.
- 2-1/2 through 4 NPT use 8 TPI.
A complete tap callout combines nominal size and pitch, such as 1/8-27 NPT, 1/4-18 NPT, or 1-11-1/2 NPT. Do not select a tap or verify a setup from the nominal fraction alone.
NPT charts commonly use three drill notations:
- Letter sizes, such as C, Q, R, and S.
- Fractional-inch sizes, such as 7/16, 45/64, and 1-9/64.
- Decimal-inch equivalents, which make close alternatives easier to compare.
Decimals matter when two recommendations appear similar. A Q drill is 0.332 inch, while an R drill is approximately 0.339 inch. The difference is only about 0.007 inch, but they remain distinct starting-hole recommendations.
The 1/8-27 row illustrates why “1/8 NPT” alone does not identify the intended preparation. The no-taper-reamer baseline is a Q drill at 0.332 inch, while another chart family lists an R drill. By contrast, 1/4-18 is the clearest conventional cutting-tap example in the supplied evidence because the compared charts consistently list 7/16 inch.
Even where charts agree, the listed drill remains a starting-hole recommendation. It does not define the finished thread’s complete geometry, acceptance status, required sealing method, or inspection result.
Why NPT tap drill charts give different answers
Third-party charts plainly disagree. Repetition across multiple websites may make a recommendation familiar, but it does not convert that recommendation into a directly verified requirement of the governing standard.
The baseline column below comes from the no-taper-reamer chart cited with the primary table. The alternatives through 1-1/2 NPT are documented by the DrillsandCutters commercial NPT chart.
| Nominal size | No-taper-reamer baseline | Documented alternative | Difference |
|---|---|---|---|
| 1/16-27 | C, 0.242 in | D, ≈0.246 in | +≈0.004 in |
| 1/8-27 | Q, 0.332 in | R, ≈0.339 in | +≈0.007 in |
| 3/8-18 | 9/16, 0.5625 in | 37/64, 0.5781 in | +0.0156 in |
| 1/2-14 | 45/64, 0.7031 in | 23/32, 0.7188 in | +0.0156 in |
| 3/4-14 | 29/32, 0.9063 in | 59/64, 0.9219 in | +0.0156 in |
| 1-11-1/2 | 1-9/64, 1.1406 in | 1-5/32, 1.1563 in | +0.0156 in |
| 1-1/4-11-1/2 | 1-31/64, 1.4844 in | 1-1/2, 1.5000 in | +0.0156 in |
| 1-1/2-11-1/2 | 1-23/32, 1.7188 in | 1-47/64, 1.7344 in | +0.0156 in |
| 2-11-1/2 | 2-7/32, 2.2188 in | 2-3/16, 2.1875 in | −0.0313 in |
| 2-1/2-8 | 2-5/8, 2.6250 in | 2-39/64, 2.6094 in | −0.0156 in |
| 3-8 | 3-1/4, 3.2500 in | 3-15/64, 3.2344 in | −0.0156 in |
The three larger-size alternatives—2-3/16, 2-39/64, and 3-15/64 inches—come from AutoDrill’s NPT table. Unlike the alternatives at many smaller sizes, these holes are smaller than the baseline values.
The baseline source expressly identifies its NPT/NPTF values as recommendations for use without a taper reamer. The available evidence does not establish the preparation method behind every alternative. It would therefore be misleading to claim that every larger recommendation assumes taper reaming—or that every smaller recommendation reflects the same engagement target.
Several variables can affect a practical starting-hole choice:
- The specific tap’s geometry and condition
- The work material and its cutting or forming behavior
- Desired thread engagement
- Actual drilled-hole size, including runout or enlargement
- Lubrication and chip control
- Machine rigidity and alignment
- Tapping depth and whether the hole is blind or through
- The inspection and performance requirements for the part
These variables do not justify an unsupported shortcut such as “always drill stainless one size larger.” Stainless grades, tap designs, machines, and production requirements differ. Use material-specific data from the tap manufacturer when available; otherwise, validate the process rather than applying a universal offset.
A practical decision rule is:
- Use the exact tap manufacturer’s recommendation first.
- If that is unavailable, choose a chart whose tap type and reaming assumption match the process.
- Treat disputed values as alternatives requiring validation, not interchangeable answers.
- Inspect or gauge the completed thread under the applicable production requirement.
The goal is not to choose the fraction repeated by the most websites. It is to select a starting hole that works with the actual tap, material, machine, preparation method, and acceptance process.
A five-step drill-selection workflow
A chart becomes safer and more useful when it is part of a controlled selection process.
1. Identify the thread form
Confirm whether the drawing, fitting, or repair procedure requires:
- NPT
- NPTF
- NPS
- NPSF
Do this before selecting a drill column. NPT and NPS differ in taper, and the fact that some third-party charts group NPT with NPTF does not prove that identical preparation is optimal for every tool or application.
If the print merely says “pipe thread,” resolve the ambiguity. A drill selected for a straight pipe thread should not automatically be used for a tapered pipe thread.
2. Identify the tap type
Determine whether the tool is:
- A conventional cutting tap, which removes material; or
- A roll-form or cold-form tap, which displaces material.
This distinction is fundamental. The cited form-tap starting holes are substantially larger than cutting-tap holes.
Check the tap body, packaging, catalog number, or manufacturer’s documentation rather than judging the tool from appearance alone.
3. Confirm the hole-preparation method
Determine whether the operation will:
- Tap directly from a straight drilled hole;
- Prepare a tapered hole by drilling, reaming, or another controlled method; or
- Use a taper reamer as a separate operation.
Do not silently carry its values into a tapered-hole process.
Likewise, do not assume that an unexplained larger chart value proves the use of a taper reamer. Unless the publisher identifies the preparation method, the reason for the difference remains uncertain.
4. Check the tooling instructions and machining conditions
Consult the documentation for the exact tap. Consider the work material, tap geometry, required engagement, lubrication, spindle alignment, machine rigidity, and likely behavior of the drilled hole.
Measure the produced hole when the result matters. A drill’s stamped size does not prove that the actual hole has that diameter. Wear, runout, point geometry, machine condition, and workpiece behavior can affect the result.
A starting hole that is too small can increase tapping torque and the risk of tap breakage. A hole that is too large can reduce thread engagement and may impair sealing performance, a tradeoff also described in 6G Tools’ NPT drill-size guidance.
That principle does not establish a universal material correction. For stainless steel or any other material, use validated tooling data and a stable process rather than enlarging or reducing the hole by habit.
5. Inspect the finished thread
The pilot-hole diameter is an input, not the acceptance criterion. Inspect or gauge the completed thread according to the applicable drawing, specification, tooling instructions, and quality procedure.
For controlled production or pressure-critical service, consult the applicable edition of ASME B1.20.1 and use the inspection process required by the job. This article does not reproduce or independently verify the standard’s tolerances, gauging procedures, or acceptance dimensions.
Worked example: 1/2-14 NPT
- 45/64 inch, or 0.7031 inch
- 23/32 inch, or 0.7188 inch
The difference is 1/64 inch, or 0.0156 inch. Do not decide by assuming that one value is universally standard and the other is wrong.
Instead:
- Confirm that the requirement is tapered NPT rather than straight NPS.
- Confirm that the tool is a cutting tap.
- Determine whether the process is direct tapping or includes tapered preparation.
- Check whether the toolmaker specifies 45/64, 23/32, or another controlled diameter.
- Drill and measure the hole.
- Tap under the recommended conditions.
- Inspect the completed thread.
Worked example: 1-11-1/2 NPT
Published charts also split between:
- 1-9/64 inch, or 1.1406 inches
- 1-5/32 inch, or 1.1563 inches
The difference is again 0.0156 inch. The appropriate choice depends on the tap and process, not on which fraction is easier to obtain. If the manufacturer’s instructions are unavailable, use a process-matched recommendation, test it in representative material, and evaluate the finished thread under the applicable inspection requirement.
NPT versus NPS—and where NPTF and NPSF fit
NPT means National Pipe Taper, while NPS means National Pipe Straight. NPT threads are tapered; NPS threads are straight. Their drill-size columns should therefore remain separate.
NPTF means National Pipe Taper Fuel, and NPSF means National Pipe Straight Fuel. Some charts group NPT with NPTF and NPS with NPSF. That documents a chart publisher’s shared drill recommendation; it does not prove identical preparation is optimal for every tool, production method, or sealing requirement.
NPS/NPSF direct-tapping comparison
The following NPS/NPSF values come from the same Allied direct-tapping chart used for the primary baseline. That chart supplies no NPS/NPSF drill value for 1/16 inch.
| Nominal size | TPI | NPS/NPSF drill | Decimal inches |
|---|---|---|---|
| 1/16 | 27* | No drill value supplied | — |
| 1/8 | 27 | S | 0.3480 |
| 1/4 | 18 | 29/64 | 0.4531 |
| 3/8 | 18 | 19/32 | 0.5938 |
| 1/2 | 14 | 47/64 | 0.7344 |
| 3/4 | 14 | 15/16 | 0.9375 |
| 1 | 11-1/2 | 1-3/16 | 1.1875 |
| 1-1/4 | 11-1/2 | 1-33/64 | 1.5156 |
| 1-1/2 | 11-1/2 | 1-3/4 | 1.7500 |
| 2 | 11-1/2 | 2-7/32 | 2.2188 |
| 2-1/2 | 8 | 2-21/32 | 2.6563 |
| 3 | 8 | 3-9/32 | 3.2813 |
| 3-1/2 | 8 | 3-25/32 | 3.7813 |
| 4 | 8 | 4-9/32 | 4.2813 |
*The 27-TPI entry reflects the corresponding 1/16 pipe-thread pitch shown in the cited NPT/NPTF column; the source does not supply a 1/16 NPS/NPSF drill recommendation.
The 1/2-inch rows provide a simple comparison:
- 1/2 NPT/NPTF baseline: 45/64 inch
- 1/2 NPS/NPSF: 47/64 inch
The values differ by 1/32 inch. Substituting the NPT number into an NPS operation—or vice versa—ignores the thread form for which each recommendation was published.
NPT and NPS components may begin to engage mechanically when nominal size and pitch correspond, but that does not establish a proper sealing joint. Connection design, mating components, sealing elements, service conditions, and applicable requirements still matter.
Similarly, the “Fuel” designation should not be interpreted as a universal promise that no application-specific sealing decision is necessary. Use the specified thread form, compatible components, and the sealing method required for the actual system.
Cutting taps and roll-form taps need different hole charts
A roll-form tap creates the profile by displacing material.
The following data are intentionally separated from the conventional cutting-tap chart.
Manufacturer-specific Jarflo roll-form NPT data—not for conventional cutting taps
These straight-hole and tapered-hole ranges are published specifically for Jarflo roll-form pipe taps by Jarvis Cutting Tools. They must not be generalized to other forming tools or substituted into a cutting-tap process.
| Jarflo NPT form tap | Straight-hole range—first | Tapered-hole range—second |
|---|---|---|
| 1/16-27 | 0.272–0.275 in | 0.281–0.284 in |
| 1/8-27 | 0.365–0.368 in | 0.374–0.377 in |
| 1/4-18 | 0.476–0.480 in | 0.490–0.494 in |
| 3/8-18 | 0.626–0.630 in | 0.640–0.644 in |
| 1/2-14 | 0.761–0.766 in | 0.779–0.784 in |
| 3/4-14 | 0.971–0.976 in | 0.990–0.995 in |
| 1-11-1/2 | 1.220–1.230 in | 1.240–1.245 in |
The 1/4-18 values show the magnitude of the process difference:
- Conventional cutting-tap baseline: 0.4375 inch
- Jarflo form-tap straight-hole range: 0.476–0.480 inch
- Jarflo form-tap tapered-hole range: 0.490–0.494 inch
The smallest listed Jarflo straight-hole value is 0.0385 inch larger than the conventional cutting-tap baseline. That is not a minor disagreement between otherwise interchangeable charts; it reflects a different thread-making process.
For these specific tools, Jarvis states that tapered preparation can provide more consistent thread height through the threaded depth. It also says straight preparation can reduce tapping torque and improve tool life, although partially formed crests may remain near the bottom.
That tradeoff is manufacturer-specific guidance, not a universal ranking of straight and tapered holes. Work material, machine, tool, required profile, and production controls still matter.
Jarvis further states that tapered starting holes for this process may be drilled, reamed, or cored. It describes NPTF and NPSF as impractical to form with these tools because those forms require fully formed crests and 100% thread. Those statements apply to the cited Jarflo system and should not be expanded into claims about every roll-form tool.
The shop-floor rule is simple: if the tool forms rather than cuts, put away the conventional cutting-tap chart and retrieve the data for that exact forming tap.
Straight versus tapered starting holes and taper reaming
An NPT thread is tapered, but that does not mean every starting hole must be taper-reamed. Engineers Edge states that holes prepared for NPT tapping may be straight or tapered, provided the completed thread meets the applicable product dimensions. It also notes that the actual pre-tapping hole size depends on multiple variables.
Keep the thread form separate from the preparation method:
- Thread form: The finished NPT thread is tapered.
- Starting-hole form: The hole before tapping may be straight or intentionally tapered in an appropriate process.
- Main-chart assumption: A conventional cutting tap is used directly without a taper reamer.
- Alternative process: The hole is intentionally prepared with a taper before tapping.
A straight pilot hole can therefore be part of an NPT tapping process. Acceptance depends on the resulting thread, not on the straightness or nominal diameter of the pilot hole by itself.
Tapered preparation may be appropriate where the toolmaker recommends it or where a validated process requires it. The dimensions must come from guidance applicable to that tool. A manufacturer-specific roll-form range cannot be inserted into a general cutting-tap chart merely because both operations produce an NPT thread.
The same caution applies to conflicting cutting-tap charts. Although several alternative values are larger than the no-taper-reamer baseline, the evidence does not identify every alternative as a taper-reamer value. Differences may reflect tool geometry, engagement targets, drilling behavior, or another undisclosed convention.
For the cited Jarflo process specifically, a tapered starting hole may be drilled, reamed, or cored. That manufacturer guidance has no established applicability to an unrelated cutting tap.
Use the primary NPT chart only when all three conditions are true:
- The required thread is NPT, or the applicable tooling chart expressly groups the intended form with NPT.
- The tool is a conventional cutting tap.
- The process is direct tapping without a taper reamer.
If any condition is false or uncertain, stop and obtain the applicable tooling instructions.
Verification, sealing, and a printable shop reference
Selecting a listed drill does not establish that the completed thread complies with a governing specification. It also does not guarantee that an assembled connection will be leak-free.
For production, controlled, or pressure-critical work:
- Follow the instructions supplied for the exact tap.
- Consult the applicable edition of ASME B1.20.1.
- Measure or otherwise control the produced starting hole where required.
- Inspect the finished thread under the applicable quality procedure.
- Verify that the mating components and sealing method suit the service.
The recommendation to consult ASME B1.20.1 is a direction to obtain the governing requirements, not a claim that this article has reproduced or independently verified them. This article intentionally omits unsupported L1 or L2 gauging sequences, tolerance limits, minimum tapping depths, blind-hole allowances, and material-specific corrections.
NPT joints commonly use an application-appropriate thread sealant or tape, but that is not a universal prescription. Fluid compatibility, pressure, temperature, fitting material, cleanliness requirements, and assembly procedure must all be considered. A drill-size chart cannot resolve those questions or guarantee sealing performance.
What must remain on a printable chart
A shop copy can become misleading when its title, scope, or warnings are cropped away. Keep the baseline, conflict table, NPS/NPSF table, and manufacturer-specific roll-form table visually separate in both screen and print layouts.
The primary chart should retain this completed footer:
Chart scope: Conventional cutting taps; direct tapping without a taper reamer. Values are recommendations, not universal requirements. Alternative published values exist. Verify the exact tap manufacturer’s instructions and inspect the finished thread. Revision: August 16, 2026. Baseline attribution: Allied Machine National Pipe Threads tap-drill chart; calculated decimal and metric conversions are labeled as such. Limitations: Not an official ASME table; not for roll-form taps; do not substitute NPS/NPSF values.
A printable page should also include this compact pre-use checklist:
- [ ] Confirm the thread form: NPT, NPTF, NPS, or NPSF.
- [ ] Confirm the tap type: cutting or roll-form.
- [ ] Confirm the preparation method: direct tapping or tapered preparation.
- [ ] Check the exact tap manufacturer’s data.
- [ ] Measure the produced hole where required.
- [ ] Inspect the finished thread under the required procedure.
An unattributed printout with no revision date or scope statement can survive in a tool cabinet long after its process assumptions have been forgotten.
Tubing Chart publishes its technical content for general information, provides it as-is, and places responsibility for application on the user under its terms of use.
What size drill should I use for a 1/8-27 NPT tap?
An R drill at approximately 0.339 inch is also a documented commercial recommendation in the DrillsandCutters NPT chart.
Use Q only when the no-reamer cutting-tap assumption matches the operation. If the tap manufacturer specifies R or another diameter, follow that tool-specific guidance and inspect the finished thread.
What is the tap drill size for 1/4-18 NPT?
Do not use that value for a roll-form tap. The documented Jarflo form-tap ranges are substantially larger and apply only to those named tools.
Why do charts list both 45/64 and 23/32 for a 1/2-14 NPT tap?
They are two documented recommendations from different chart families:
- 45/64 inch = 0.7031 inch
- 23/32 inch = 0.7188 inch
The difference is 0.0156 inch. The 45/64 value is explicitly documented as a no-taper-reamer baseline, while a separate engineering chart lists 23/32 without establishing that every such alternative assumes the same preparation method.
Do not assume the larger value necessarily means taper reaming. Check the exact tap maker’s instructions, match the chart to the tap and process, and inspect the resulting thread.
Can I use an NPT drill size for an NPS thread?
Not by default. NPT is tapered and NPS is straight, and direct-tapping charts provide separate drill columns.
For example, the cited 1/2-inch recommendations are 45/64 inch for NPT/NPTF and 47/64 inch for NPS/NPSF. The separate values and the warning that NPT and NPS should not be assumed to create a proper sealing combination are summarized in Fractory’s NPT and NPS chart.
Use the value for the required thread form and exact tool rather than substituting based on nominal size alone.
Can I use this NPT chart for a roll-form tap?
No. The primary chart is for conventional cutting taps.
For example, the Jarflo 1/4-18 form-tap data specify a 0.476–0.480-inch straight-hole range or a 0.490–0.494-inch tapered-hole range, compared with the 0.4375-inch cutting-tap baseline. Use the forming-tap maker’s data for the exact tool; do not generalize those Jarflo ranges to every form tap.
Start with the no-taper-reamer cutting-tap chart only when it matches the tool and process. Treat conflicting rows as choices requiring tap-maker confirmation, never substitute NPS or roll-form dimensions, and judge critical work by the inspected finished thread—not the pilot-hole number alone.