Guide
What dimensions, TPI and tap drill sizes are in an NPT thread chart?
An NPT thread chart is the table of basic dimensions for American National Standard Taper Pipe threads: for each nominal pipe size it gives the threads per inch, the pipe outside diameter, the pitch diameters at the small end and at the gauge plane, the hand-tight, effective and wrench-makeup lengths, and, in shop versions, the tap drill. The dimensions come from ASME B1.20.1; the tap drills and port depths are shop practice built on top of them.
By Saola Manufacturing engineering team · Published · Updated
This guide gives the full chart from 1/16 through 2 NPT in inches and millimeters, then turns it into the numbers a design engineer actually puts on a machined part: the tap drill, the depth of a tapped port, the wall around it and the callout. How NPT differs from the Dryseal and British tapers is covered separately in NPT vs NPTF and NPT vs BSPT.
On this page
What do the columns in an NPT thread chart mean?
Each column fixes one point on a cone: the thread tapers 1 in 16 on diameter, so a chart only needs a pitch diameter at one plane plus a set of lengths to define the whole thread. E0, E1, L1, L2 and L3 are those reference points.
TPI and pitch. Threads per inch are set per pipe size, not per diameter: 1/16 and 1/8 share one pitch, 1/4 and 3/8 another, 1/2 and 3/4 another, and every size from 1 through 2 shares the coarsest. The thread form is a 60-degree V with flat-truncated crests and roots, and per ASME B1.20.1 its height is 0.8 times the pitch.
D, the pipe OD. The outside diameter of the pipe, which is why the nominal size never matches any measured diameter. A 1/4 NPT male thread is a little over half an inch across.
E0 and E1. E0 is the pitch diameter at the small end of the external thread. E1 is the pitch diameter at the gauge plane, which for an internal thread is the face of the port. Because of the taper, per ASME B1.20.1 E1 equals E0 plus L1 divided by 16.
L1, L2, L3, L4. L1 is hand-tight engagement, the length a male thread enters a female thread by hand. L2 is the effective thread length, the full-form portion of the male thread. L3 is wrench makeup, the further travel when the joint is tightened, set per ASME B1.20.1 at three threads for sizes up to 2 NPT. L4 is the overall external thread length including the vanish thread left by the die chamfer.
What are the NPT thread dimensions from 1/16 to 2 NPT?
The table below lists the ASME B1.20.1 basic dimensions for the ten sizes most often tapped into machined parts, in inches. The second table repeats the same values converted to millimeters.
These are basic dimensions, not limits. NPT is toleranced by gauging rather than by plus-or-minus values on each diameter: the product passes if a standard L1 gauge sits within one turn of its basic position, per ASME B1.20.1. That is why a chart shows a single number per column. The pipe OD, E0 and the lengths are values the standard derives from formulas in D and the pitch, so they can be checked against each other: the effective length is the pitch times the sum of 0.8D and 6.8, per ASME B1.20.1.
| NPT size | TPI | Pipe OD D (in) | E0 pitch dia. small end (in) | E1 pitch dia. gauge plane (in) | L1 hand-tight (in) | L2 effective (in) | L3 wrench makeup (in) | L4 overall external (in) |
|---|---|---|---|---|---|---|---|---|
| 1/16 | 27 | 0.3125 | 0.27118 | 0.28118 | 0.160 | 0.2611 | 0.1111 | 0.3896 |
| 1/8 | 27 | 0.405 | 0.36351 | 0.37360 | 0.1615 | 0.2639 | 0.1111 | 0.3924 |
| 1/4 | 18 | 0.540 | 0.47739 | 0.49163 | 0.2278 | 0.4018 | 0.1667 | 0.5946 |
| 3/8 | 18 | 0.675 | 0.61201 | 0.62701 | 0.240 | 0.4078 | 0.1667 | 0.6006 |
| 1/2 | 14 | 0.840 | 0.75843 | 0.77843 | 0.320 | 0.5337 | 0.2143 | 0.7815 |
| 3/4 | 14 | 1.050 | 0.96768 | 0.98887 | 0.339 | 0.5457 | 0.2143 | 0.7935 |
| 1 | 11.5 | 1.315 | 1.21363 | 1.23863 | 0.400 | 0.6828 | 0.2609 | 0.9845 |
| 1-1/4 | 11.5 | 1.660 | 1.55713 | 1.58338 | 0.420 | 0.7068 | 0.2609 | 1.0085 |
| 1-1/2 | 11.5 | 1.900 | 1.79609 | 1.82234 | 0.420 | 0.7235 | 0.2609 | 1.0252 |
| 2 | 11.5 | 2.375 | 2.26902 | 2.29627 | 0.436 | 0.7565 | 0.2609 | 1.0582 |
| NPT size | Pitch (mm) | Pipe OD D (mm) | E1 pitch dia. (mm) | L1 hand-tight (mm) | L2 effective (mm) | L3 wrench makeup (mm) |
|---|---|---|---|---|---|---|
| 1/16 | 0.941 | 7.94 | 7.14 | 4.06 | 6.63 | 2.82 |
| 1/8 | 0.941 | 10.29 | 9.49 | 4.10 | 6.70 | 2.82 |
| 1/4 | 1.411 | 13.72 | 12.49 | 5.79 | 10.21 | 4.23 |
| 3/8 | 1.411 | 17.15 | 15.93 | 6.10 | 10.36 | 4.23 |
| 1/2 | 1.814 | 21.34 | 19.77 | 8.13 | 13.56 | 5.44 |
| 3/4 | 1.814 | 26.67 | 25.12 | 8.61 | 13.86 | 5.44 |
| 1 | 2.209 | 33.40 | 31.46 | 10.16 | 17.34 | 6.63 |
| 1-1/4 | 2.209 | 42.16 | 40.22 | 10.67 | 17.95 | 6.63 |
| 1-1/2 | 2.209 | 48.26 | 46.29 | 10.67 | 18.38 | 6.63 |
| 2 | 2.209 | 60.33 | 58.33 | 11.07 | 19.22 | 6.63 |
What tap drill size does each NPT size need?
Use the smaller drill of the pair when the hole will be taper-reamed before tapping, and the larger one when the pipe tap has to cut the taper on its own; the two typically differ by 1/64 to 1/32 in. The most widely repeated pair for 1/4 NPT is 7/16 without a reamer and 27/64 with one.
A straight drilled hole does not match a tapered thread. The federal pipe-thread handbook (NBS H28, Part II) lists K0, the basic minor diameter at the small end of the pipe, as information for selecting tap drills. The common unreamed drills sit just above K0 (23/32, or 0.719 in, against 0.701 in for 1/2 NPT), so the pipe tap shaves the cone toward the face on its own and torque climbs as it goes deeper. The common reamed drills sit just below K0, because the taper pipe reamer opens the cone first and the tap then only forms the thread. The reamer is ground to the thread taper, 3/4 inch per foot on diameter or 1 in 16 per ASME B1.20.1.
Tap drills are shop data, not basic dimensions of the thread, and published charts disagree by a drill size in places: for unreamed holes, 3/8 NPT appears as 37/64 in some charts and 9/16 in others, 1/2 NPT as 23/32 or 45/64, and 1/8 NPT as letter R or Q. The values below follow the charts that list a reamed and an unreamed drill side by side. Treat them as a starting point and let the shop confirm them against the tap maker’s data and an L1 plug gauge on first parts.
| NPT size | Drill, no reamer | Drill, with taper reamer | Drill no reamer (mm) | Drill with reamer (mm) | Min. full thread L1+L3+1 pitch (in) | Min. full thread (mm) |
|---|---|---|---|---|---|---|
| 1/16-27 | D (0.246 in) | 15/64 (0.2344 in) | 6.25 | 5.95 | 0.31 | 7.8 |
| 1/8-27 | R (0.339 in) | 21/64 (0.3281 in) | 8.61 | 8.33 | 0.31 | 7.9 |
| 1/4-18 | 7/16 (0.4375 in) | 27/64 (0.4219 in) | 11.11 | 10.72 | 0.45 | 11.4 |
| 3/8-18 | 37/64 (0.5781 in) | 9/16 (0.5625 in) | 14.68 | 14.29 | 0.46 | 11.7 |
| 1/2-14 | 23/32 (0.7188 in) | 11/16 (0.6875 in) | 18.26 | 17.46 | 0.61 | 15.4 |
| 3/4-14 | 59/64 (0.9219 in) | 57/64 (0.8906 in) | 23.42 | 22.62 | 0.62 | 15.9 |
| 1-11.5 | 1-5/32 (1.1563 in) | 1-1/8 (1.1250 in) | 29.37 | 28.58 | 0.75 | 19.0 |
| 1-1/4-11.5 | 1-1/2 (1.5000 in) | 1-15/32 (1.4688 in) | 38.10 | 37.31 | 0.77 | 19.5 |
| 1-1/2-11.5 | 1-47/64 (1.7344 in) | 1-23/32 (1.7188 in) | 44.05 | 43.66 | 0.77 | 19.5 |
| 2-11.5 | 2-7/32 (2.2188 in) | 2-3/16 (2.1875 in) | 56.36 | 55.56 | 0.78 | 19.9 |
How deep should an NPT port be tapped in a machined part?
A practical design rule is a minimum full-thread depth of at least L1 plus L3 plus one pitch, so the male fitting can be wrench-tightened in a port that gauges a full turn deep. Then add room below it for the tap chamfer and the drill point.
The reasoning is the chart itself. L1 is where the fitting stops by hand and L3 is the further travel when it is wrenched up, so a port shorter than L1 plus L3 lets the male thread bottom on unformed metal before the joint is tight. ASME B1.20.1 also accepts an internal thread that gauges up to one turn deep, which moves the fitting one pitch further in; adding that pitch keeps a port at the deep edge of tolerance from running out of thread. If the mating fitting may also sit at its small limit, a second pitch of margin is cheap insurance. The resulting minimum full-thread depths are in the last two columns of the tap drill table.
Below the last full thread, a pipe tap leaves its chamfered lead, a few threads long and set by the tap maker’s catalog, and the drill leaves its point. On a blind port in a manifold, those three lengths stacked together set how close the port can come to a cross-drilled passage or the opposite face. The same stack-up logic for straight threads is in the blind tapped hole depth guide.
A taper port has one more depth trap: the deeper the tap is driven, the larger the port face diameter becomes, and the fitting sits deeper. A tap run in too far makes a port that gauges deep and lets the fitting bottom early; a tap run short makes a port that the fitting cannot enter far enough to seal. That is why the gauge, not a depth micrometer, accepts the port.
How much wall does an NPT port need around it?
No thread standard sets a minimum wall for an NPT port, so size it from the internal major diameter at the port face and the pressure and assembly torque the joint will see. A taper wedges outward as it is tightened, so give it more wall than a straight thread of the same size.
The largest diameter of the port is at its face, where the internal major diameter is roughly E1 plus 0.8 pitch per ASME B1.20.1; for every size that lands close to the pipe OD. Measure wall thickness from there, not from the tap drill. Each extra turn of makeup on a 1:16 taper drives the cone deeper and raises the hoop stress in the boss, which is why an overtightened pipe fitting can split a boss or crack a thin aluminum wall long before the thread strips.
Check the wall at the port face against the fitting’s assembly torque and the working pressure rather than a fixed ratio, and keep the port face clear of edges and neighboring ports so the wrench and the fitting hex also fit. Engagement and wall logic for straight tapped holes is in the thread engagement guide. For high-pressure hydraulic manifolds, a straight-thread O-ring port is usually a better choice than NPT, because the seal no longer depends on wedging the thread.
How do you call out an NPT thread on a drawing?
Write the nominal size, the threads per inch and the series, as in “1/4-18 NPT”, and add a minimum full-thread depth for blind ports. If the port must seal without sealant, call out NPTF instead, because plain NPT relies on a sealant or tape to seal.
The designation is the ASME B1.20.1 format: size, pitch, series. On an internal feature, the gauge does the acceptance, so a note restating the one-turn gauge tolerance is redundant but harmless; what the drawing does need is anything the standard leaves open, such as depth, a countersink or chamfer at the face, and whether the port is tapped before or after coating. The table gives callout patterns a buyer can copy onto a print or an RFQ.
| Situation | Callout on the drawing | Why |
|---|---|---|
| Through port, sealant or tape at assembly | 1/4-18 NPT | Gauge controls fit; depth is the wall |
| Blind port in a manifold | 1/4-18 NPT ↓ .45 MIN FULL THREAD | L1 + L3 + 1 pitch from the chart |
| Blind port near a cross hole | 1/2-14 NPT ↓ .61 MIN FULL THREAD, DRILL ↓ .95 MAX | Caps tap lead and drill point |
| Port must seal without sealant | 1/8-27 NPTF-2 | Class 2 requires crest and root truncation inspection |
| Chamfered port face | 1/2-14 NPT, C'SINK ø.88 X 90° | Protects the first thread from burrs and damage |
| Port in a coated part | 3/8-18 NPT, MASK OR TAP AFTER COATING | Coating changes gauge standoff |
When we review a drawing with pipe ports, we check that each port has a depth that clears cross holes after the tap lead and drill point, that the wall around the face suits the fitting torque, and whether the note says NPT or NPTF. Tapered ports in bar and plate are standard CNC milling work, and ports on the axis of round parts are cut on CNC turning centers. On the RFQ, list the fitting each port receives, the working pressure and whether sealant is used, and send the model through the quote form.
How is an NPT thread checked against the chart?
With an L1 taper plug gauge for internal threads and an L1 ring gauge for external threads: the gauge is screwed on hand-tight and its step or notch must fall within one turn of the part face. Calipers and pin gauges cannot accept a taper thread.
The L1 gauge checks the pitch diameter at the gauge plane, which is the E1 column of the chart. A port that lets the plug in too far is oversize and the fitting will bottom; one that stops short is undersize and the fitting will stand proud. The federal pipe-thread standard in NBS Handbook H28, Part II describes the same L1 gauging as a functional size check; where leaks have to be prevented without sealant, ASME B1.20.3 Dryseal Class 2 adds crest and root truncation inspection. General inspection practice for tapped holes is in the go/no-go thread gauge guide.
Frequently asked
Why is a 1/4 NPT thread bigger than its nominal size?
Because NPT sizes name the nominal pipe, not any thread diameter. Per ASME B1.20.1 the 1/4 size has a pipe OD of 0.540 in, so the male thread measures a little over half an inch.
Is the tap drill the same with or without a taper reamer?
No. A reamed hole uses a smaller drill because the reamer cuts the cone, and the tap only forms the thread. An unreamed hole uses a slightly larger drill so the tap can cut the taper without excessive torque. In the published two-column charts, the two drills typically differ by 1/64 to 1/32 in.
How many turns does an NPT fitting take to make up?
Per ASME B1.20.1 the chart allows about four to five turns hand-tight (L1) and three more turns of wrench makeup (L3) for sizes up to 2 NPT. A fitting that runs in well past that is a sign the port, the fitting or both are out of gauge, most often a port tapped too deep.
Can a metric drawing call out an NPT port?
Yes. The callout stays in inch form, such as 1/4-18 NPT, even on a millimeter drawing; only the depth and position dimensions are metric. Use the millimeter table here for the depth note and keep the designation unchanged.
Does an NPT chart apply to NPTF Dryseal threads?
The pitch diameters, TPI and lengths are the same basic values, but NPTF also limits crest and root truncation so the flanks and crests close up and seal without sealant; Class 2 is the class that requires that truncation to be inspected. Use NPTF gauges and the NPT vs NPTF guide if the port must be dry-sealing.
Sources
- ASME B1.20.1-2013 (R2018), Pipe Threads, General Purpose, Inch, ASME, 2013, reaffirmed 2018
- ASME B1.20.3-1976 (R2023), Dryseal Pipe Threads, Inch, ASME, 1976, reaffirmed 2023
- Handbook H28, Screw-Thread Standards for Federal Services, Part II: Pipe Threads, National Bureau of Standards (now NIST), 1957