1/8-27 NPT NPT (taper pipe thread) External Thread Specifications

1/8-27 NPT · 0.125-27 NPT

ASMESTD: ASME B1.20.1SOURCE: Table 2 · p. 9TYPE: External (bolt/screw)CLASS: —SERIES: NPT (taper pipe thread)TAPER 1:16
Major diameter
0.40554 – 0.39625
max – min, at the gage plane
Pitch diameter
0.37591 – 0.37129
max – min, at the gage plane
Minor diameter
0.35095 – 0.34166
max – min, at the gage plane
Pitch
0.03704
27 threads per inch

Thread profile

gage plane: hand-tight, L1 from small endthread axisD1 = 0.4055–0.3962major dia (crest)E1 = 0.37591–0.37129pitch dia, basic 0.37360K1 = 0.3509–0.3417minor dia (root)P = 0.0370460°⟂ axis30°crest flat = 0.0014–0.0041crest truncation 0.0012–0.0036root flat = 0.0014–0.0041root truncation 0.0012–0.0036H0.032075h0.029630 max0.024960 mintaper 1:16 on dia (1:32 on radius) · β = 1° 47′ 24″, included 2β = 3° 34′ 47″ · bisector ⟂ axisexternal thread (male)

External profile over three pitches at the gage plane, drawn at maximum material (sharpest crest, flattest root); hatched bands are the truncation zones. Diameters shortened.

Thread length, taper and gage planes

plane 0 · E00.36351plane 1 · E10.37360plane 2 · E20.38000pipe OD D 0.4050MALE · 1/8-27 NPTtrue scale (lengths and diameters); axis below, not drawnL1 0.1615 (4.36 thd)full thread 0.2639 (7.13 thd)V 0.1285

Lengths, taper and gage planes to scale; diameters shortened. D and K at the planes are calculated with the mean truncations.

Limits of size, class —

DimensionBasicMaxMinTolerance
Major diameter D1Crest of the external thread at the gage plane0.405540.396250.00929
Pitch diameter E1Gage plane E1: L1 from the small end (external) or at the large end of the fitting (internal); gaging tolerance converted to diameter0.373600.375910.371290.00462
Minor diameter K1Root of the external thread at the gage plane0.350950.341660.00929
Allowance esNone: taper threads are made up to interference (NPT/NPTR); NPSC uses sealant—
Crest truncation fcTable 1 (both threads)0.0035560.001222
Root truncation frTable 1 (both threads)0.0035560.001222
Crest flat FcEquivalent width of flat0.0041110.001407
Root flat FrEquivalent width of flat0.0041110.001407

Dimensions in inches. Values are those of the standard (ASME B1.20.1); calculated cells follow the standard’s formulas. Where the standard prints a value that differs from its formula, the printed value is shown with a note.

Taper, lengths and gaging

Taper, gage and sealing paras. 3, 5; Tables 1, 3

Taper on the diameter T
1 in 16 (0.75 in./ft)
Half apex angle of the pitch cone β
1.789911 °
Taper tolerance on the diameter, max / min
0.1250 / -0.0625 in/ft
Lead tolerance in the effective length ±
0.003 in
60° angle tolerance ±
2.5 °
Lead angle at the basic PD λ
1° 48′
Gaging method
L1 ring gage (external) / L1 plug gage (internal), or L1 indicating gage (para. 3.2)
Gaging tolerance
±1 turn
Gaging position
Small end of pipe flush with small end of ring
Sealing
Pressure-tight: wrench-tight with sealant (para. 1.4)

Basic pitch diameters and lengths Table 3

Pitch dia at the small end of the external thread E0
0.36351 in
Hand-tight engagement L1
0.1615 · 4.36 in, threads
Pitch dia at the hand-tight plane E1
0.37360 in
Effective external thread L2
0.2639 · 7.13 in, threads
Pitch dia at L2 E2
0.38000 in
Wrench make-up length L3
0.1111 · 3 in, threads
Pitch dia at the wrench make-up plane E3
0.35656 informula gives 0.35657
Complete external threads L5
0.1898 in
Pitch dia at L5 E5
0.37537 in
Vanish thread V
0.1285 · 3.47 in, threads
Overall external thread length L4
0.3924 in
Basic minor dia at the small end K0
0.3338 informula gives 0.3339
Change of diameter per turn
0.00231 in
Hand-tight engagement range, ± 1 turn
0.0874 – 0.2356 in
Wrench make-up interference on the PD
0.00694 in
Hand-tight plus wrench make-up length L1+L3
0.2726 in

L1 plug and ring gages Tables 8 and 9

Plug gage major dia at the E0 plane
0.38521 in
Plug gage major dia at the gaging notch (E1)
0.39531 in
Plug gage major dia at the large end (E2)
0.40171 informula gives 0.40170
Gage pitch dia at the E0 plane
0.36351 in
Gage pitch dia at the gaging notch (E1)
0.37360 in
Gage pitch dia at the large end (E2)
0.38000 in
Ring gage minor dia at the E0 plane
0.34180 in
Ring gage minor dia at the gaging notch (E1)
0.35190 in
Thickness of the L1 ring or gage elements
0.1615 in
Working gage crest truncation
0.005185 in
Maximum standoff between plug and ring gages
0.032 in

Working gage tolerances Table 7

On the pitch diameter ±
0.0002 in
Lead, plugs / rings
0.0002 / 0.0003 in
Half angle ±, plugs / rings
15 / 20 min
Taper, plugs / rings
0.0003 / 0.0006 in
Major dia of plugs
0.0004 in
Minor dia of rings
0.0004 in
Total cumulative tolerance on the PD, plugs / rings
0.00080 / 0.00118 in

Notes

  • Table 2 Notes: L1 is also the length of the L1 ring gage and the distance from the gaging notch to the small end of the L1 plug; E1 is also the PD at the gaging notch of the L1 plug; L2 is also the threaded length of the L1 plug; L4 is a reference dimension; L5 = L2 − 2P (the next two threads are complete at the root); K0 is given for selecting tap drills (Appendix B). Gaging: ±1 turn from basic (Fig. 4 / Fig. 6); with indicating gages ±0.0625/n on diameter (paras. 3.2.1.2, 3.2.2.2). The gaging tolerance can lengthen or shorten engagement by up to two turns; gage wear reduces it further (para. 3.1.9.1). Diameter limits here are calculated at the gage plane (E1 ± 0.0625/n on PD; major and minor add the Table 1 truncation limits). The standard itself tabulates basic dimensions only. Chamfer: external threads with chamfer smaller than the minor dia are gaged from the last scratch of the chamfer cone (para. 3.2.4.1, Fig. 5); otherwise from the end of the pipe (para. 3.2.4.3).
  • E3 (col. 16): printed 0.35656, formula 0.35657 (1 in last place, rounding); K0 (col. 24): printed 0.3338, formula 0.3339 (1 in last place, rounding); Gage plug major at E2 (col. 7): printed 0.40171, formula 0.4017 (1 in last place, rounding)

Mates

1/8-27 NPT screws into a thread of the same standard at the same size and pitch. The standard mating internal thread is class Std. Check the fit with the standard pair →

Working out the mating options…

Different pitch or a different thread family never mate; the fit check runs over both tolerance zones.

Related thread sizes

Same diameter other pitch, and neighbouring sizes

→ 1/8-27 NPSM→ 1/8-27 NPSL→ 1/16-27 NPT→ 1/4-18 NPT→ 1/8-27 NPT internal
All values for 1/8-27 NPTevery column of the workbook row
All values of the workbook row 1/8-27 NPT
Designation1/8-27 NPT
Decimal designation0.125-27 NPT
Threads per inch27
Thread formTaper
SeriesNPT
Series nameAmerican National Standard Taper Pipe Thread
External / internalExternal
Class—
Nominal pipe size1/8
Pitch P (in.)0.03703704
O.D. of pipe D, basic major dia (in.)0.4050
Size category (Table 3 group)1/16, 1/8 (27 TPI)
Source tableTable 2
Printed page9
Where the diameter limits applyGage plane E1: L1 from the small end (external) or at the large end of the fitting (internal); gaging tolerance converted to diameter
Major dia max (in.)0.40554
Major dia min (in.)0.39625
Major dia tolerance (in.)0.00929
Major dia statusCalculated at the gage plane: PD limits with Table 1 truncation limits (para. 2.4)
Basic pitch dia at gage plane E1 (in.)0.37360
Pitch dia max (in.)0.37591
Pitch dia min (in.)0.37129
PD tolerance (in.)0.00462
Minor dia max (in.)0.35095
Minor dia min (in.)0.34166
Minor dia tolerance (in.)0.00929
Minor dia statusCalculated at the gage plane: PD limits with Table 1 truncation limits (para. 2.4)
Allowance sourceNone: taper threads are made up to interference (NPT/NPTR); NPSC uses sealant
Root radius max (in.)not given in the standard
Root radius min (in.)not given in the standard
Lead tolerance ± in effective length (in.)0.003
60° angle tolerance ± (deg)2.5
Taper tolerance max (in./ft)0.1250
Taper tolerance min (in./ft)-0.0625
Lead / angle / taper sourceTable 3, p. 10
Lead angle at basic PD (deg)1.807416
Lead angle (deg min)1° 48′
Tensile stress area As (sq in.)not given in the standard
Thread angle (deg)60
Flank angle (deg)30
Sharp V height H (in.)0.0320750
Thread height h max (in.)0.02963
Thread height h min (in.)0.02496
Crest truncation min (in.)0.001222
Crest truncation max (in.)0.003556
Root truncation min (in.)0.001222
Root truncation max (in.)0.003556
Crest flat min (in.)0.001407
Crest flat max (in.)0.004111
Root flat min (in.)0.001407
Root flat max (in.)0.004111
Sweep cut: start diameter (in.)0.39314
Sweep cut: tip diameter (in.)0.33388
Sweep cut: depth (in.)0.029630
Sweep cut: profile width at start diameter (in.)0.035630
Sweep cut: tip flat (in.)0.001407
Sweep cut: start diameter isMajor dia at the small end (E0 plane), basic position; the blank is a 1:16 cone
Taper half-angle β (deg)1.789911
Taper on diameter1 in 16 (0.75 in./ft)
E0 pitch dia at small end (in.)0.36351
L1 hand-tight engagement (in.)0.1615
L1 (threads)4.36
E1 pitch dia at hand-tight plane (in.)0.37360
L2 effective thread, external (in.)0.2639
L2 (threads)7.13
E2 pitch dia at L2 plane (in.)0.38000
L2 − L1 (in.)0.1024
L2 − L1 (threads)2.76
L3 wrench make-up, internal (in.)0.1111
L3 (threads)3
E3 pitch dia at wrench make-up plane (in.)0.35656
V vanish thread (in.)0.1285
V (threads)3.47
L4 overall external thread length, reference (in.)0.3924
L5 complete external threads (in.)0.1898
E5 pitch dia at L5 plane (in.)0.37537
Height of thread h, Table 2 col. 22 (in.)0.02963
Change in dia per turn 0.0625/n (in.)0.00231
K0 basic minor dia at small end (in.)0.3338
E0 by formula D − (0.05D + 1.1)P0.36351
E1 by formula E0 + 0.0625·L10.37360
L2 by formula (0.80D + 6.8)P0.2639
E2 by formula E0 + 0.0625·L20.38000
E3 by formula E0 − 0.0625·L30.35657
L5 by formula L2 − 2P0.1898
E5 by formula E0 + 0.0625·L50.37537
V by formula 3.47P0.1285
L4 by formula L2 + V0.3924
K0 by formula E0 − 0.8P0.3339
Gaging methodL1 ring gage (external) / L1 plug gage (internal), or L1 indicating gage (para. 3.2)
Gaging tolerance (turns)±1 turn
Gaging limit large (turns)1
Gaging limit small (turns)1
Basic gaging positionSmall end of pipe flush with small end of ring
Hand-tight engagement min, ±1 turn both parts (in.)0.0874
Hand-tight engagement max, ±1 turn both parts (in.)0.2356
Wrench make-up interference on PD, basic L3/16 (in.)0.00694
Hand-tight + wrench make-up length L1 + L3 (in.)0.2726
SealingPressure-tight: wrench-tight with sealant (para. 1.4)
Gage: plug major dia at E0 plane (in.)0.38521
Gage: plug major dia at E1 gaging notch (in.)0.39531
Gage: plug major dia at E2 large end (in.)0.40171
Gage: pitch dia at E0 plane (in.)0.36351
Gage: pitch dia at E1 gaging notch (in.)0.37360
Gage: pitch dia at E2 large end (in.)0.38000
Gage: ring minor dia at E0 plane (in.)0.34180
Gage: ring minor dia at E1 gaging notch (in.)0.35190
Gage: thickness of L1 ring or gage elements (in.)0.1615
Gage: working gage crest truncation (in.)0.005185
Gage: root must clear 0.0381P width (in.)0.001411
Gage: plug major at E0 by formula E0 + (H − 2·trunc)0.38521
Working gage tol. on PD ± (in.)0.0002
Working gage tol. lead, plugs (in.)0.0002
Working gage tol. lead, rings (in.)0.0003
Working gage tol. half angle, plugs ± (min)15
Working gage tol. half angle, rings ± (min)20
Working gage tol. taper, plugs + (in.)0.0003
Working gage tol. taper, rings − (in.)0.0006
Working gage tol. major dia, plugs − (in.)0.0004
Working gage tol. minor dia, rings + (in.)0.0004
Working gage total cumulative tol. on PD, plugs (in.)0.00080
Working gage total cumulative tol. on PD, rings (in.)0.00118
Max standoff between plug and ring gages (in.)0.032
Check against formulasE3 (col. 16): printed 0.35656, formula 0.35657 (1 in last place, rounding); K0 (col. 24): printed 0.3338, formula 0.3339 (1 in last place, rounding); Gage plug major at E2 (col. 7): printed 0.40171, formula 0.4017 (1 in last place, rounding)
NotesTable 2 Notes: L1 is also the length of the L1 ring gage and the distance from the gaging notch to the small end of the L1 plug; E1 is also the PD at the gaging notch of the L1 plug; L2 is also the threaded length of the L1 plug; L4 is a reference dimension; L5 = L2 − 2P (the next two threads are complete at the root); K0 is given for selecting tap drills (Appendix B). Gaging: ±1 turn from basic (Fig. 4 / Fig. 6); with indicating gages ±0.0625/n on diameter (paras. 3.2.1.2, 3.2.2.2). The gaging tolerance can lengthen or shorten engagement by up to two turns; gage wear reduces it further (para. 3.1.9.1). Diameter limits here are calculated at the gage plane (E1 ± 0.0625/n on PD; major and minor add the Table 1 truncation limits). The standard itself tabulates basic dimensions only. Chamfer: external threads with chamfer smaller than the minor dia are gaged from the last scratch of the chamfer cone (para. 3.2.4.1, Fig. 5); otherwise from the end of the pipe (para. 3.2.4.3).
Lookup key1/8-27 NPT · External

Related tools

ASME B1.20.1: Pipe Threads, General Purpose, Inch. Source: workbook data (2026-10-11). Confirm against your licensed copy of the standard before use.