1/4-18 NPT NPT (taper pipe thread) External Thread Specifications

1/4-18 NPT · 0.250-18 NPT

ASMESTD: ASME B1.20.1SOURCE: Table 2 · p. 9TYPE: External (bolt/screw)CLASS: —SERIES: NPT (taper pipe thread)TAPER 1:16
Major diameter
0.53955 – 0.52649
max – min, at the gage plane
Pitch diameter
0.49510 – 0.48816
max – min, at the gage plane
Minor diameter
0.45677 – 0.44371
max – min, at the gage plane
Pitch
0.05556
18 threads per inch

Thread profile

gage plane: hand-tight, L1 from small endthread axisD1 = 0.5395–0.5265major dia (crest)E1 = 0.49510–0.48816pitch dia, basic 0.49163K1 = 0.4568–0.4437minor dia (root)P = 0.0555660°⟂ axis30°crest flat = 0.0021–0.0057crest truncation 0.0018–0.0049root flat = 0.0021–0.0057root truncation 0.0018–0.0049H0.048113h0.044440 max0.038330 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 · E00.47739plane 1 · E10.49163plane 2 · E20.50250pipe OD D 0.5400MALE · 1/4-18 NPTtrue scale (lengths and diameters); axis below, not drawnL1 0.2278 (4.10 thd)full thread 0.4018 (7.23 thd)V 0.1928

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 plane0.539550.526490.01306
Pitch diameter E1Gage plane E1: L1 from the small end (external) or at the large end of the fitting (internal); gaging tolerance converted to diameter0.491630.495100.488160.00694
Minor diameter K1Root of the external thread at the gage plane0.456770.443710.01306
Allowance esNone: taper threads are made up to interference (NPT/NPTR); NPSC uses sealant—
Crest truncation fcTable 1 (both threads)0.0048890.001833
Root truncation frTable 1 (both threads)0.0048890.001833
Crest flat FcEquivalent width of flat0.0056670.002111
Root flat FrEquivalent width of flat0.0056670.002111

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 ±
2.0 °
Lead angle at the basic PD λ
2° 04′
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
0.47739 in
Hand-tight engagement L1
0.2278 · 4.10 in, threads
Pitch dia at the hand-tight plane E1
0.49163 in
Effective external thread L2
0.4018 · 7.23 in, threads
Pitch dia at L2 E2
0.50250 in
Wrench make-up length L3
0.1667 · 3 in, threads
Pitch dia at the wrench make-up plane E3
0.46697 in
Complete external threads L5
0.2907 in
Pitch dia at L5 E5
0.49556 in
Vanish thread V
0.1928 · 3.47 in, threads
Overall external thread length L4
0.5946 in
Basic minor dia at the small end K0
0.4329 in
Change of diameter per turn
0.00347 in
Hand-tight engagement range, ± 1 turn
0.1167 – 0.3389 in
Wrench make-up interference on the PD
0.01042 in
Hand-tight plus wrench make-up length L1+L3
0.3945 in

L1 plug and ring gages Tables 8 and 9

Plug gage major dia at the E0 plane
0.51339 in
Plug gage major dia at the gaging notch (E1)
0.52763 in
Plug gage major dia at the large end (E2)
0.53850 in
Gage pitch dia at the E0 plane
0.47739 in
Gage pitch dia at the gaging notch (E1)
0.49163 in
Gage pitch dia at the large end (E2)
0.50250 in
Ring gage minor dia at the E0 plane
0.44139 in
Ring gage minor dia at the gaging notch (E1)
0.45563 informula gives 0.45562
Thickness of the L1 ring or gage elements
0.2278 in
Working gage crest truncation
0.006056 in
Maximum standoff between plug and ring gages
0.036 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
0.0004 / 0.0007 in
Major dia of plugs
0.0006 in
Minor dia of rings
0.0006 in
Total cumulative tolerance on the PD, plugs / rings
0.00092 / 0.00134 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).
  • Gage ring minor at E1 (col. 12): printed 0.45563, formula 0.45562 (1 in last place, rounding)

Mates

1/4-18 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

→ 1/4-18 NPSM→ 1/4-18 NPSL→ 1/8-27 NPT→ 3/8-18 NPT→ 1/4-18 NPT internal
All values for 1/4-18 NPTevery column of the workbook row
All values of the workbook row 1/4-18 NPT
Designation1/4-18 NPT
Decimal designation0.250-18 NPT
Threads per inch18
Thread formTaper
SeriesNPT
Series nameAmerican National Standard Taper Pipe Thread
External / internalExternal
Class—
Nominal pipe size1/4
Pitch P (in.)0.05555556
O.D. of pipe D, basic major dia (in.)0.5400
Size category (Table 3 group)1/4, 3/8 (18 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.)0.53955
Major dia min (in.)0.52649
Major dia tolerance (in.)0.01306
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.)0.49163
Pitch dia max (in.)0.49510
Pitch dia min (in.)0.48816
PD tolerance (in.)0.00694
Minor dia max (in.)0.45677
Minor dia min (in.)0.44371
Minor dia tolerance (in.)0.01306
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)2.0
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)2.060036
Lead angle (deg min)2° 04′
Tensile stress area As (sq in.)not given in the standard
Thread angle (deg)60
Flank angle (deg)30
Sharp V height H (in.)0.0481125
Thread height h max (in.)0.04444
Thread height h min (in.)0.03833
Crest truncation min (in.)0.001833
Crest truncation max (in.)0.004889
Root truncation min (in.)0.001833
Root truncation max (in.)0.004889
Crest flat min (in.)0.002111
Crest flat max (in.)0.005667
Root flat min (in.)0.002111
Root flat max (in.)0.005667
Sweep cut: start diameter (in.)0.52184
Sweep cut: tip diameter (in.)0.43294
Sweep cut: depth (in.)0.044450
Sweep cut: profile width at start diameter (in.)0.053445
Sweep cut: tip flat (in.)0.002111
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.)0.47739
L1 hand-tight engagement (in.)0.2278
L1 (threads)4.10
E1 pitch dia at hand-tight plane (in.)0.49163
L2 effective thread, external (in.)0.4018
L2 (threads)7.23
E2 pitch dia at L2 plane (in.)0.50250
L2 − L1 (in.)0.1740
L2 − L1 (threads)3.13
L3 wrench make-up, internal (in.)0.1667
L3 (threads)3
E3 pitch dia at wrench make-up plane (in.)0.46697
V vanish thread (in.)0.1928
V (threads)3.47
L4 overall external thread length, reference (in.)0.5946
L5 complete external threads (in.)0.2907
E5 pitch dia at L5 plane (in.)0.49556
Height of thread h, Table 2 col. 22 (in.)0.04444
Change in dia per turn 0.0625/n (in.)0.00347
K0 basic minor dia at small end (in.)0.4329
E0 by formula D − (0.05D + 1.1)P0.47739
E1 by formula E0 + 0.0625·L10.49163
L2 by formula (0.80D + 6.8)P0.4018
E2 by formula E0 + 0.0625·L20.50250
E3 by formula E0 − 0.0625·L30.46697
L5 by formula L2 − 2P0.2907
E5 by formula E0 + 0.0625·L50.49556
V by formula 3.47P0.1928
L4 by formula L2 + V0.5946
K0 by formula E0 − 0.8P0.4329
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.)0.1167
Hand-tight engagement max, ±1 turn both parts (in.)0.3389
Wrench make-up interference on PD, basic L3/16 (in.)0.01042
Hand-tight + wrench make-up length L1 + L3 (in.)0.3945
SealingPressure-tight: wrench-tight with sealant (para. 1.4)
Gage: plug major dia at E0 plane (in.)0.51339
Gage: plug major dia at E1 gaging notch (in.)0.52763
Gage: plug major dia at E2 large end (in.)0.53850
Gage: pitch dia at E0 plane (in.)0.47739
Gage: pitch dia at E1 gaging notch (in.)0.49163
Gage: pitch dia at E2 large end (in.)0.50250
Gage: ring minor dia at E0 plane (in.)0.44139
Gage: ring minor dia at E1 gaging notch (in.)0.45563
Gage: thickness of L1 ring or gage elements (in.)0.2278
Gage: working gage crest truncation (in.)0.006056
Gage: root must clear 0.0381P width (in.)0.002117
Gage: plug major at E0 by formula E0 + (H − 2·trunc)0.51339
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.)0.0004
Working gage tol. taper, rings − (in.)0.0007
Working gage tol. major dia, plugs − (in.)0.0006
Working gage tol. minor dia, rings + (in.)0.0006
Working gage total cumulative tol. on PD, plugs (in.)0.00092
Working gage total cumulative tol. on PD, rings (in.)0.00134
Max standoff between plug and ring gages (in.)0.036
Check against formulasGage ring minor at E1 (col. 12): printed 0.45563, formula 0.45562 (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 key1/4-18 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.