1/4-18 NPSM NPSM (straight, mechanical) Internal Thread Specifications
1/4-18 NPSM · 0.250-18 NPSM
ASMESTD: ASME B1.20.1SOURCE: Table 6 · p. 17TYPE: Internal (nut/tapped hole)CLASS: 2BSERIES: NPSM (straight, mechanical)STRAIGHT
Major diameter
— – 0.5277
max – min, at the gage plane
Pitch diameter
0.4974 – 0.4916
max – min, at the gage plane
Minor diameter
0.481 – 0.468
max – min, at the gage plane
Pitch
0.05556
18 threads per inch
Thread profile
Internal profile over three pitches at the gage plane, drawn at maximum material (sharpest crest, flattest root); hatched bands are the truncation zones. Diameters shortened.
Limits of size, class 2B
| Dimension | Basic | Max | Min | Tolerance |
|---|---|---|---|---|
| Major diameter D1Root of the internal thread | — | 0.5277ref. | — | |
| Pitch diameter E1Straight thread: the limits apply along the whole length | 0.49163 | 0.4974 | 0.4916 | 0.0058 |
| Minor diameter K1Crest of the internal thread | 0.481 | 0.468 | 0.013 | |
| Allowance esTable 6 col. 4 (external, Class 2A) | 0.0013 | |||
| Crest truncation fcTable 1 (both threads) | 0.012028 | 0.012028 | ||
| Root truncation frTable 1 (both threads) | 0.006014 | 0.006014 | ||
| Crest flat FcEquivalent width of flat | 0.013889 | 0.013889 | ||
| Root flat FrEquivalent width of flat | 0.006944 | 0.006944 |
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
- none (straight)
- Half apex angle of the pitch cone β
- 0 °
- Lead angle at the basic PD λ
- 2° 04′
- Gaging method
- GO / NOT GO thread gages or indicating gages to ASME B1.2 (para. 6.4); NOT GO may enter up to 3 turns
- Gaging tolerance
- limits of size
- Gaging position
- not applicable (straight thread)
- Sealing
- Mechanical joint, no sealing requirement (section 6)
Notes
- Table 6 General Note (a): NPSM has a form similar to UN with tolerances similar to 2A/2B; the internal thread minimum PD equals E1 of NPT (Note 1). Values were not recalculated in 2013 (Foreword (d)). Gages: GO / NOT GO to the PD limits of Table 6 per ASME B1.2 (para. 6.4.4): GO plug min major = int PD min + 0.64951905P; NOT GO plug max major = int PD max + 0.43301270P; GO ring max minor = ext PD max − 0.43301300P; NOT GO ring min minor = ext PD min − 0.21650635P.
Mates
1/4-18 NPSM takes a thread of the same standard at the same size and pitch. The standard mating external 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
All values for 1/4-18 NPSMevery column of the workbook row
| Designation | 1/4-18 NPSM |
|---|---|
| Decimal designation | 0.250-18 NPSM |
| Threads per inch | 18 |
| Thread form | Straight |
| Series | NPSM |
| Series name | Straight Pipe Threads for Free-fitting Mechanical Joints for Fixtures |
| External / internal | Internal |
| Class | 2B |
| Nominal pipe size | 1/4 |
| Pitch P (in.) | 0.05555556 |
| O.D. of pipe D, basic major dia (in.) | 0.540 |
| Size category (Table 3 group) | 1/4, 3/8 (18 TPI) |
| Source table | Table 6 |
| Printed page | 17 |
| Where the diameter limits apply | Straight thread: the limits apply along the whole length |
| Major dia min (in.) | 0.5277 |
| Major dia status | Reference only: produced by commercial taps (General Note (b)); min = PD min + (H − 2 × 0.10825P) |
| Basic pitch dia at gage plane E1 (in.) | 0.49163 |
| Pitch dia max (in.) | 0.4974 |
| Pitch dia min (in.) | 0.4916 |
| PD tolerance (in.) | 0.0058 |
| Minor dia max (in.) | 0.481 |
| Minor dia min (in.) | 0.468 |
| Minor dia tolerance (in.) | 0.013 |
| Minor dia status | Toleranced (Table 6) |
| Allowance (in.) | 0.0013 |
| Allowance source | Table 6 col. 4 (external, Class 2A) |
| Root radius max (in.) | not given in the standard |
| Root radius min (in.) | not given in the standard |
| Lead / angle / taper source | Not given in the standard for straight threads |
| 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.) | 0.030070 |
| Thread height h min (in.) | 0.030070 |
| Crest truncation min (in.) | 0.012028 |
| Crest truncation max (in.) | 0.012028 |
| Root truncation min (in.) | 0.006014 |
| Root truncation max (in.) | 0.006014 |
| Crest flat min (in.) | 0.013889 |
| Crest flat max (in.) | 0.013889 |
| Root flat min (in.) | 0.006944 |
| Root flat max (in.) | 0.006944 |
| Sweep cut: start diameter (in.) | 0.468 |
| Sweep cut: tip diameter (in.) | 0.528140 |
| Sweep cut: depth (in.) | 0.030070 |
| Sweep cut: profile width at start diameter (in.) | 0.041667 |
| Sweep cut: tip flat (in.) | 0.006944 |
| Sweep cut: start diameter is | Minor dia min (hole) |
| Taper half-angle β (deg) | 0 |
| Taper on diameter | none (straight) |
| Gaging method | GO / NOT GO thread gages or indicating gages to ASME B1.2 (para. 6.4); NOT GO may enter up to 3 turns |
| Gaging tolerance (turns) | limits of size |
| Basic gaging position | not applicable (straight thread) |
| Sealing | Mechanical joint, no sealing requirement (section 6) |
| PD limit by formula (NPSM ext: E1 − allowance; int: E1) | 0.4916 |
| Minor dia min by formula (int) | 0.468 |
| Check against formulas | OK |
| Notes | Table 6 General Note (a): NPSM has a form similar to UN with tolerances similar to 2A/2B; the internal thread minimum PD equals E1 of NPT (Note 1). Values were not recalculated in 2013 (Foreword (d)). Gages: GO / NOT GO to the PD limits of Table 6 per ASME B1.2 (para. 6.4.4): GO plug min major = int PD min + 0.64951905P; NOT GO plug max major = int PD max + 0.43301270P; GO ring max minor = ext PD max − 0.43301300P; NOT GO ring min minor = ext PD min − 0.21650635P. |
| Lookup key | 1/4-18 NPSM · Internal 2B |
Related tools
- Thread standardsEvery ISO and ASME thread series and size, with limits and drawings.
- Thread Spec CalculatorDiameters, tolerances and geometry for ISO metric and unified threads.
- Thread Fit CheckPick a bolt and a nut and see the clearance or interference.
- Twist Drill SizesASME B94.11M drills: sizes, lengths, shanks and tolerances.
- Thread Designation GuideHow to read M10×1.5, 1/4-20 UNC-2A, NPT and more.
- Thread Notations & TermsSymbols and terms of a thread profile, explained.
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.