NPT Thread Dimensions: Complete Reference Table for 1/16 to 4 Inch
NPT (National Pipe Taper) dimensions are defined by ANSI/ASME B1.20.1. The nominal pipe size is not the outside diameter. A 3/4" NPT fitting has an OD of 1.050 inches, not 0.750. Every size follows this pattern. If your supplier feed puts the nominal value in the OD field, the data is wrong for all 14 standard sizes.
What the nominal size actually means
Here's where catalog data breaks. The nominal pipe size traces back to the approximate inside bore of Schedule 40 steel pipe. It has nothing to do with the thread OD. Never has.
A 3/4" NPT fitting has an outside diameter of 1.050 inches. A 1" NPT? 1.315 inches. These aren't close. The nominal value has never described the actual OD.
When a supplier feed says thread_od = 0.750 for a 3/4" NPT product, that's wrong. Not off by a little. Wrong by 40%. This single misunderstanding causes more attribute errors than incorrect TPI or pitch values combined.
Complete NPT dimensions table
All 14 standard sizes per ASME B1.20.1. These are external thread dimensions.
| Nominal | OD (in) | TPI | Pitch (in) | E1 Pitch Dia (in) | L1 Hand-Tight (in) | L2 Effective (in) |
|---|---|---|---|---|---|---|
| 1/16" | 0.3125 | 27 | 0.03704 | 0.27118 | 0.1600 | 0.2611 |
| 1/8" | 0.405 | 27 | 0.03704 | 0.36351 | 0.1615 | 0.2639 |
| 1/4" | 0.540 | 18 | 0.05556 | 0.47739 | 0.2278 | 0.4018 |
| 3/8" | 0.675 | 18 | 0.05556 | 0.61201 | 0.2400 | 0.4078 |
| 1/2" | 0.840 | 14 | 0.07143 | 0.75843 | 0.3200 | 0.5337 |
| 3/4" | 1.050 | 14 | 0.07143 | 0.96768 | 0.3390 | 0.5457 |
| 1" | 1.315 | 11.5 | 0.08696 | 1.21363 | 0.4000 | 0.6828 |
| 1-1/4" | 1.660 | 11.5 | 0.08696 | 1.55713 | 0.4200 | 0.7068 |
| 1-1/2" | 1.900 | 11.5 | 0.08696 | 1.79609 | 0.4200 | 0.7235 |
| 2" | 2.375 | 11.5 | 0.08696 | 2.26902 | 0.4360 | 0.7565 |
| 2-1/2" | 2.875 | 8 | 0.12500 | 2.71953 | 0.6820 | 1.1375 |
| 3" | 3.500 | 8 | 0.12500 | 3.34062 | 0.7660 | 1.2000 |
| 3-1/2" | 4.000 | 8 | 0.12500 | 3.83750 | 0.8210 | 1.2500 |
| 4" | 4.500 | 8 | 0.12500 | 4.33438 | 0.8440 | 1.3000 |
E1 is the pitch diameter at the gauge plane. L1 is hand-tight engagement length. L2 is the effective thread length you'll actually use.
The taper: one number for all sizes
Every NPT thread tapers at 1:16. One inch of diameter change per 16 inches of length. That works out to 0.75 inches per foot, or 3/4" per foot if you prefer fractions.
Thread angle is 60 degrees. The half-angle between the taper surface and the centerline is 1°47'.
These are constants. You don't need a per-size lookup.
When gauging NPT threads with an L1 ring or plug gauge per B1.20.1, the tolerance is ±1 turn from the gauge notch or face. That's tight.
NPT vs NPTF: same dimensions, different attribute
NPTF (dryseal, per ASME B1.20.3) uses the same OD, TPI, and taper as NPT. The difference is tighter manufacturing tolerances that create a metal-to-metal seal without sealant.
For your PIM, NPTF and NPT share the same dimensional fields. The distinction lives in the thread_series attribute, not in OD or TPI.
| Standard | Thread Angle | Taper | Sealant Required |
|---|---|---|---|
| NPT | 60° | 1:16 | Yes |
| NPTF | 60° | 1:16 | No (dryseal) |
| BSPT | 55° | 1:16 | Yes |
BSP threads (BSPT tapered, BSPP parallel) look similar but aren't interchangeable. Different thread angle. Different OD for the same nominal size. A 1/2" BSPT has an OD around 0.825 inches versus 0.840 for NPT. Small gap, but the threads won't mate properly.
Common catalog data errors
Before: Supplier feed with wrong OD
nominal_size: 3/4thread_od: 0.750thread_type: NPT
After: Corrected attributes
nominal_pipe_size: 3/4thread_od: 1.050thread_type: NPTtpi: 14
Supplier gives OD matching the nominal value: Wrong. Look up actual OD in the B1.20.1 table above. Supplier says "NPTF": Same dimensional fields as NPT. Add thread_series = NPTF. Supplier says "BSP" or "BSPT": Different standard entirely. Do not merge with NPT records. No thread type given but OD matches NPT table: Flag as probable NPT, request confirmation before cataloging.
Measure or verify the outside diameter. Match it to the B1.20.1 table to identify the nominal size. If the supplier says 1/2" NPT but the OD is 1.050, they mean 3/4".
Each nominal size has exactly one correct TPI. A 1/2" NPT is 14 TPI. If a supplier says 1/2" NPT at 18 TPI, that's wrong. The 18 TPI value belongs to 1/4" or 3/8".
Don't leave it blank. NPT, NPTF, and NPS (straight pipe) are distinct. NPTF products need the dryseal callout even though the dimensional fields match NPT.
Quick lookup: hand-tight engagement for common sizes
For the six sizes you'll see most often:
| Nominal | L1 Hand-Tight (in) | L1 in Threads |
|---|---|---|
| 1/4" | 0.228 | ~4.1 |
| 3/8" | 0.240 | ~4.3 |
| 1/2" | 0.320 | ~4.5 |
| 3/4" | 0.339 | ~4.7 |
| 1" | 0.400 | ~4.6 |
| 2" | 0.436 | ~5.0 |
This is how far the male thread should engage by hand before you reach for a wrench. If your fitting bottoms out before hitting these numbers, something's off with the thread cut or the mating port.