— Pipe, Tube & Thread Standards
NPS & Schedule (ASME B36.10)
Outside diameter (OD) is fixed per NPS regardless of schedule — NPS is a label, not a measurement (1/2" pipe has a 0.840" OD, not 0.500"). Wall thickness increases with schedule number, reducing the inside diameter (ID).
| NPS | OD (in) | Sch 40 Wall | Sch 40 ID | Sch 80 Wall | Sch 80 ID |
|---|---|---|---|---|---|
| 1/4" | 0.540 | 0.088 | 0.364 | 0.119 | 0.302 |
| 1/2" | 0.840 | 0.109 | 0.622 | 0.147 | 0.546 |
| 3/4" | 1.050 | 0.113 | 0.824 | 0.154 | 0.742 |
| 1" | 1.315 | 0.133 | 1.049 | 0.179 | 0.957 |
| 1-1/2" | 1.900 | 0.145 | 1.610 | 0.200 | 1.500 |
| 2" | 2.375 | 0.154 | 2.067 | 0.218 | 1.939 |
| 3" | 3.500 | 0.216 | 3.068 | 0.300 | 2.900 |
| 4" | 4.500 | 0.237 | 4.026 | 0.337 | 3.826 |
| 6" | 6.625 | 0.280 | 6.065 | 0.432 | 5.761 |
| 8" | 8.625 | 0.322 | 7.981 | 0.500 | 7.625 |
See the Pipe Flow & Pressure Drop calculator to compute flow velocity and pressure drop from a pipe's actual ID.
Tube OD & Wall Gauge
Round tube is sized by its actual OD directly (unlike pipe/NPS) — 1/2" tube has a real 0.500" OD. Wall thickness is specified in decimal inches, which map to standard BWG (Birmingham Wire Gauge) numbers; heavier wall = lower BWG number = smaller ID for a given OD.
| Tube OD | Common Wall Thicknesses (in) | Equivalent BWG |
|---|---|---|
| 1/8" | 0.028, 0.035 | 22, 20 |
| 1/4" | 0.028, 0.035, 0.049, 0.065 | 22, 20, 18, 16 |
| 3/8" | 0.035, 0.049, 0.065 | 20, 18, 16 |
| 1/2" | 0.035, 0.049, 0.065, 0.083 | 20, 18, 16, 14 |
| 3/4" | 0.049, 0.065, 0.083, 0.095, 0.109 | 18, 16, 14, 13, 12 |
| 1" | 0.035, 0.049, 0.065, 0.083, 0.095, 0.109 | 20, 18, 16, 14, 13, 12 |
Thread Standard Comparison
| Standard | Taper/Straight | Sealing Method | Angle | Typical Use |
|---|---|---|---|---|
| NPT (National Pipe Taper) | Tapered (3/4"/ft) | Thread interference + sealant (PTFE tape/pipe dope) | 60° | North America, general industrial/plumbing |
| NPTF (Dryseal) | Tapered, same as NPT | Metal-to-metal crest/root interference — no sealant needed | 60° | US hydraulic, pneumatic, fuel/lubrication lines |
| BSPP (British Standard Pipe Parallel, "G") | Straight/parallel | Bonded washer, O-ring, or gasket at a flat face | 55° (Whitworth) | Europe, UK, Asia, marine, international equipment |
| BSPT (British Standard Pipe Taper) | Tapered | Thread interference (55° form, not NPT-compatible) | 55° (Whitworth) | Europe, UK, Asia |
| Metric straight w/ O-ring boss (ISO 6149) | Straight metric | O-ring compressed against a machined boss/flat face | 60° (metric) | European/international hydraulics |
| SAE ORB (O-Ring Boss) | Straight (UN/UNF form) | O-ring seated against machined boss face | 60° (UN) | US hydraulics, mobile/industrial equipment |
| UN/UNF (straight mechanical) | Straight | None inherent — mechanical fastening only | 60° | General mechanical fastening |
NPT (60°) and BSPP/BSPT (55°) fittings look similar but don't cross-mate — e.g. 1/4" NPT is 18 TPI vs. 1/4" BSPP is 19 TPI. 1/2" and 3/4" NPT vs. BSPT are especially easy to mistake visually.
NPT Thread Sizes (ASME B1.20.1)
| NPT Size | Major Diameter (in) | TPI |
|---|---|---|
| 1/8 | 0.405 | 27 |
| 1/4 | 0.540 | 18 |
| 3/8 | 0.675 | 18 |
| 1/2 | 0.840 | 14 |
| 3/4 | 1.050 | 14 |
| 1 | 1.315 | 11.5 |
| 1-1/4 | 1.660 | 11.5 |
| 1-1/2 | 1.900 | 11.5 |
| 2 | 2.375 | 11.5 |
NPT major diameters match the corresponding pipe NPS outside diameters — thread size is keyed to the pipe it's cut into, not to its nominal label. Standard taper is 3/4" per foot (1°47′ on diameter).