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NPT Thread Chart
Complete NPT pipe thread dimensions per ASME B1.20.1 — major, minor and pitch diameter, TPI, tap drill size and hand-tight engagement, 1/16" to 4".
NPT is the North American tapered pipe thread. The number in the size is nominal bore, not a measured dimension — a 1/2″ NPT thread measures 0.840″ across. That single fact accounts for most pipe thread confusion.
| Size | TPI | Major dia | Major (mm) | Minor dia | Pitch dia (E1) | Tap drill | Hand-tight |
|---|---|---|---|---|---|---|---|
| 1/16″ | 27 | 0.3125″ | 7.94 | 0.2611″ | 0.27118″ | C | 0.16″ |
| 1/8″ | 27 | 0.405″ | 10.29 | 0.3537″ | 0.36351″ | R | 0.16″ |
| 1/4″ | 18 | 0.54″ | 13.72 | 0.4723″ | 0.47739″ | 7/16 | 0.228″ |
| 3/8″ | 18 | 0.675″ | 17.14 | 0.6073″ | 0.61201″ | 37/64 | 0.24″ |
| 1/2″ | 14 | 0.84″ | 21.34 | 0.7565″ | 0.76843″ | 23/32 | 0.32″ |
| 3/4″ | 14 | 1.05″ | 26.67 | 0.9665″ | 0.97843″ | 59/64 | 0.339″ |
| 1″ | 11.5 | 1.315″ | 33.4 | 1.2135″ | 1.21363″ | 1-5/32 | 0.4″ |
| 1-1/4″ | 11.5 | 1.66″ | 42.16 | 1.5585″ | 1.55863″ | 1-1/2 | 0.42″ |
| 1-1/2″ | 11.5 | 1.9″ | 48.26 | 1.7985″ | 1.79863″ | 1-47/64 | 0.42″ |
| 2″ | 11.5 | 2.375″ | 60.32 | 2.2735″ | 2.27363″ | 2-7/32 | 0.436″ |
| 2-1/2″ | 8 | 2.875″ | 73.02 | 2.7194″ | 2.76216″ | 2-5/8 | 0.682″ |
| 3″ | 8 | 3.5″ | 88.9 | 3.3444″ | 3.38716″ | 3-1/4 | 0.766″ |
| 4″ | 8 | 4.5″ | 114.3 | 4.3444″ | 4.38716″ | 4-1/4 | 0.844″ |
All of these threads are tapered 3/4″ per foot on diameter (1:16), which is 1°47′ of included taper. Tap drill sizes are
starting points; material and tap maker both shift them slightly.