British Std Pipe

Size Tpi Pitch mm Major Dia Inch Major Dia Mm Core Dia Inch Core Dia Mm Guage Length Inch Guage Length Mm Tapping Drill Size Mm R95%
​1⁄16 28 0.907 0.304 7.723 0.258 6.561 5⁄32 4.000 6.6
​1⁄8 28 0.907 0.383 9.728 0.337 8.566 5⁄32 4.000 8.6
​1⁄4 19 1.337 0.518 13.157 0.451 11.445 0.236 6.000 11.5
​3⁄8 19 1.337 0.656 16.662 0.589 14.950 1⁄4 6.400 15.0
​1⁄2 14 1.814 0.825 20.955 0.734 18.631 0.321 8.200 18.7
​5⁄8 14 1.814 0.902 22.911 0.811 20.587 0.321 8.200 20.7
​3⁄4 14 1.814 1.041 26.441 0.950 24.117 3⁄8 9.500 24.2
​7⁄8 14 1.814 1.189 30.201 1.098 27.877 3⁄8 9.500 28.0
1 11 2.309 1.309 33.249 1.193 30.291 0.409 10.400 30.4
​1 1⁄8 11 2.309 1.492 37.897 1.376 34.939 0.409 10.400 35.1
​1 1⁄4 11 2.309 1.650 41.910 1.534 38.952 1⁄2 12.700 39.1
​1 3⁄8 11 2.309 1.745 44.323 1.629 41.365 1⁄2 12.700 41.5
​1 1⁄2 11 2.309 1.882 47.803 1.766 44.845 1⁄2 12.700 45.0
​1 5⁄8 11 2.309 2.082 52.883 1.966 49.926 5⁄8 15.900 50.1
​1 3⁄4 11 2.309 2.116 53.746 2.000 50.788 5⁄8 15.900 50.9
​1 7⁄8 11 2.309 2.244 56.998 2.128 54.041 5⁄8 15.900 54.2
2 11 2.309 2.347 59.614 2.231 56.656 5⁄8 15.900 56.8
​2 1⁄4 11 2.309 2.587 65.710 2.471 62.752 11⁄16 17.500 62.9
​2 1⁄2 11 2.309 2.960 75.184 2.844 72.226 11⁄16 17.500 72.4
​2 3⁄4 11 2.309 3.210 81.534 3.094 78.576 13⁄16 20.600 78.7
3 11 2.309 3.460 87.884 3.344 84.926 13⁄16 20.600 85.1
​3 1⁄4 11 2.309 3.700 93.980 3.584 91.022 7⁄8 22.200 91.2
​3 1⁄2 11 2.309 3.950 100.330 3.834 97.372 7⁄8 22.200 97.5
​3 3⁄4 11 2.309 4.200 106.680 4.084 103.722 7⁄8 22.200 103.9
4 11 2.309 4.450 113.030 4.334 110.072 1 25.400 110.2
​4 1⁄2 11 2.309 4.950 125.730 4.834 122.772 1 25.400 122.9
5 11 2.309 5.450 138.430 5.334 135.472 1 1⁄8 28.600 135.6
​5 1⁄2 11 2.309 5.950 151.130 5.834 148.172 1 1⁄8 28.600 148.3
6 11 2.309 6.450 163.830 6.334 160.872 1 1⁄8 28.600 161.0
7 10 2.540 7.450 189.230 7.322 185.979 1 3⁄8 34.900 186.1
8 10 2.540 8.450 214.630 8.322 211.379 1 1⁄2 38.100 211.5
9 10 2.540 9.450 240.030 9.322 236.779 1 1⁄2 38.100 236.9
10 10 2.540 10.000 265.430 10.000 262.179 1 5⁄8 41.300 262.3
11 8 3.175 10.000 290.830 10.000 286.766 1 5⁄8 41.300 287.0
12 8 3.175 10.000 316.230 10.000 312.166 1 5⁄8 41.300 312.4
13 8 3.175 10.000 347.472 10.000 343.408 1 5⁄8 41.300 343.6
14 8 3.175 10.000 372.872 10.000 368.808 1 3⁄4 44.500 369.0
15 8 3.175 10.000 398.272 10.000 394.208 1 3⁄4 44.500 394.4
16 8 3.175 10.000 423.672 10.000 419.608 1 7⁄8 47.600 419.8
17 8 3.175 10.000 449.072 10.000 445.008 2 50.800 445.2
18 8 3.175 10.000 474.472 10.000 470.408 2 50.800 470.6
The size number was originally based on the inner diameter (measured in inches) of a steel tube for which the thread was intended, but contemporary pipes tend to use thinner walls to save material, and thus have an inner diameter larger than this nominal size.