Thread pitch
20 TPI
1.27 mm pitch
Thread pitch
- Pitch
- 1.27 mm20 TPI
- 10 threads
- 0.500" · 12.70 mm
Used by
1/4"-20, 7/16"-20, 1/2"-20
Full chart 39 rows
Sort any column, filter by series, print a shop sheet. Click a row to load it on the gauge.
Thread pitch conversion chart
39 rowsThread pitch conversion chart · ScrewBoltChart
screwboltchart.com · inch-first · tap drills 75% (soft metals) · torque dry, K=0.20 · verify against the governing standard
| 80.00 | 0.3175 | 0.125" · 3.18 mm | #0-80 | ||
| 72.57 | 0.3500 | 0.138" · 3.5 mm | M1.6x0.35 | ||
| 72.00 | 0.3528 | 0.139" · 3.53 mm | #1-72 | ||
| 64.00 | 0.3969 | 0.156" · 3.97 mm | #1-64, #2-64 | ||
| 63.50 | 0.4000 | 0.157" · 4 mm | M2x0.4 | ||
| 56.44 | 0.4500 | 0.177" · 4.5 mm | M2.5x0.45 | ||
| 56.00 | 0.4536 | 0.179" · 4.54 mm | #2-56, #3-56 | ||
| 50.80 | 0.5000 | 0.197" · 5 mm | M3x0.5, M4x0.5, M5x0.5 | ||
| 48.00 | 0.5292 | 0.208" · 5.29 mm | #3-48, #4-48 | ||
| 44.00 | 0.5773 | 0.227" · 5.77 mm | #5-44 | ||
| 42.33 | 0.6000 | 0.236" · 6 mm | M3.5x0.6 | ||
| 40.00 | 0.6350 | 0.250" · 6.35 mm | #4-40, #5-40, #6-40 | ||
| 36.29 | 0.7000 | 0.276" · 7 mm | M4x0.7 | ||
| 36.00 | 0.7056 | 0.278" · 7.06 mm | #8-36 | ||
| 33.87 | 0.7500 | 0.295" · 7.5 mm | M6x0.75 | ||
| 32.00 | 0.7938 | 0.313" · 7.94 mm | #6-32, #8-32, #10-32 | ||
| 31.75 | 0.8000 | 0.315" · 8 mm | M5x0.8 | ||
| 28.00 | 0.9071 | 0.357" · 9.07 mm | #12-28, 1/4"-28, 1/16" BSP, 1/8" BSP | ||
| 27.00 | 0.9407 | 0.370" · 9.41 mm | 1/16" NPT, 1/8" NPT | ||
| 25.40 | 1.0000 | 0.394" · 10 mm | M6x1.0, M8x1.0, M10x1.0 | ||
| 24.00 | 1.0583 | 0.417" · 10.58 mm | #10-24, #12-24, 5/16"-24, 3/8"-24 | ||
| 20.32 | 1.2500 | 0.492" · 12.5 mm | M8x1.25, M10x1.25, M12x1.25 | ||
| 20.00 | 1.2700 | 0.500" · 12.7 mm | 1/4"-20, 7/16"-20, 1/2"-20 | ||
| 19.00 | 1.3368 | 0.526" · 13.37 mm | 1/4" BSP, 3/8" BSP | ||
| 18.00 | 1.4111 | 0.556" · 14.11 mm | 5/16"-18, 9/16"-18, 5/8"-18, 1/4" NPT, 3/8" NPT | ||
| 16.93 | 1.5000 | 0.591" · 15 mm | M10x1.5, M12x1.5, M14x1.5, M16x1.5, M18x1.5, M20x1.5, M22x1.5 | ||
| 16.00 | 1.5875 | 0.625" · 15.88 mm | 3/8"-16, 3/4"-16 | ||
| 14.51 | 1.7500 | 0.689" · 17.5 mm | M12x1.75 | ||
| 14.00 | 1.8143 | 0.714" · 18.14 mm | 7/16"-14, 7/8"-14, 1/2" NPT, 3/4" NPT, 1/2" BSP, 3/4" BSP | ||
| 13.00 | 1.9538 | 0.769" · 19.54 mm | 1/2"-13 | ||
| 12.70 | 2.0000 | 0.787" · 20 mm | M14x2.0, M16x2.0, M24x2.0 | ||
| 12.00 | 2.1167 | 0.833" · 21.17 mm | 9/16"-12, 1"-12 | ||
| 11.50 | 2.2087 | 0.870" · 22.09 mm | 1" NPT, 1-1/4" NPT, 1-1/2" NPT, 2" NPT | ||
| 11.00 | 2.3091 | 0.909" · 23.09 mm | 5/8"-11, 1" BSP, 1-1/4" BSP, 1-1/2" BSP, 2" BSP | ||
| 10.16 | 2.5000 | 0.984" · 25 mm | M18x2.5, M20x2.5, M22x2.5 | ||
| 10.00 | 2.5400 | 1.000" · 25.4 mm | 3/4"-10 | ||
| 9.00 | 2.8222 | 1.111" · 28.22 mm | 7/8"-9 | ||
| 8.47 | 3.0000 | 1.181" · 30 mm | M24x3.0 | ||
| 8.00 | 3.1750 | 1.250" · 31.75 mm | 1"-8 |
05 / Field guide
Count the
threads.
Pitch is the distance from one thread crest to the next. It is the most reliable way to tell look-alike screws apart, and you can measure it by laying the screw against a comb.
TPI and pitch are the same thing
Inch threads count threads per inch; metric threads give the pitch in millimetres. Convert with 25.4. 20 TPI is a 1.27 mm pitch; a 1.25 mm pitch is about 20.3 TPI. Close, but not the same.
TPI = 25.4 ÷ pitch pitch = 25.4 ÷ TPI
Why ten threads
One thread can’t tell 24 TPI (1.058 mm) from a 1.0 mm pitch: the difference is 0.06 mm. Over ten threads it grows to 0.58 mm, which you can see easily. That is why every comb here carries a red ten-thread bar.
drift over n threads = |pitch₁ − pitch₂| × n
Using the comb
Lay the screw against the comb and look straight down. If the teeth seat evenly across ten threads, it is a match. If they line up at one end and climb out at the other, try the neighbouring pitch. For pipe, check 27, 18, 14 and 11.5 TPI.
Common look-alike pairs
A handful of pairs cause most mix-ups. 1/4"-20 (1.27 mm) and M6 × 1.0 sit 0.35 mm apart in diameter; their combs drift 2.7 mm apart over ten threads. 5/16"-18 (1.41 mm) and M8 × 1.25 differ by 1.6 mm over ten. #10-24 (1.06 mm) and M5 × 0.8 differ by 2.6 mm. 1/2"-13 (1.95 mm) and M12 × 1.75 drift 2 mm apart over ten threads and sit 0.7 mm apart in diameter. Check both numbers every time.
Questions
Answered plainly.
01How do I find the thread pitch of a bolt?+−
Measure the length of ten threads and divide by ten for metric pitch, or count the threads in one inch for TPI. On screen, hold the bolt against each comb until ten teeth seat without drift.
02Is 1.25 mm pitch the same as 20 TPI?+−
No. 20 TPI is 1.27 mm. The difference is only 0.02 mm per thread, but it adds up to 0.2 mm over ten threads, enough to bind in a nut.
03What is the pitch of an M6 bolt?+−
1.0 mm for the standard coarse thread. The fine pitch is 0.75 mm.
04What TPI is a 1/2" bolt?+−
13 TPI coarse (UNC) or 20 TPI fine (UNF).
05Is the on-screen gauge accurate enough?+−
After calibration, yes for telling standard pitches apart, because neighbouring pitches drift far apart over ten threads. For quality inspection use a calibrated thread gauge.
06Can I print the comb?+−
Yes. The 1:1 print page has inch and metric combs and a drill gauge with a check ruler. Print at 100% (Actual size) and check the ruler before use.
07What is the difference between 1/4-20 and 1/4-28?+−
Both are 0.250" in diameter. 1/4-20 is the coarse thread (1.27 mm pitch) and 1/4-28 the fine one (0.91 mm). Over ten threads they differ by 3.6 mm.
08How do I measure pitch without a gauge?+−
Press the threads into paper or modelling clay, then measure across ten crests with a ruler and divide by ten. Or calibrate this page and hold the screw against the on-screen comb.
09What is the pitch of an M8 bolt?+−
1.25 mm for the standard coarse thread, about 20.3 TPI. The common fine pitch is 1.0 mm.
10Why do some fine threads look coarse?+−
Pitch scales with diameter. A 1"-12 fine thread (2.12 mm) is coarser than a #8-32 coarse thread (0.79 mm). Compare pitches within the same diameter, not across sizes.
11What is the pitch of a 3/8" bolt?+−
16 TPI (1.59 mm) for the coarse thread and 24 TPI (1.06 mm) for the fine thread.