Holemaking and tapping
Thread percentage calculator
Percent of thread = 76.98 x TPI x (major diameter - drill); an F drill in 5/16-18 gives about 77%.
Technical review pending. Formulas and values are cited. This notice comes down after a machinist review.
Percent of thread
%
%thread = 76.98 x TPI x (D - drill) = 76.98 x 18 x (0.3125 - 0.257) = 76.9%
Drill 0.2570 in (6.53 mm). Holes drill a little oversize, so the real percent runs slightly lower.
Drill for 75%: drill = D - (0.01299 x 75 / TPI) = 0.3125 - (0.01299 x 75 / 18) = 0.2584 in.
| Row | Drill | Decimal (in) | Metric (mm) | Percent of thread (%) |
|---|---|---|---|---|
| Next drill down | 1/4 | 0.2500 | 6.35 | 86.6 |
| Your drill | F | 0.2570 | 6.53 | 76.9 |
| Next drill up | G | 0.2610 | 6.63 | 71.4 |
| Nearest to 75% | F | 0.2570 | 6.53 | 76.9 |
Source: Machinery's Handbook (percentage of thread); ASME B1.1; ISO 261.
How it works
Inch
%thread = 76.98 x TPI x (D - drill)
| Symbol | Meaning | Unit |
|---|---|---|
| %thread | Percent of full thread height | % |
| TPI | Threads per inch | 1/in |
| D | Basic major diameter | in |
| drill | Drill diameter, same units as D | in |
| 76.98 | Constant: 1 / 0.01299 | none |
Metric
%thread = 76.98 x (D - drill) / P
| Symbol | Meaning | Unit |
|---|---|---|
| D | Basic major diameter | mm |
| drill | Drill diameter; an inch drill converts at 25.4 mm per inch | mm |
| P | Pitch, the number after the x in the callout | mm |
The inch form is the same as (D - drill) x TPI / 0.01299. Both assume 60° Unified or ISO metric threads and cut taps.
Worked example: 5/16-18 with an F drill
5/16-18: D = 0.3125 in, 18 TPI; F drill = 0.257 in.
- Subtract: 0.3125 - 0.2570 = 0.0555 in.
- Multiply: 76.98 x 18 x 0.0555 = 76.9%.
Result: 76.9%, about 77% thread.
Worked example: M6 x 1 with 5.1 mm and 5.0 mm drills
M6 x 1: D = 6.0 mm, P = 1 mm.
- 5.1 mm drill: 76.98 x (6.0 - 5.1) / 1 = 76.98 x 0.9 = 69.3%.
- 5.0 mm drill: 76.98 x (6.0 - 5.0) / 1 = 77.0%.
Result: 69.3% with 5.1 mm, 77.0% with 5.0 mm.
More checks
- 1/4-28 with a #3 drill (0.213 in): 76.98 x 28 x (0.250 - 0.213) = 76.98 x 28 x 0.037 = 79.8%.
- M3 x 0.5 with a #40 drill (0.098 in = 2.4892 mm): 76.98 x (3 - 2.4892) / 0.5 = 76.98 x 0.5108 / 0.5 = 78.6%.
Shop notes
- Rule of thumb from forum practice, not a standard: about 60 to 65% in hard steel, about 70% in steel, 75 to 80% in aluminum.
- Higher percentages add little strength and raise tapping torque and tap breakage.
- Holes drill a little oversize, so the real percent of thread runs slightly lower than computed.
- Form (roll) taps are sized differently. Use the tap maker's drill chart.
- Percent of thread checks the drill. The finished thread is inspected against the class 2B (inch) or 6H (metric) internal thread limits.
FAQ
What percent thread does a G drill give in 5/16-18?
About 71%: 76.98 x 18 x 0.0515 = 71.4%. An F drill (0.257 in) gives 76.9%.
What percent of thread is right for steel?
About 70% in steel, 60 to 65% in hard steel and 75 to 80% in aluminum. That is a forum rule of thumb, not a standard.
Is 100% thread stronger?
Barely. Past about 75% the gain in strength is small and tap breakage rises.
What is close to 2.5 mm for M3?
A #40 drill (0.098 in, 2.49 mm) gives 79% thread in M3 x 0.5; a #39 (0.0995 in, 2.53 mm) gives 73%.
Last updated