Thread Calculator
Solve the basic dimensions of a 60° V-thread (Unified inch or ISO metric geometry). Enter the major diameter in inches, threads per inch, and a percent-of-thread target to get pitch, pitch diameter, minor diameter, and tap drill size.
Example: with Major diameter (in) 0.25 · Threads per inch (TPI) 20 · Percent of thread for tap drill (%) 75 → Tap drill size: 0.2013 in (5.11 mm) for 75% thread.
- Pitch0.0500 in (1.270 mm)
- Basic pitch diameter0.2175 in
- Basic minor diameter0.1959 in
Computed by the calculator below using its default values. Change any input to see your own numbers.
ASME B1.1 basic profile: pitch dia = D − 0.6495P, minor dia = D − 1.0825P. Tap drill = D − 0.01299 × %thread ÷ TPI (75% is the machinist default).
The geometry behind every 60° thread
Unified inch threads and ISO metric threads share one profile: a 60° V with flats at crest and root. Every basic dimension falls out of the pitch P (1 ÷ TPI for inch threads). The fundamental triangle height is H = 0.866P; the basic pitch diameter sits 0.6495P below the major diameter, and the basic minor diameter 1.0825P below it. A 1/4-20 screw therefore has a 0.2175 in pitch diameter and 0.1959 in minor diameter, matching the ASME B1.1 tables.
Tap drill choice is a trade-off. Percent of thread compares the drilled hole to a theoretical full-depth thread: drill = major − 0.01299 × % ÷ TPI. Around 75%, a fastener already develops nearly the strength of a full thread, but the tap turns far easier and breaks less. That is why published charts — #7 drill for 1/4-20, 5/16 for 3/8-16 — hover near 75%; harder materials often drop to 60–65%.
How it’s calculated
P = 1 ÷ TPI. Basic pitch diameter = D − 0.649519 × P; basic minor diameter = D − 1.082532 × P (ASME B1.1 / ISO 68 basic 60° profile, H = 0.866025 × P). Tap drill = D − 0.01299 × percent-of-thread ÷ TPI, the standard machinist formula; 75% is the default engagement. Millimeter values at 25.4 mm per inch (exact).
Gives basic (zero-tolerance) dimensions — actual parts are made to class fits (2A/2B, 6g/6H) with allowances a few thousandths off basic.
Common UNC threads and their chart tap drills (~75%)
| Thread | Pitch (in) | Basic minor dia | Chart tap drill |
|---|---|---|---|
| #10-24 | 0.0417 | 0.1449 in | #25 (0.1495 in) |
| 1/4-20 | 0.0500 | 0.1959 in | #7 (0.2010 in) |
| 5/16-18 | 0.0556 | 0.2524 in | F (0.2570 in) |
| 3/8-16 | 0.0625 | 0.3073 in | 5/16 (0.3125 in) |
| 1/2-13 | 0.0769 | 0.4167 in | 27/64 (0.4219 in) |
Pitch and minor diameters computed with the B1.1 basic-profile formulas above; drills are the standard chart picks nearest ~75% thread.
Common mistakes
- Confusing pitch with TPI: inch threads are specified in threads per inch, and pitch is its reciprocal — a 20 TPI thread has 0.050 in pitch.
- Drilling for 100% thread — it roughly doubles tapping torque for only ~5% more strength and snaps taps in steel.
- Using the minor diameter as the tap drill; basic minor is the zero-allowance limit, smaller than any practical drill choice.
- Applying these numbers to pipe (NPT) or Acme threads — they use different profiles and tables.
Frequently asked questions
What are the thread formulas?
With pitch P = 1/TPI: pitch diameter = major − 0.6495 × P, minor diameter = major − 1.0825 × P, and tap drill = major − 0.01299 × percent-of-thread ÷ TPI. All from the 60° basic profile in ASME B1.1.
What tap drill do I use for 1/4-20?
For 75% thread: 0.25 − 0.01299 × 75 ÷ 20 = 0.2013 in, which is a #7 drill (0.201 in). In soft aluminum you might stay at 75%; in stainless many machinists drop to 65% and drill slightly larger.
What does percent of thread mean?
How much of the theoretical full thread height the tapped hole engages. 75% is the standard compromise: close to full strength, far less tapping torque. 100% threads gain little strength but break taps.
Does this work for metric threads?
The geometry does — ISO metric uses the same 60° profile, and the classic metric tap drill shortcut (drill = diameter − pitch) equals about 77% thread. Enter dimensions in inches here, or convert at 25.4 mm per inch.