Screw Thread Calculator
Select a Unified inch (UNC or UNF) or ISO metric thread and its tolerance class to see the basic major, pitch and minor diameters, the limits of size for the bolt and the nut, thread depth, lead angle and the right tap drill.
Last updated:
75% is the usual choice
How to use the Screw Thread Calculator
- Choose the thread series: UNC, UNF, metric coarse or metric fine.
- Pick the size from the list.
- Select the class: 2A/2B or 6g/6H for most fasteners, 3A/3B for close fits.
- Set the thread engagement you want for the tap drill (75% is typical).
- Read the dimensions and the limits table, and download it as CSV if needed.
Basic thread formulas (60° Unified and ISO metric)
| Dimension | Formula | 1/4-20 UNC | M10 × 1.5 |
|---|---|---|---|
| Pitch | P = 1 ÷ TPI (inch) or mm value | 0.0500 in | 1.5 mm |
| Fundamental triangle height | H = 0.866025 P | 0.0433 in | 1.299 mm |
| Pitch diameter | E = D − 0.649519 P | 0.2175 in | 9.026 mm |
| Internal minor diameter | D1 = D − 1.082532 P | 0.1959 in | 8.376 mm |
| External minor (rounded root) | d3 = D − 1.226869 P | 0.1887 in | 8.160 mm |
| External thread height | h = 0.613435 P | 0.0307 in | 0.920 mm |
| Engagement height | H1 = 0.541266 P | 0.0271 in | 0.812 mm |
How the tolerance classes work
| Class | Applies to | Fit | Typical use |
|---|---|---|---|
| 1A / 1B | Bolt / nut | Loose, with allowance | Quick assembly, dirty or damaged threads |
| 2A / 2B | Bolt / nut | Standard, with allowance on 2A | Most commercial screws, bolts and nuts |
| 3A / 3B | Bolt / nut | Close, no allowance | Precision and aerospace fasteners |
| 6g / 6H | Metric bolt / nut | Medium, small allowance on 6g | General metric fasteners |
| 6h / 6H | Metric bolt / nut | Medium, no allowance | Close fit or parts plated after threading with care |
For Unified threads the class 2A pitch diameter tolerance is 0.0015∛D + 0.0015√LE + 0.015∛P² (here LE, the length of engagement, is taken as D). Class 3A uses 75% of that, class 1A 150%, and the matching internal classes are 30% larger. The 1A and 2A allowance is 30% of the class 2A tolerance. Metric limits use the ISO 965-1 grade 6 tolerances and the g fundamental deviation.
Practical tips
- Measuring pitch diameter: use thread wires or a thread micrometer; the outside diameter alone does not tell you whether a thread will fit.
- Plating: a coating adds roughly four times its thickness to the pitch diameter, so the 2A or 6g allowance is what leaves room for it.
- Tap drills: 75% thread engagement gives nearly full strength; going higher makes tapping much harder for little gain.
- Identifying a thread: measure the major diameter with calipers and the pitch with a thread gauge, then pick the matching size here.
Frequently asked questions
How do you calculate the pitch diameter of a thread?
For 60° Unified and ISO metric threads the basic pitch diameter is E = D − 0.649519 × P, where D is the basic major diameter and P the pitch (1 ÷ TPI for inch threads). For a 1/4-20 thread: 0.25 − 0.649519 × 0.05 = 0.2175 in.
What is the minor diameter of a thread?
The basic minor diameter of an internal thread is D1 = D − 1.082532 × P, which is also the smallest allowed hole size for classes 2B and 6H. For M10 × 1.5 it is 10 − 1.6238 = 8.376 mm. The external thread's minor diameter at a rounded root is about D − 1.226869 × P.
What do thread classes 2A and 2B mean?
A is for external (bolt) threads and B for internal (nut) threads. Class 2 is the standard for commercial fasteners; class 2A includes a small allowance so plated or dirty parts still assemble. Class 3 is a tighter fit with no allowance, and class 1 is a loose fit for quick assembly.
What does 6g/6H mean on a metric thread?
The number is the tolerance grade and the letter the tolerance position. Lowercase letters are for external threads and uppercase for internal threads. 6H has no allowance on the nut; 6g leaves a small gap (for example 32 µm on a 1.5 mm pitch) on the bolt. 6g/6H is the standard medium fit for metric fasteners.
How deep is a thread?
The basic height of an external Unified or metric thread is about 0.6134 × P, and the overlap between a bolt and nut (thread engagement height) is 0.5413 × P. For a 1/4-20 thread that is 0.0307 in and 0.0271 in.
What is the difference between UNC and UNF?
UNC (Unified National Coarse) has fewer threads per inch and is the general-purpose choice; it assembles quickly and resists stripping in softer materials. UNF (fine) has more threads per inch, a larger minor diameter and finer adjustment, and is used where vibration resistance or thin walls matter.
Are these values exact enough for inspection?
The basic dimensions follow directly from the standard formulas. The limits are calculated from the ASME B1.1 and ISO 965-1 tolerance rules and match typical table values, but for gauging and inspection always confirm against the official published tables.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.