Calculator Inputs
Formula Used
Diameter in inches = diameter in millimeters ÷ 25.4
Pitch in inches = pitch in millimeters ÷ 25.4
Threads per inch = 25.4 ÷ metric pitch
How to Use This Calculator
- Select a conversion mode.
- Choose a metric preset or enter custom values.
- Select thread position, class, direction, and precision.
- Choose an imperial comparison series when needed.
- Press Calculate to review all conversion details.
Metric Thread and Unified Thread Comparison
Metric threads use millimeters for diameter and pitch. Unified threads use inch diameters and TPI. Similar values can still have incompatible thread forms.
UNC uses coarse pitches for general fastening. UNF uses finer pitches for stronger clamping. UNEF provides still finer standardized pitches.
Always confirm the physical thread before replacing hardware. Use a pitch gauge for final verification.
Common Metric Thread Examples
| Metric thread | Diameter in inches | Pitch in inches | Approximate TPI |
|---|---|---|---|
| M3 × 0.5 | 0.1181 | 0.0197 | 50.8 |
| M6 × 1.0 | 0.2362 | 0.0394 | 25.4 |
| M8 × 1.25 | 0.3150 | 0.0492 | 20.32 |
| M10 × 1.5 | 0.3937 | 0.0591 | 16.93 |
| M12 × 1.75 | 0.4724 | 0.0689 | 14.51 |
| M20 × 2.5 | 0.7874 | 0.0984 | 10.16 |
Worked Example
An M10 × 1.5 thread has a 10 millimeter diameter. Dividing by 25.4 gives 0.3937 inches. Dividing 25.4 by 1.5 gives 16.933 TPI.
The calculator then compares standard Unified threads. It ranks matches using diameter and pitch differences. The result remains an engineering approximation.
Never force an approximate match into existing hardware. Verify both parts before final assembly.
Frequently Asked Questions
What does M10 × 1.5 mean?
M10 is the nominal diameter. The 1.5 value is pitch. Both values use millimeters.
How do I convert metric pitch to TPI?
Divide 25.4 by the pitch. A 1 millimeter pitch equals 25.4 TPI. Smaller pitches produce larger TPI values.
Can metric and imperial threads interchange?
Usually they cannot interchange safely. Profiles and pitches may differ. Always verify the exact standard.
What is the nearest fractional inch size?
It is a rounded diameter fraction. It does not identify a compatible thread. Use it only for quick reference.
What is UNC?
UNC means Unified National Coarse. It uses relatively coarse thread pitches. It is common in general fastening.
What is UNF?
UNF means Unified National Fine. It uses finer pitches than UNC. It can provide stronger clamping.
What is UNEF?
UNEF means Unified National Extra Fine. It provides very fine standardized pitches. It suits specialized applications.
How accurate is the tap drill estimate?
The estimate uses diameter minus pitch. Actual drill choice depends on engagement. Material and tolerance also matter.
What do 6g and 6H mean?
They are ISO tolerance classes. Lowercase classes normally describe external threads. Uppercase classes normally describe internal threads.
Why are several imperial matches listed?
One metric thread may resemble multiple standards. Ranked matches help compare compromises. They do not guarantee interchangeability.
Should I verify the result physically?
Yes, always use a thread gauge. Check diameter with suitable measuring tools. Confirm specifications before machining or assembly.