Metric to SAE Calculator

If you work with tools that mix metric sizes and fractional inches, you often need a fast bridge between mm and SAE-style inches. This calculator converts millimeters to decimal inches and also shows a fractional inch value rounded to the nearest 1/64.

How the conversion works

The inch is defined exactly:

$$1\,\text{in} = 25.4\,\text{mm}$$

So the base conversion is:

$$\text{in} = \frac{\text{mm}}{25.4}$$

The fractional inch display rounds the decimal inch value to a nearby fraction with denominator up to 64.

Units and conversions

Worked examples

1. Converting 12.7 mm to inches

   $$\text{in} = \frac{12.7}{25.4} = 0.5\,\text{in}$$

   Result: 0.5 in, which is 1/2 in in fractional form.

2. Converting 25 mm to inches

   $$\text{in} = \frac{25}{25.4} = 0.984252\,\text{in}$$

   Result: 0.984252 in; the nearest 1/64 is 63/64 in (0.984375 in).

Tips and pitfalls

• Fractional rounding can hide small differences; use decimal inches when tolerance matters.
• Many common tool sizes are not exact conversions. Treat "nearest SAE size" as a convenience, not a guarantee of fit.
• If you already have inches, switch to the fractional inch view for a readable shop format.

References and further reading

• International yard and pound agreement - Wikipedia
• Inch - Wikipedia