# Metric to SAE Calculator (mm to Fractional Inches)

URL: https://speedycalculator.com/calculators/conversion/metric-to-sae/
Last updated: 2026-10-25

> The metric to SAE calculator converts a metric size in millimetres into decimal inches and the nearest fractional (SAE) size, and shows how far apart they are. Use it to see whether a metric wrench or socket will fit an SAE fastener.

## Inputs

- Metric size (mm)

## Results

- Decimal inches
- Nearest 1/64″
- Nearest 1/16″ (common wrench sizes)
- Difference from 1/16″ size

## Formula

```
inches = mm ÷ 25.4;  nearest SAE size = round(inches × 64) ÷ 64
```

## Explanation

SAE (imperial) tools are sized in fractions of an inch, metric tools in millimetres. Some pairs are near-identical, such as 19 mm and 3/4″ (19.05 mm); others, like 13 mm and 1/2″ (12.7 mm), are too far apart and will round off a bolt head.

### How to use the Metric to SAE Calculator

- **Metric size**
- Every result field is editable: type into **Decimal inches** and **Metric size** plus every other field update to match.

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Millimetres are divided by 25.4. The result is rounded to the nearest 1/64″ and 1/16″, and the difference is reported in millimetres.

### Metric and SAE wrench equivalents

mmDecimal inNearest SAEDifference60.23620.25″+0.35 mm80.31500.3125″-0.06 mm100.39370.375″-0.48 mm110.43310.4375″+0.11 mm120.47240.5″+0.70 mm130.51180.5″-0.30 mm140.55120.5625″+0.29 mm150.59060.5625″-0.71 mm160.62990.625″-0.12 mm170.66930.6875″+0.46 mm190.74800.75″+0.05 mm220.86610.875″+0.22 mm240.94490.9375″-0.19 mm271.06301.0625″-0.01 mm301.18111.1875″+0.16 mm321.25981.25″-0.25 mm

### What each result means

- **Decimal inches** = [Metric size] ÷ 25.4
- **Nearest 1/64″** = round([Metric size] ÷ 25.4 × 64) ÷ 64
- **Nearest 1/16″ (common wrench sizes)** = round([Metric size] ÷ 25.4 × 16) ÷ 16
- **Difference from 1/16″ size** = (round([Metric size] ÷ 25.4 × 16) ÷ 16) × 25.4 - [Metric size]

## Worked example

Inputs — Metric size: 19 mm.

Results — **Decimal inches = 0.748 in**; Nearest 1/64″ = 3/4; Nearest 1/16″ (common wrench sizes) = 3/4; Difference from 1/16″ size = 0.05 mm.

## FAQ

**Is 19 mm the same as 3/4″?**
Nearly: 3/4″ is 19.05 mm, a 0.05 mm difference.

**Can I use a 13 mm socket on a 1/2″ bolt?**
It will be loose by 0.3 mm and may round the bolt; use the correct tool.

**What formula does the Metric to SAE Calculator use?**
inches = mm ÷ 25.4;  nearest SAE size = round(inches × 64) ÷ 64.


## Sources

- [NIST Special Publication 811 – Guide for the Use of the International System of Units](https://www.nist.gov/pml/special-publication-811)
- [NIST Handbook 44 Appendix C – General tables of units of measurement](https://www.nist.gov/pml/owm/nist-handbook-44-current-edition)