About

In manufacturing and engineering, blueprints often mix standards. While modern science uses metric (mm), legacy machinery, drill bits, and fasteners in the US often rely on fractional inches (e.g., 3/8"). A simple decimal conversion (0.375) is useful for CNC programming, but less helpful when searching for a wrench or bolt in a toolbox.

This tool provides a dual output: exact decimal inches for precision work, and the nearest fractional inch (reduced to the lowest common denominator) for practical shop floor application. It calculates down to 1/64th of an inch precision, displaying the error margin to ensure the part fits within tolerance.

Formulas

The standard conversion factor is exact.

L_(in) = L_(mm)25.4

Reference Data

Millimeters (mm)	Decimal Inch	Fractional Inch
1 mm	0.0394"	~3/64"
3.175 mm	0.1250"	1/8"
6.35 mm	0.2500"	1/4"
10 mm	0.3937"	~25/64"
12.7 mm	0.5000"	1/2"
19.05 mm	0.7500"	3/4"
25.4 mm	1.0000"	1"
50.8 mm	2.0000"	2"

Frequently Asked Questions

The fractional output rounds to the nearest 1/64th of an inch. We display the "Error" value so you know exactly how far off the fraction is from the true millimeter measurement.

In 1959, the International Yard and Pound Agreement defined the international inch as exactly 25.4 millimeters. Before that, slight variations existed between countries.

Yes. This is particularly useful for finding the closest Imperial drill bit size for a Metric hole specification.