CM to inches converter

Where the centimeter and inch come from

The inch is an old English unit, once defined by lining up barleycorns and later by royal decree, long before it had a fixed metric equivalent. One early rule laid three barleycorns end to end for an inch, while another used the width of a man's thumb at the base of the nail, so the same word covered noticeably different lengths from town to town. The centimeter is a hundredth of a meter, part of the decimal metric system France adopted in 1795, with no folk measurements behind it at all.

The two units only lined up exactly in 1959, when the US, UK and four other countries signed the international yard and pound agreement and fixed 1 inch at exactly 2.54 cm. Every conversion here uses that same exact figure.

Common sizes in both units

A credit card is a handy pocket ruler: its long edge is 8.56 cm, close enough to 3.5 inches to check a rough size on the go without pulling out a tape measure. A US quarter is a smaller stand-in, at just under an inch across.

A quick way to estimate without a calculator

• Divide centimeters by 2.5 for a fast estimate. It runs a little high: 160 cm divided by 2.5 gives 64, versus the exact 62.99 inches.
• For a closer mental estimate, take that rough number and shave off about 1.5 percent.
• Going the other way, multiply inches by 2.5 to land within a few percent of the exact centimeter figure.
• The gap between the shortcut and the exact figure grows as the length grows, so for anything over a meter or so, check the exact calculator above instead of estimating.

Where each unit is used

The centimeter is the everyday length unit almost everywhere outside the US, and the standard for medical charts, product packaging and scientific work worldwide. The US still measures height, screen size, tools and building materials in inches, so cm figures from imported products, recipes or manuals often need converting before they mean much.

Even within the US, science and medicine mostly work in centimeters: a doctor's chart records height in cm, and manufacturing drawings for parts sold worldwide are often dimensioned in cm or mm alongside inches.