Inches to cm:
convert screens, waists and fabric lengths

How many centimetres are in an inch?

One inch is exactly 2.54 centimetres. Unlike many conversions, this is not a rounded figure: the inch is defined as 25.4 millimetres.

That makes mental conversion easy. Ten inches is 25.4 cm, twelve inches (one foot) is 30.48 cm, and a rough rule of thumb is to multiply by 2.5 and add a little.

This article focuses on things you buy or have stitched in inches: televisions, monitors, phone screens and trousers. If you are starting from centimetres, such as a height on a form, see the cm to inches article.

Screens and clothing are sold in inches, while flat-pack instruction sheets, furniture catalogues and many tape measures use centimetres. Knowing the factor lets you check a size without opening the box.

Inches to cm formula and fractions

Tape measures and tailors use fractions such as 3/8 or 7/16. Turn the fraction into a decimal first, then multiply.

1. 1Divide the numerator by the denominator: 3 ÷ 8 = 0.375.
2. 2Add the whole inches, for example 12 + 0.375 = 12.375.
3. 3Multiply by 2.54: 12.375 × 2.54 = 31.4325 cm.
4. 4Round to the nearest millimetre: 31.4 cm.

On an inch tape, the finest marks are usually sixteenths. One sixteenth of an inch is 1.5875 mm, so it falls between the millimetre marks of a metric tape. Five-eighths of an inch, for instance, is 1.5875 cm, which most people round to 1.6 cm.

Paper and doors are everyday cases. A4 paper is 8.27 × 11.69 in, which is 21.0 × 29.7 cm, and US Letter is 8.5 × 11 in, which is 21.59 × 27.94 cm. A 32-inch door is 81.28 cm wide, so it will not pass through an 80 cm gap.

A neat mental route is to multiply by 2.5 and then add 1.6%, because 2.54 is exactly 2.5 × 1.016. For 55 in, 55 × 2.5 = 137.5, and 137.5 × 1.016 = 139.7 cm. It is useful when you are standing in a showroom without a calculator.

A short list is enough to convert almost anything by combining values: 1 in = 2.54 cm, 2 in = 5.08 cm, 5 in = 12.7 cm and 10 in = 25.4 cm. Seven inches is 5 + 2, so 12.7 + 5.08 = 17.78 cm.

For small parts, work in millimetres. One inch is 25.4 mm, so a quarter inch is 6.35 mm and three-quarters of an inch is 19.05 mm.

Worked example: will a 55-inch TV fit?

A 55-inch TV is a diagonal measurement, so the width on your wall is much less. Assume a 16:9 panel and a media unit that is 130 cm wide.

Size jumps matter more than they look. Going from 55 to 65 inches raises the diagonal by 18%, but the screen area by about 40%, because area grows with the square of the diagonal.

A 32-inch screen shows the same effect: it is 81.28 cm on the diagonal but only 70.8 cm wide, about 10 cm less than the figure on the box.

The key lesson is that the diagonal figure on the box, 139.7 cm, is longer than either side of the screen. Width and height come from the 16:9 shape using the Pythagorean theorem, because 16² + 9² = 337.

TV size chart in centimetres

Monitors follow the same rule. A 24-inch monitor is 60.96 cm on the diagonal, a 27-inch monitor is 68.58 cm, and a 32-inch monitor is 81.28 cm. A desk 120 cm wide holds a 27-inch monitor with room to spare, but it is tight for two.

Use the width column for the wall and unit, and the diagonal column when you compare against a dimension printed in centimetres on a box.

Waist sizes, collars and alterations

Trouser labels use inches for the waist. A size 34 is 34 × 2.54 = 86.4 cm, which is what you would read on a metric tape.

Alterations use the same factor in smaller steps. Taking a waistband in by 1.5 inches is 3.81 cm, and a kurta length of 40 inches is 101.6 cm. For practical alterations, 3.81 cm is rounded to 4 cm.

Shirt collars follow the same rule. A 15.5-inch collar is 15.5 × 2.54 = 39.37 cm, which sits between the centimetre collar sizes 39 and 40. Check the label for the exact size and compare it with your own neck measurement.

Brands cut differently, so treat the table as a conversion of the number, not a promise of fit. For tailoring, measure with a flexible tape in centimetres, then compare against the garment's flat measurements converted the same way.

Phones, laptops and other small sizes

A 6.1-inch phone screen has a diagonal of 15.494 cm, and a 15.6-inch laptop panel measures 39.624 cm corner to corner. As with TVs, neither tells you the width of the device itself.

Carry-on bag limits and suitcases are another place the conversion shows up. A 22-inch cabin bag is 55.88 cm tall, which is close to the 55 cm height limit often quoted for cabin bags, so check your airline before you travel.

Mistakes with inches to cm

Fractions and tape-measure marks produce most of the errors in this conversion, so go through the common slips below.

• Assuming the screen size is the width. It is the diagonal, so the physical width is about 87% of it on a 16:9 panel.
• Multiplying by 2.5. It looks harmless, but a 40-inch waist becomes 100 cm instead of 101.6 cm.
• Treating nominal sizes such as pipe sizes or plank thicknesses as exact. Trade sizes are labels, so check the actual dimension.
• Forgetting to convert fractions to decimals before multiplying.
• Rounding too early. Carry the full 2.54 through the sum and round only the final answer, since five or six small roundings can add a centimetre.

As a last check, divide your answer by 2.54. If you do not get the number of inches you started with, redo the multiplication.