How to Convert Cm to Inches (Why the TV Size Isn't the Width)
To convert centimeters to inches, multiply the centimeter figure by 0.393701. A 30 cm measurement becomes 30 × 0.393701 = 11.81 inches. The reverse direction is simpler: multiply inches by 2.54 to get centimeters. Those two numbers aren’t quite mirror images of each other, and the reason why matters more than it looks.
Why the factor is 0.393701, and not a round 0.39
The inch has a fixed, legal definition: 1 inch equals exactly 2.54 cm. That figure isn’t a rounded approximation or an old imperial holdover, it comes from the 1959 International Yard and Pound Agreement, the treaty that also fixed the pound at exactly 0.45359237 kg. The US, UK, Canada, Australia, South Africa and New Zealand all signed on, which is why an inch measures identically whether you’re in Texas or Yorkshire.
Going from inches to centimeters is therefore exact: multiply by 2.54 and the decimal terminates. Going the other way doesn’t work out so cleanly. 1 cm equals 1 ÷ 2.54 inches, and that division produces 0.3937007874…, a decimal that repeats forever. There’s no way to write it exactly in decimal form, only to truncate it somewhere. This tool and this article use 0.393701, six decimal places, which is precise enough for anything from a shoe size to a shipping container and leaves the rounding error too small to matter for a single conversion.
Worked examples: height, TV screens and paper size
Start with something everyday: converting a height of 175 cm. Multiply by the factor: 175 × 0.393701 = 68.897675 inches. Heights in the US are usually given in feet and inches, so divide that result by 12: 68.897675 ÷ 12 = 5.7415 feet. The whole number, 5, is the feet. Take the remainder, 0.7415, and multiply by 12 to get the leftover inches: 0.7415 × 12 = 8.9 inches. So 175 cm is 5 feet, 8.9 inches, commonly rounded up and written as 5’9”.
Now the trap that catches far more people. TV and monitor sizes are always advertised in inches, and always measured as the diagonal, the distance corner to corner across the screen, even in countries that use the metric system for everything else. A “55-inch TV” means the diagonal is 55 × 2.54 = 139.7 cm. It does not mean the screen is 139.7 cm wide. For a standard 16:9 widescreen panel, that 55-inch diagonal works out to roughly 121.9 cm of actual width and about 68.6 cm of height. Someone measuring a 130 cm gap in a media cabinet, converting “55 inches” straight to 139.7 cm, and concluding the TV won’t fit has made an expensive mistake: the real width, 121.9 cm, would have fit with room to spare. Diagonal and width are two different numbers, and only one of them is what most conversions actually need.
Paper size hides a similar mismatch. A4, the standard sheet size across most of the world, measures 21.0 × 29.7 cm, which converts to 8.27 × 11.69 inches. US Letter, the standard in the United States and parts of Canada, measures 8.5 × 11 inches, which converts to 21.59 × 27.94 cm. These are not the same rectangle:
| Size | Centimeters | Inches |
|---|---|---|
| A4 | 21.0 × 29.7 cm | 8.27 × 11.69 in |
| US Letter | 21.59 × 27.94 cm | 8.5 × 11 in |
Letter is 0.59 cm wider than A4 and 1.76 cm shorter. Print a Letter-formatted PDF on an A4 printer without telling it to scale, and the content gets cropped along one edge or the margins come out uneven. The reverse causes the same problem in the other direction. Anyone exchanging documents internationally runs into this the first time a resume or contract comes out looking slightly wrong on paper.
Calculate with your own numbers
Run any of the figures above, or your own, through the converter below.
Common mistakes and edge cases
- Reading a TV or monitor spec as a width. As shown above, the advertised inch figure is always the diagonal. Converting it directly to centimeters gives the diagonal in centimeters, not the width. Check the manufacturer’s spec sheet for actual width and height before assuming a screen fits a space.
- Rounding the factor too early on large quantities. Using 0.39 instead of 0.393701 is fine for a single tape-measure reading, but the error compounds with scale. Take a 100-meter (10,000 cm) bolt of fabric: 10,000 × 0.39 = 3,900 inches, while the correct figure is 10,000 × 0.393701 = 3,937.01 inches. That’s a 37-inch shortfall on a single roll, real money when fabric, lumber or cable is priced per inch or per foot.
- A quick mental estimate, for when precision doesn’t matter. Divide the centimeter figure by 2.5, then subtract 2% of that result. For 100 cm: 100 ÷ 2.5 = 40, and 2% of 40 is 0.8, giving 39.2. The actual value is 39.37, close enough for comparing furniture dimensions or clothing sizes while shopping, but not close enough for anything you’re going to cut, order or bill.
Frequently asked questions
Why isn’t the cm-to-inches factor a round number? Because it’s a division, not a multiplication. The inch is legally defined as exactly 2.54 cm, so converting inches to centimeters is exact and terminates. Converting the other way means dividing 1 by 2.54, which produces the repeating decimal 0.3937007874… There’s no round equivalent, only a truncation of it, and 0.393701 is precise enough for everyday and most professional use.
Is a 55-inch TV 139.7 cm wide? No. 139.7 cm is the diagonal measurement, corner to corner, which is how TVs and monitors are always sized. The actual width of a 55-inch, 16:9 screen is roughly 121.9 cm, with a height of about 68.6 cm. Measure the cabinet or wall space against the width and height from the manufacturer’s spec sheet, not against the inch number on the box.
Is A4 paper the same size as US Letter once converted? No. A4 is 21.0 × 29.7 cm (8.27 × 11.69 in), and Letter is 21.59 × 27.94 cm (8.5 × 11 in). Letter is slightly wider and slightly shorter than A4. Printing one format on the other’s paper size without scaling crops content or produces uneven margins.
How many inches is 1 cm exactly? 1 cm equals 0.3937007874… inches, an infinitely repeating decimal. This tool rounds it to 0.393701, six decimal places, which is accurate enough for any measurement from clothing to construction.