Inches to cm: the common sizes and the traps

One inch equals exactly 2.54 centimeters. That figure isn't a rounding: since 1959 the international inch has been defined that way by agreement, so the raw math is never really in question. To convert your own number, type it into the inches to cm converter: decimals and fractions such as 3/4 or 5 1/2 both work, and you get centimeters, millimeters and the nearest tape-measure fraction at once. What actually trips people up is everything around that number: screens measured diagonally, areas confused with lengths, feet-and-inches notation misread as a decimal, and a computing quirk that makes a "1 TB" drive look smaller than promised. That's what the rest of this guide covers.

How many centimeters are in these common sizes?

Here are the ones that come up constantly, all built on the same exact factor, 2.54 cm per inch:

• 1 inch = 2.54 cm
• 2 inches = 5.08 cm (common hardware and pipe-fitting sizes)
• 6 inches = 15.24 cm (a standard ruler's half-length, a common phone height)
• 8.5 inches = 21.59 cm (the width of US Letter paper; A4 is a different size entirely)
• 12 inches = 30.48 cm (exactly 1 foot)
• 13 inches = 33.02 cm (compact laptop screen diagonal)
• 27 inches = 68.58 cm (a typical desktop monitor diagonal)
• 55 inches = 139.7 cm (a common living-room TV diagonal)
• 60 inches = 152.4 cm (5 feet, and a frequent bolt-of-fabric width)

For anything not on this list, the formula never changes: inches × 2.54 = centimeters, and centimeters ÷ 2.54 = inches. The converter also includes a chart from 1 to 12 inches and a table of frequent measurements, so you can read a value off without typing anything.

Tape measures and drill bits use fractions rather than decimals, and those convert exactly too: 1/16 inch is 1.5875 mm, 1/8 is 3.175 mm, 1/4 is 6.35 mm, 1/2 is 12.7 mm and 3/4 is 19.05 mm. The converter rounds to the nearest sixteenth of an inch, the usual tape-measure marking, and marks the result as approximate when the value does not land exactly on one.

Why are screen sizes measured diagonally?

When a manufacturer advertises a "55-inch" TV, that number isn't the width or the height: it's the line connecting two opposite corners. Diagonal measurement lets one figure describe screen size regardless of aspect ratio (16:9, 21:9, 4:3), which is why it became the industry standard for quick comparison.

The catch shows up when two screens share a diagonal but not a shape. A 34-inch monitor at 16:9 measures about 75.3 × 42.3 cm. A 34-inch ultrawide at a nominal 21:9 measures about 79.4 × 34.0 cm. It is a little wider and noticeably shorter, and its visible area is roughly 15% smaller. If what you actually care about is desk space or wall space, diagonal inches alone won't answer that: you need the aspect ratio too, to work out width and height separately. The converter's Screens tab does exactly that, taking a diagonal and an aspect ratio (16:9, 21:9, 16:10 or 4:3) and returning the width and height in centimeters, for the visible image area without the bezel or stand.

The classic mistake: linear inches when you need square inches

This is probably the costliest error of the bunch, because it doesn't show up until the material doesn't fit. A 10 × 10 inch panel has an area of 100 square inches, and converting that to square centimeters does not mean multiplying by 2.54. It means multiplying by 2.54 squared, or 6.4516. The correct answer is 645.16 cm², not 254 cm².

The reason is geometric: converting a length applies the conversion factor once; converting an area applies it twice, because area is length × length. This trips up anyone ordering glass cut to size, fabric by the square foot converted to metric, or comparing panel and screen specs listed in different unit systems.

How do I convert 5'11" to centimeters?

American height notation uses an apostrophe for feet and a double quote for inches: 5'11" means 5 feet and 11 inches, two separate units written side by side, not one decimal number. The common mistake is reading the "11" as a decimal fraction of a foot and calculating 5.11 feet, which gives the wrong result.

The correct conversion treats each unit on its own: 5 feet (1 foot = 30.48 cm) plus 11 inches (2.54 cm each).

• 5 feet × 30.48 cm = 152.4 cm
• 11 inches × 2.54 cm = 27.94 cm
• Total: 152.4 + 27.94 = 180.34 cm

The converter takes inches only, so for a mixed height first turn everything into inches: 5 × 12 + 11 = 71 inches, which the tool converts to the same 180.34 cm. It's the same class of trap as reading a date in the wrong format (05/12/2026 means May 12th in the US and December 5th almost everywhere else), where two familiar-looking notations produce different results depending on which one you assume. If you deal with dates across regions, the date calculator avoids that exact kind of mix-up.

The computing case: kB and KiB aren't the same thing

It isn't an inches problem, but it's the same underlying issue: two measurement systems that share a name and confuse everyone. A kB (kilobyte, decimal) is 1,000 bytes; a KiB (kibibyte, binary) is 1,024 bytes. The gap looks small until it compounds: a MB is 1,000,000 bytes while a MiB is 1,048,576, and by the time you reach TB vs. TiB the difference is already past 9%.

That's the actual reason a drive sold as "1 TB" shows less space once you plug it in: the manufacturer counts in decimal TB (1,000,000,000,000 bytes), but the operating system typically displays binary TiB. Same physical bytes, two different rulers, and structurally the same mistake as mixing up feet and inches, just swapped from base 10 vs. base 12 to base 10 vs. base 2. If you need to move directly between number systems, the number base converter handles binary, decimal, and hexadecimal.

Temperature: multiplication alone won't save you

Every conversion above is a fixed multiplier: multiply and you're done. Temperature doesn't work that way, because Celsius and Fahrenheit don't share a zero point, and 0°C is not 0°F. The formula is °F = °C × 9/5 + 32 (and inversely, °C = (°F − 32) × 5/9): you have to shift before you scale.

That's why "twice as many degrees" means nothing physically: 68°F isn't twice as hot as 34°F, because the zero on that scale is arbitrary (true zero, with no thermal energy at all, sits at −459.67°F). It's a useful reminder that a conversion can look trivial and still hide an offset rather than a plain ratio. The same kind of gotcha shows up with Unix timestamps, where mixing up seconds and milliseconds throws a date off by thousands of days; the timestamp converter catches that instantly.

Convert it in a few seconds

For inches, centimeters, millimeters and screen dimensions, use the inches to cm converter. For anything else in this guide (square inches, temperature, and more than twenty other units) the unit converter covers it. Both run entirely in your browser, so nothing you type (a spec sheet, a blueprint measurement, a shipping label) ever leaves your machine.

Frequently asked questions

Related tools

• Inches to cm converter: inches, centimeters and millimeters, with fractions and a screen-size calculator.
• Unit converter: length, area, weight, temperature and more, fully local.
• Number base converter: binary, decimal and hex, for when the mix-up is kB vs. KiB.
• Timestamp converter: Unix dates without mixing up seconds and milliseconds.