cm to Inches Converter

The cm to Inches converter is a fast, exact length conversion tool that moves a measurement between centimetres and inches in both directions. Type a number in either field and the other updates live, with no rounding ambiguity and no measurement uncertainty, because the relationship between the two units is defined, not measured. It is built for the everyday situations where you are handed a centimetre figure and need the inch reading (or the reverse): a 30 cm laptop listed in a US shop, a 6'2" height on a medical form, a 50 cm fabric panel, a 17.78 cm phone width quoted in inches on a US forum, a 187.96 cm body measurement that needs to be read against an imperial sizing chart, or a screen diagonal that the manufacturer publishes in centimetres but your comparison sheet keeps in inches.

The converter assumes the modern international inch, which has been exactly 2.54 centimetres since 1 July 1959. Before that date the United States, the United Kingdom, and the Commonwealth countries each used slightly different "inches" inherited from older Imperial standards, and conversions between national inches could differ by a few parts per ten thousand. The international yard and pound agreement of 1959 unified those values; today every legally defined inch in commerce, science, and engineering equals exactly 2.54 cm. This converter uses that value, so every answer it produces is correct under modern law and modern standards.

How to Use the cm to Inches Converter

1. Type the numeric value you want to convert into the Centimetres field or the Inches field. Decimal values are accepted, including sub-centimetre values such as 0.5 cm or 12.75 cm, and small inch values such as 0.25 in or 0.125 in.
2. For the Cm To Inches, The opposite field updates automatically as you type. For the Cm To Inches, The update is recomputed on every keystroke, so the panel always reflects the latest value.
3. Read the Current value row to see both sides of the conversion displayed together, for example 30 cm = 11.811024 in.
4. Read the Show your work row to see the conversion written out step-by-step as <value> cm × (1 in / 2.54 cm) = <value> in, so you can paste the math into a calculation sheet or a homework solution without retyping it.
5. Use the ⇄ swap button when you want to flip the orientation: the value in the inches field moves to the centimetres field and vice versa. This is useful if you have an answer in the "wrong" unit and want to keep the rest of the page oriented in your preferred unit.
6. Use the Quick presets to jump to one of the common everyday values (1 cm, 30 cm, 50 cm, 100 cm, 187.96 cm) and read the equivalent inch value at full six-decimal precision.
7. Click Copy cm, Copy inches, or Copy result to put the value on the clipboard, then paste it into a document, email, or spreadsheet.

the Cm To Inches runs entirely in the browser. For the Cm To Inches, Inputs are not stored, not transmitted, and not associated with any account.

The Formulas

The single conversion factor that drives this tool is:

1 inch = exactly 2.54 centimetres

This is the value published in NIST Special Publication 811 (the SI Units Guide) and in the BIPM SI Brochure. It is exact because the international yard was defined in 1959 as exactly 0.9144 metres, and the international inch is one thirty-sixth of that yard. No rounding is involved: 0.9144 m / 36 = 0.0254 m = 2.54 cm exactly.

From that single equality, the two working formulas fall out by elementary algebra:

To convert centimetres to inches:

inches = centimetres × (1 inch / 2.54 cm)

The dimensionless fraction (1 in / 2.54 cm) equals exactly 0.39370078740157477... A useful rounded form is 0.3937, accurate to about 4 parts in 10 million, which is more than enough for everyday work.

To convert inches to centimetres:

centimetres = inches × 2.54 cm/in

The number 2.54 has no fraction to keep track of: 5 inches is exactly 12.70 cm, 10 inches is exactly 25.40 cm, and 100 inches is exactly 254.00 cm. Anything expressed in inches can be converted to centimetres by moving the decimal point and writing a "2.54" in front.

The two formulas are exact inverses. If you convert a centimetre value to inches and then back to centimetres, you get the original value to machine precision (about 15 significant digits in JavaScript's IEEE 754 doubles). For the Cm To Inches, There is no drift and no accumulated error from repeated round-trips, because the factor is exact.

If you want a rough mental shortcut, remember that 1 cm ≈ 0.4 in (close enough for a quick sanity check) and that 10 cm ≈ 4 in. For more precision, remember that 25 cm = 9.8425 in and 30 cm = 11.8110 in, both are useful anchors because they correspond to common physical sizes (a sheet of A4 paper is 29.7 cm long, and 30 cm is the diagonal of a small laptop).

Worked Examples

Example 1, 1 cm. The single centimetre is exactly 1 × (1 / 2.54) = 0.3937007874... inches, usually rounded to 0.3937 in. So 1 cm = 0.3937 in (four-decimal precision), and a fingertip-thickness object quoted as 1 cm is roughly thirteen thirty-seconds of an inch thick.

Example 2, 50 cm. 50 cm × (1 / 2.54) = 50 / 2.54 = 19.6850393700... inches, rounded to 19.685 in. This is a typical fabric bolt width in continental Europe; the equivalent imperial width is approximately 19 and eleven-sixteenths inches. A tailor converting between metric and imperial yardage would use this exact value.

Example 3, 100 cm. 100 cm × (1 / 2.54) = 100 / 2.54 = 39.3700787401... inches, rounded to 39.37 in. This is the conversion most people know by heart: one metre is about thirty-nine and three-eighths inches, or roughly three feet three and three-eighths inches. A child's height of 100 cm, a kitchen-counter depth, and a metre stick are all 39.37 in.

Example 4, 6'2" person. A person who is 6 feet 2 inches tall is 6 × 12 + 2 = 74 inches tall. 74 in × 2.54 cm/in = 187.96 cm exactly. So 6'2" = 187.96 cm. This is the value used in many medical forms, driver-license applications, and online dating profiles that toggle between the metric and imperial height systems.

Example 5, 30 cm screen. A laptop screen with a 30 cm diagonal is 30 / 2.54 = 11.8110236220... inches, rounded to 11.81 in. Real laptop screens are quoted to the nearest tenth of an inch and cluster around common sizes such as 11.6 in, 13.3 in, 14 in, 15.6 in, and 17.3 in, which correspond to 29.46 cm, 33.78 cm, 35.56 cm, 39.62 cm, and 43.94 cm respectively.

Example 6, round-trip check. Take 187.96 cm. Divide by 2.54 to get 74.0 in exactly. Multiply 74.0 in by 2.54 to get 187.96 cm exactly. The round trip returns the same value to machine precision, which is the formal proof that the two formulas are exact inverses.

Where It Shows Up

Centimetre-to-inch conversions appear in a surprising number of everyday settings, almost always at the boundary between a metric source and an imperial consumer (or vice versa).

Engineering and machining. Drawings for parts manufactured in Asia or continental Europe are published in millimetres and centimetres, while drawings for parts manufactured in the United States are published in inches. A 12.7 mm shaft is 0.5 in; a 25.4 mm shaft is 1.0 in. Engineers converting tolerances between the two systems need exact values.

Sewing, tailoring, and pattern-making. Commercial fabric is sold by the metre in much of the world and by the yard in the United States. A 150 cm bolt of wool is about 59 inches wide. Pattern measurements published in centimetres must be re-stated in inches for customers used to imperial measurement.

Industrial and product design. Consumer electronics (phone sizes, tablet sizes, TV screen diagonals) are published in centimetres in Asia and in inches in North America. A 17.78 cm tablet is a 7-inch tablet; a 165.1 cm television is a 65-inch television. The conversion is mechanical.

Screen and monitor sizing. Display diagonals are almost always quoted in inches for marketing reasons, but the actual physical dimensions (length, width, bezel size) are usually quoted in centimetres or millimetres in the engineering specification. A 27-inch monitor has a diagonal of 68.58 cm.

Bicycle sizing. Road-bike and mountain-bike frame sizes are quoted in centimetres in most of the world and in inches in the US market (a 54 cm frame is approximately a 21-inch frame, and a 56 cm frame is approximately a 22-inch frame). Riders shopping internationally need to bridge the two systems.

Clothing and apparel. Shirt and jacket chest measurements, trouser inseams, and shoe sizes are all expressed in centimetres in many countries and in inches elsewhere. A 91.44 cm chest (the imperial 36-inch chest) is the boundary between M and L in many sizing charts.

Common Mistakes

Rounding 2.54 to 2.5. Using 1 in ≈ 2.5 cm introduces an error of about 1.6 percent (the true ratio is 2.54, not 2.5). On a 100-inch measurement this is 1.6 inches off, which is enough to mis-size a curtain, a shelf, or a piece of furniture. Always use the exact 2.54 factor.

Rounding the inch-to-cm factor the other way. Using 1 cm ≈ 0.4 in introduces the same 1.6 percent error in the other direction. Use 0.3937 (or 0.3937007874 for higher precision).

Mixing international and survey inches. The US survey inch (1/39.37 of a metre, defined separately for land survey) is 2.54000508 cm, not 2.54 cm. It is used only for land surveying in the United States and only for measurements referenced to NAD 27 datum. For everyday length conversions (the scope of this tool), use the international inch of 2.54 cm exactly.

Confusing the diagonal of a screen with its width or height. A 13-inch laptop screen has a 13-inch diagonal, but its width and height are roughly 11.5 in and 7.2 in respectively (for a 16:10 panel) or 11.55 in and 6.48 in (for a 16:9 panel). The cm-to-inch conversion only converts the diagonal.

Forgetting that 1 foot = 12 inches = 30.48 cm exactly. When converting a height like 5'10" (5 feet 10 inches) to centimetres, remember that 1 foot is exactly 30.48 cm and 1 inch is exactly 2.54 cm. So 5'10" = 5 × 30.48 + 10 × 2.54 = 152.4 + 25.4 = 177.8 cm. Do not multiply 70 inches by 2.54 and forget that the original was 5'10", not 70".

Frequently Asked Questions

Is 1 inch exactly 2.54 cm? Yes. The international yard and pound agreement of 1959 defined the international yard as exactly 0.9144 metres, which makes the international inch exactly 0.0254 metres, i.e. 2.54 centimetres. NIST SP 811, the BIPM SI Brochure, and the legal metrology acts of the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa all cite this exact value. There is no rounding error in the conversion factor.

How many centimetres are in an inch again? Exactly 2.54 cm. For rough mental math, 1 in is about 2.5 cm plus a hair, or about a fingernail-and-a-half longer than 2.5 cm. For exact work, use 2.54.

How do I convert cm to inches in my head? A useful trick: 100 cm is exactly 39.37 in (a little under 3 ft 3 in). So 1 cm is about 0.39 in, 10 cm is about 3.94 in, and 50 cm is about 19.7 in. For a quick estimate, divide the centimetres by 2.5 and call the result "approximately this many inches", but remember the estimate is 1.6 percent too low.

How many inches is 1 cm? 1 cm = 1 / 2.54 in = 0.3937007874... in. Rounded to four decimal places, 1 cm = 0.3937 in. Rounded to two decimal places, 1 cm ≈ 0.39 in.

Why are screens measured in inches but phones in centimetres? Different industries adopted different conventions at different times. Television and computer monitor diagonals have been quoted in inches in the United States since the 1950s, and that convention spread globally. Smartphone manufacturers in Asia originally quoted display sizes in millimetres or centimetres; the inch diagonal has gradually become a secondary metric used in marketing materials. A 6.1-inch phone display is a 15.49 cm display, and a 6.7-inch phone display is a 17.02 cm display.

Are US inches the same as UK inches? Yes, in everyday use. The United Kingdom officially adopted the international inch of 2.54 cm on 1 July 1959, and the United States did the same. Before that date the UK inch (the Imperial inch) was fractionally different from the US customary inch, but those small differences have been legal non-entities for more than sixty years. Today, every legally defined inch in both countries equals exactly 2.54 cm.

What about the "survey inch"? The US survey foot is defined as 1200 / 3937 metres, which makes the US survey inch exactly 2.54000508 cm, about 50 parts per million larger than the international inch. It is used only for geodetic surveying referenced to the NAD 27 datum, and it was effectively retired on 1 January 2023 in favour of the international foot for most US federal surveying applications. For everyday length conversions (clothing, furniture, electronics, body measurements), the international inch of 2.54 cm is correct.

References

• NIST Special Publication 811 (2008 edition, with current online amendments): Guide for the Use of the International System of Units (SI). Annex B lists 1 in = 2.54 cm exactly as the conversion factor for the international inch.
• BIPM SI Brochure (9th edition, 2019): The International System of Units (SI). Section 2.3.3 on non-SI units accepted for use with the SI cites the international yard (0.9144 m) and the international inch (0.0254 m) as exact.
• United States Code, Title 15, § 206 (Uniform Standards for Weights and Measures), and the 1959 international yard and pound agreement between the United States, the United Kingdom, Canada, Australia, New Zealand, and South Africa.
• ISO 80000-1:2009 / ISO 80000-3:2006 (Quantities and units, Space and time), which lists the inch as a non-SI unit accepted for use with the SI.
• IEEE/ASTM SI 10-2016 (American National Standard for Metric Practice), which confirms 1 in = 2.54 cm exactly and warns against the older US survey inch in non-survey applications.