Inches to cm Converter

1 inch is exactly 2.54 cm, and 1 cm is about 0.3937 inch

The 2.54 between inches and centimeters is not a rounded figure. It is a definition. Since July 1, 1959, the inch has been fixed at exactly 25.4 millimeters, and the NIST conversion tables list inch to centimeter as 2.54 with no rounding. That is why inches to centimeters always ends in a finite decimal: 27 in × 2.54 = 68.58 cm. Going the other way you divide by 2.54, which never ends (1 cm = 0.393700787… in), so you round it.

The two fields above follow each other. Whichever you typed last is the one in charge, and the other shows ≈ when it was rounded or = when it is exact. The inch field takes decimals like 2.5, fractions like 1 1/2, and feet and inches like 5′ 8″. The two-sided tape shows where the same length falls on an inch scale and on a centimeter scale, and the two panels below read your number as a screen diagonal or as a waist measurement.

When you need to switch between inches and centimeters

• Screens: phones, laptops, monitors and TVs are sold in inches, but your desk, shelf or wall is measured in centimeters. The inch figure is the diagonal, so you need the aspect ratio to get width and height.
• Height in feet and inches: US profiles, medical forms and sports rosters give 5′ 8″. Convert to total inches first, then multiply by 2.54.
• Jeans and trousers: US jeans are often labeled by waist and leg length in inches, such as W30 L32, while many size charts and tape measures work in centimeters.
• Tools, parts and plans: US wrenches, drill bits, lumber and furniture plans use fractions such as 3/8 in or 1 1/4 in. Metric tools and drawings need those in millimeters.
• Bikes, pipes and other named sizes: some inch names are labels, not lengths. A "26 inch" bike wheel is not 660.4 mm across where the tire seats (see Common mistakes).

How to use the converter

1. Type a value in the inch field or the centimeter field. Commas in 1,234.5 are fine. The inch field also accepts fractions (3/8, 1 1/2), feet and inches (5′ 8″ or 5' 8"), and an inch mark as in 27".
2. Pick the number of decimals (0, 1, 2, 4 or 6). The fields, result rows and common values all round to that place. The result sentence shows the full exact value when it terminates and six or more places when it does not.
3. Pick a fraction mark (1/8, 1/16 or 1/32) and the "nearest mark" row shows how far that tape-measure reading is from the real length, in mm. 10 cm reads as 3 15/16 in on a 1/16 scale, and that mark is 0.0125 mm longer than 10 cm.
4. On the two-sided tape, tap or drag the top edge (inches) to move by your chosen fraction, or the bottom edge (centimeters) to move by 1 mm. With the tape focused, the arrow keys move one mark and Page Up and Page Down move 1 inch or 1 centimeter.
5. In "As a screen", choose a ratio (16:9, 16:10, 21:9, 4:3, 3:2 or 20:9) to read your length as the diagonal and get width, height and area. "As a waist" reads it as a waist circumference and gives the flat width (circumference ÷ 2).
6. Tap a common value to load it into the field above. "Copy result" copies the result sentence; "Copy link" copies an address that carries your value and settings.

How the math works

1. Inches to centimeters: inches × 2.54. 32 in × 2.54 = 81.28 cm, or × 25.4 = 812.8 mm.
2. Centimeters to inches: centimeters ÷ 2.54. 82 cm ÷ 2.54 = 32.283464…, so 32.28 in to two decimals.
3. Fractions: turn the fraction into a decimal, then multiply. 3/8 in = 0.375 in, and 0.375 × 25.4 = 9.525 mm. 1 1/2 in = 1.5 × 2.54 = 3.81 cm.
4. Feet and inches: 1 ft = 12 in. 5′ 8″ = 5 × 12 + 8 = 68 in, and 68 × 2.54 = 172.72 cm. Going back, 174 cm ÷ 2.54 = 68.50 in, which is 5 ft 8.5 in.
5. Nearest fraction mark: multiply the inches by 16, round to the nearest whole number, divide by 16. 10 cm = 3.937… in, 3.937 × 16 = 62.99…, so 63/16 = 3 15/16 in.
6. Rounding: the converter works out the exact value as a fraction first and rounds once, at the place you chose (half rounds up). If a positive value would show as 0, it adds places until three significant digits appear. Feet and inches are rounded in total inches before splitting, so you never see "5 ft 12 in".

Inch to centimeter conversion tables

Screen sizes are diagonals: width and height by ratio

A screen size in inches is the diagonal, from one corner of the picture to the opposite corner. With diagonal D and aspect ratio a:b, width = D × a ÷ √(a² + b²) and height = D × b ÷ √(a² + b²). For 16:9, √(16² + 9²) = √337 ≈ 18.36, so the width is about 87.2% of the diagonal and the height about 49%. The table uses that formula for 16:9 and leaves out bezel and stand.

Screens with the same diagonal fit inside the same circle, and the closer a rectangle is to a square, the more area it has. Among 27 in screens, 4:3 has about 2,258 cm², 16:9 about 2,010 cm² and 21:9 about 1,703 cm². An ultrawide looks wide but is shorter and smaller in area than a 16:9 of the same diagonal. A phone is usually taller than it is wide: a 6.5 in screen at 20:9 comes to about 15.1 × 6.8 cm.

Quoting the diagonal of the visible picture has a long history in the United States. The Federal Trade Commission's Picture Tube Rule (16 CFR Part 410) treated it as deceptive to state a TV picture size unless it was the actual viewable picture, and when the figure was not the horizontal width it required the manner of measurement to be shown, as in its example "20 inch (50.80 cm) diagonal". The FTC repealed that rule effective January 7, 2019, finding it no longer necessary, so the diagonal is now an industry convention rather than a federal requirement. For shelf or wall fit, use the product's listed dimensions, because the frame and stand are extra.

Jeans, waist size and flat width

A waist size in inches is the distance around the waist. In a size such as W30 L32, W is the waist and L the leg length, both in inches. Waist 30 in is 30 × 2.54 = 76.2 cm around, and laid flat the garment measures half that, 38.1 cm across. Going back, a 41 cm flat waistband means 82 cm around, or 32.28 in.

A label does not guarantee the measured waist. Brands cut the same number differently, and stretch fabric gives. Measure your own waist with a tape, convert it here, then compare it with the garment's chart rather than the label alone.

Reading fractions on a tape measure

An inch tape divides each inch into 2, 4, 8 and 16 parts, with shorter lines for smaller parts. The longest line in the middle is 1/2 in, the next are 1/4 in and 3/4 in, then the eighths, then the sixteenths. That is why plans say 7/16 in rather than 0.4375 in. Every fraction of an inch is a terminating decimal, so each converts to millimeters exactly.

Common mistakes

• Using the diagonal as the width: a 55 in TV is 139.7 cm on the diagonal, but a 16:9 one is only about 121.8 cm wide. Measuring a cabinet with the diagonal overshoots by almost 18 cm.
• Reading a flat width as the waist: a 38 cm flat waistband is 76 cm around, about 29.92 in, not 15 in.
• Treating 5.8 ft as 5′ 8″: 5.8 ft is 69.6 in (176.784 cm). 5′ 8″ is 68 in (172.72 cm). The number after the foot mark is inches, not a decimal.
• Multiplying area or volume by 2.54: 1 in² = 2.54² = 6.4516 cm², and 1 in³ = 2.54³ = 16.387064 cm³.
• Converting a size name as if it were a length: Sheldon Brown lists the common mountain-bike "26 x 1.00 to 5.0" tires as fitting a 559 mm ISO rim, while an old 26 x 1 3/8 tire is a different 590 mm size, and 650B is 584 mm. Multiplying the name by 2.54 gives none of them, so check the standard size chart.
• Feeding a rounded value back in: 10 cm rounded to 3.94 in and converted back is 10.0076 cm. Keep your original number, and use four or more decimals or the mm row for small parts.

Limits

• Only the international inch (25.4 mm) is used. Older or regional inches, and the US survey foot, are not covered.
• Results are rounded at the decimals you pick, and long values show at most 10 significant digits. The result sentence and formula show the full value.
• "As a screen" gives estimates from the diagonal and a ratio. Real panel ratios, rounded corners, bezels, stands and wall brackets are not included.
• "As a waist" simply halves the circumference. Ease, stretch and each brand's sizing are not included.
• The tape on screen is not a real ruler. Its size changes with your display and zoom, so do not measure with it.

Basis: 1 in = 2.54 cm is a definition (NIST SP 811 Appendix B.8, 1959 agreement), so it does not change from year to year. The FTC repeal notice (83 FR 50484, effective January 7, 2019), the eCFR version history showing 16 CFR 410.1 removed on that date, the rule's earlier text and the Sheldon Brown tire sizes were checked on 2026-10-21.

Sources

• 01NIST SP 811, Appendix B.8: Factors for units listed alphabeticallyinch to centimeter 2.54 E+00 and inch to meter 2.54 E-02, both exact. Checked 2026-10-21
• 02NIST: SI Units, LengthFederal Register Notice 59-5442, 1 in = 25.4 mm exactly from July 1, 1959
• 03Federal Register 83 FR 50484: FTC repeals the Picture Tube RuleFinal rule of 2018-10-09; the rule is no longer necessary and is repealed, effective January 7, 2019. Checked 2026-10-21
• 0416 CFR 410.1 (2012 edition, repealed in 2019)Earlier text: size must be the actual viewable picture; if not the horizontal width, state the manner of measurement, for example "20 inch (50.80 cm) diagonal"
• 05Sheldon Brown: Tire Sizing Systems26 x 1.00 to 5.0 = 559 mm ISO; 26 x 1 3/8 (E.A.3) = 590 mm; 650B = 584 mm. Checked 2026-10-21

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