Article — Metric to Inches Converter
Metric to Inches: convert mm, cm, and m using one calculator
One inch equals exactly 2.54 centimeters by international treaty. To convert centimeters to inches, divide by 2.54. To convert millimeters to inches, divide by 25.4. To convert meters to inches, multiply by 39.3701. The factors are exact, not approximate. So 100 cm equals 39.37 in, 50 mm equals 1.969 in, and 1.83 m equals 72 in (six feet). The calculator at the top of this page handles all three metric units with one toggle.
The 2.54 cm-per-inch number is fixed by the 1959 International Yard and Pound Agreement, signed by the US, UK, Canada, Australia, New Zealand, and South Africa. Before the treaty, the US and imperial inches differed by about 2 parts per million — close enough to ignore for daily work, but enough to matter in precision manufacturing. The treaty made the inch the same number on both sides of the Atlantic for the first time.
Why metric to inches uses the 2.54 factor
Because that is the definition. The inch was historically defined first — barleycorns, thumbs, and king's feet all contributed to its size over centuries. The metric system was created later, with the meter defined originally as one ten-millionth of the distance from the equator to the North Pole. When the two systems had to interoperate in the 20th century, a single conversion factor was needed.
The 1959 agreement picked 2.54 cm as the exact equivalent of one inch. The choice was a compromise: the US inch had been 2.540005 cm, the imperial inch slightly different. Rounding both to 2.54 simplified manufacturing and trade. Every metric-to-inches conversion since 1959 uses this exact factor.
The same 1959 treaty defined the pound at exactly 0.45359237 kg and the mile at 1.609344 km. The three numbers fix US-customary length and mass to the metric system without any measurement uncertainty — pure mathematical relationships.
The metric-to-inches formulas
Each metric unit has its own factor, but all derive from the 2.54 cm definition. Pick the right factor for the unit you have.
mm ÷ 25.4 = inchescm ÷ 2.54 = inchesm × 39.3701 = inchesin × 2.54 = cmin × 25.4 = mmin ÷ 39.3701 = mMental shortcuts: cm × 0.4 gives inches with about 1.6% overstatement. m × 39 gives inches with about 0.9% understatement. mm × 0.04 also works with 1.6% overstatement. Use these for sanity checks; use the exact factor for any spec.
mm, cm, m: when each metric unit is used
Millimeters dominate precision engineering, drilling, fasteners, and most CAD work. A 6 mm bolt, a 25 mm drill bit, a 3 mm sheet of plywood — the metric trades default to mm because the numbers are usually small whole values. Centimeters appear in tape measures, body height, furniture dimensions, and most school-level math. Meters cover larger distances: rooms, sports fields, vehicle dimensions, running tracks.
The metric prefixes step in factors of 10, which makes unit switching trivial: 1.83 m = 183 cm = 1830 mm. Inches do not. A 0.5 m measurement reads cleanly as 19.685 in — awkward fractional inches that have to be approximated for everyday use. This is one of the reasons the rest of the world adopted metric: the math is easier when conversions are powers of ten.
Metric to inches for body height
Body height is the most-converted metric-to-inches value worldwide. Passports, drivers' licenses, and medical records use cm in most countries and feet-and-inches in the US. The calculator above runs either direction. Typical adult heights:
- 150 cm = 59.06 in = 4 ft 11 in (short adult)
- 160 cm = 62.99 in = 5 ft 3 in (median adult female, many countries)
- 170 cm = 66.93 in = 5 ft 7 in (typical adult, mixed)
- 175 cm = 68.90 in = 5 ft 9 in (median adult male, many European countries)
- 180 cm = 70.87 in = 5 ft 11 in (tall adult male)
- 183 cm = 72.05 in = 6 ft 0 in (a clean conversion)
- 190 cm = 74.80 in = 6 ft 3 in (very tall adult)
- 200 cm = 78.74 in = 6 ft 7 in (basketball-player range)
To convert cm directly to feet-and-inches: divide cm by 2.54 to get total inches, divide that by 12 to get whole feet, and the remainder is leftover inches. 175 / 2.54 = 68.90, 68.90 / 12 = 5 with 8.90 inches left. So 175 cm = 5 ft 8.90 in, or about 5'9".
Metric to inches for screens and devices
Monitor, TV, and laptop screen sizes are always quoted in inches worldwide, regardless of which measurement system the country otherwise uses. The number refers to the diagonal of the active display area, not the bezel-to-bezel size. So a 27" monitor has a 27-inch (68.6 cm) diagonal, with an actual width around 60 cm and height around 34 cm.
The inch convention for screens dates to the 1940s and stuck because TV and computer manufacturers were predominantly American. Even Sony, Samsung, and LG — companies based in countries that otherwise use metric — label their screens in inches for export consistency. The metric equivalent is printed on the spec sheet but rarely on the box.
Metric to inches in engineering and CAD
Mechanical engineering is bilingual. European and Japanese drawings use mm throughout; US drawings use inches and decimal-inch tolerances. CAD software handles both, often switching the display unit per drawing. The conversion math is the same as for body height — multiply or divide by 25.4 — but the precision requirements are much higher.
Tolerances of ±0.001 inch (about 0.025 mm) are routine. A 0.5-inch shaft hole on a US print becomes a 12.7 mm hole on a metric print. The conversion is exact, but the drill bit sizes typically don't match perfectly: the closest metric drill might be 12.5 mm or 13 mm. Engineering practice is to convert the spec and let the bit selection follow the closest standard size, then add or subtract a clearance fit.
For woodworking with metric plans on imperial tools, the 2.5 cm ≈ 1 in approximation is usually fine. A 2.4-inch board cut to 6 cm is wrong by 0.04 in (1 mm) — below most home-shop tolerances. For metal or cabinetmaking, use the exact 2.54 factor.
The countries that still use inches
The United States, Liberia, and Myanmar (Burma) are the only countries that have not officially adopted the metric system. The UK is metric by law but uses imperial on road signs (miles, yards), in pubs (pints), and in some everyday speech (a person's height in feet and inches). Canada, Australia, and New Zealand are formally metric but routinely use inches for body height and TV screens in conversation.
US scientific work uses metric internally; FDA drug doses are in grams and milligrams, NASA missions in meters and kilograms. The metric-to-inches conversion happens at the boundary between government and consumer, between research and commerce, between original-design European parts and the US market. The calculator above is for that boundary work.
Reading 175 cm as 175 mm gives 6.89 in instead of 68.90 in — a 10× error. Drawing scales make this worse if the dimension labels are unclear. Always confirm the unit label before converting. The 2.54 factor is correct only when both sides agree on what "metric" means.
Common metric-to-inches mistakes
The most frequent error is using a rounded factor on a high-precision job. The mental shortcut "1 inch ≈ 2.5 cm" is off by 1.6% — usable for everyday measurements but unacceptable in machining. The second is mixing up mm and cm. The third is forgetting that screen sizes refer to the diagonal, not the width.
- 1 in = 2.54 cm (exact)
- 1 in = 25.4 mm (exact)
- 1 in = 0.0254 m (exact)
- 1 ft = 30.48 cm (exact)
- 1 m = 39.3701 in (3 ft 3.37 in)
- 1 yard = 0.9144 m (exact)
- 1 mile = 1.609344 km (exact)
- Adult height converts: 175 cm = 5'9", 180 cm = 5'11", 183 cm = 6'0"