Length Converter

How to use this calculator#

1. 1
   Pick the unit you actually measured inThe 'from' unit must match your tape or ruler, not the unit you want. Reading 175 on a metric tape means 175 cm, not 175 mm — a factor-of-ten slip here is the single most common source of a wildly wrong answer.
2. 2
   Turn feet and inches into one number firstThe converter takes a single value, so collapse mixed units before you type. 5 ft 9 in is (5 × 12) + 9 = 69 inches. Never enter 5.9 — that means five and nine tenths of a foot, which is 5 ft 10.8 in.
3. 3
   Choose the target unit and read the full precisionThe result carries six significant figures. Keep them all if the number feeds another calculation, and round only once, at the very end.
4. 4
   Convert back to checkFlip the two units and re-enter the answer. If you do not land back on your starting number, you converted the wrong direction — the classic multiply-instead-of-divide error.

The formula#

The exact conversion factors#

Every length conversion on this page runs through the metre, the SI base unit. The inch has been defined as exactly 2.54 centimetres since the 1959 international yard and pound agreement, and every other imperial length follows from it: 1 foot is 12 inches or 30.48 cm, 1 yard is 3 feet or 0.9144 m, and 1 mile is 5,280 feet or 1,609.344 m exactly.

Keep this short table in your head and you can handle most conversions without a calculator: 1 in = 2.54 cm, 1 ft = 30.48 cm, 1 yd = 0.9144 m, 1 m = 3.28084 ft, 1 m = 39.3701 in, 1 km = 0.621371 mi, and 1 mi = 1.609344 km. The nautical mile is different again, at exactly 1,852 m, because it represents one minute of latitude.

The conversions people search for most#

Height and distance dominate the searches. 170 cm is 66.93 in, which reads as 5 ft 7 in; 180 cm is 70.87 in, or 5 ft 11 in. For running, a 5K is 3.107 miles and a 10K is 6.214 miles, while a marathon distance of 42.195 km works out to 26.219 miles. A 6-foot person is 182.88 cm, and a 100-metre track is 109.36 yards.

Converting feet-and-inches into a single number trips people up. Turn the whole thing into inches first: 5 ft 9 in is (5 x 12) + 9 = 69 in, then multiply by 2.54 to get 175.26 cm. Going the other way, divide centimetres by 2.54 for total inches, then divide that by 12 — the whole number is feet and the remainder is the leftover inches.

Metric, imperial and rounding#

The metre began in 1793 as one ten-millionth of the distance from the equator to the North Pole. Since 1983 it has been defined by the speed of light, as the distance light travels in 1/299,792,458 of a second. The imperial units were re-based on the metre in 1959, which is why factors such as 2.54 and 0.9144 are exact definitions rather than measured approximations.

Rounding is the usual source of error. Because 1 m is 3.28084 ft, using the rough figure 3.3 introduces roughly a 0.6% error — on a 100 m span that is about 60 cm of drift. Round only at the final step, never partway through a multi-step conversion, and carry at least five significant figures whenever the result feeds into construction, machining or engineering work.

Worked examples#

Reference tables#

Common mistakes#

• Converting a length when you needed an areaA room 3 m by 4 m is 12 m². Converting each side to feet gives 9.84 ft by 13.12 ft, which is 129.17 sq ft. Converting the area 12 with the length factor 3.28084 gives 39.4 — off by a factor of 3.3. Square the factor for areas and cube it for volumes.
• Writing 5 ft 9 in as 5.9 feetFeet and inches are base 12, not base 10. 5 ft 9 in is 5.75 feet, not 5.9. Entering 5.9 ft yields 179.83 cm instead of the correct 175.26 cm — a 4.6 cm error, enough to change a clothing size or fail a height restriction.
• Using 3.3 ft per metre as though it were exactThe true figure is 3.280840 ft. The shortcut is 0.58% high, which is fine for guessing a ceiling height and disastrous over distance: across a 100 m running track it adds 1.9 ft, and on a 500 m survey line it puts you nearly 3 ft out.
• Rounding partway through a multi-step conversionGoing mm to inches to feet, rounding 68.8976 in to 69 before dividing by 12 turns 5 ft 8.9 in into 5 ft 9 in. On one measurement that is trivial; repeated across twenty joists in a framing take-off, the accumulated error is a whole inch.

Frequently asked questions#

Key terms#

SI base unit

The metre is the SI base unit of length, defined since 1983 as the distance light travels in vacuum in 1/299,792,458 of a second. Every other length unit on this page is defined as a fixed multiple of it.

International yard and pound agreement (1959)

The treaty in which the US, UK and four Commonwealth nations fixed the yard at exactly 0.9144 m. It is the reason 1 in = 2.54 cm and 1 mi = 1,609.344 m are exact definitions rather than measurements.

US survey foot

A slightly longer legacy foot (1200/3937 m) used in older US land surveys, about 2 parts per million larger than the international foot. It was officially retired at the end of 2022 but still appears in historic deeds and state plane coordinates.

Nautical mile

Exactly 1,852 metres, chosen to approximate one minute of latitude along a meridian. Used in aviation and at sea; it is not interchangeable with the statute mile.

Significant figures

The digits in a number that carry real information. A measurement read to the nearest centimetre supports three significant figures, so quoting a converted result to six implies a precision you did not measure.

Sources#