Length Converter

What this converter answers

What a length is in another unit, and how exact that answer is. Type one number and you get it in millimeters, centimeters, meters, kilometers, inches, feet, feet and inches, yards, miles and nautical miles at once. Heights come out as feet and inches, and inch results can be rounded to the nearest 1/2 to 1/64 in, with the rounding error shown in inches and millimeters.

How to use it

• Length: the number to convert, in the unit you pick under From. Commas between thousands and e-notation both work: 1,250, 3e-6.
• Feet and inches: appears when From is feet and inches. Type 5 ft 11 in, 5'11", 5 11 or 5-11. More than 11 inches carries over, so 5 ft 13 in is read as 6 ft 1 in.
• From and To: the unit you measured in and the unit you need. Micrometers and nanometers come after kilometers, and the old US survey foot after miles.
• Significant figures: Auto shows up to 12 significant figures and whole numbers in full, so short exact values such as 2.54 or 1,609.344 appear complete. Choose fewer for a shorter answer.
• Inch fractions (shown when To is inches or feet and inches): the precision of the fraction results, from the nearest whole inch to the nearest 1/64 in.

Under the answer, Swap turns the conversion around, and each unit name in the table makes that unit the main result. The four Try buttons load a height, a metric size as an inch fraction, a race distance and a nautical mile, each explained further down.

How many centimeters are in an inch?

Exactly 2.54. Since 1959 the foot has been exactly 0.3048 m, which makes the inch, yard and mile exact metric lengths too. These factors are definitions, not measurements:

Going from metric to US units, the factors never end: 1 cm ≈ 0.393701 in, 1 m ≈ 3.28084 ft, 1 m ≈ 1.09361 yd and 1 km ≈ 0.621371 mi. So inches to centimeters can be exact while centimeters to inches is almost always rounded. The converter marks every value Exact or Rounded for that reason.

Length conversion formula

Each unit has a fixed length in meters, $f$. To convert a length $x$ from one unit to another:

• $x_{\text{from}}$ is the length you have, $x_{\text{to}}$ the length in the new unit.
• $f_{\text{from}}$ and $f_{\text{to}}$ are the two units’ lengths in meters: 0.0254 for an inch, 0.3048 for a foot, 0.01 for a centimeter.

For inches to centimeters that is $x \times 0.0254 \div 0.01$, the familiar $x \times 2.54$. When neither direction is a short exact decimal, as with nautical miles to miles, the steps go through meters: $x \times 1{,}852 \div 1{,}609.344$.

Worked example: 178 cm in feet and inches

1. Centimeters to inches: 178 ÷ 2.54 = 70.0787401575 in. The division doesn’t end, so this is rounded.
2. Whole feet: 70.0787401575 ÷ 12 = 5.83989501312, so 5 ft, which is 60 in.
3. Inches left over: 70.0787401575 − 60 = 10.0787401575 in, shown as 5 ft 10.08 in.
4. Nearest 1/16 in: 70.0787401575 × 16 = 1,121.2598, which rounds to 1,121 sixteenths, so 70 1/16 in or 5 ft 10 1/16 in. That is 0.0162 in (0.41 mm) shorter than the exact length.
5. To the nearest whole inch the height is 5 ft 10 in.

The table under the result gives the same length as 1,780 mm, 1.78 m and 1.94663167104 yd.

Feet and inches to cm (height conversion)

Turn the feet into inches, add the inches, then multiply by 2.54: (feet × 12 + inches) × 2.54. For 5 ft 7 in, 5 × 12 + 7 = 67 in, and 67 × 2.54 = 170.18 cm. Because 2.54 is exact, every whole-inch height has an exact centimeter value:

The other way round, centimeters to feet and inches, the inches are rounded:

mm to inches as a fraction (1/16, 1/32, 1/64)

Tape measures and drill sets use fractions, so a metric size has to be rounded to the nearest mark. Multiply the inches by the denominator, round to a whole number, and put it over the denominator. For 25 mm: 25 ÷ 25.4 = 0.984252 in, and 0.984252 × 64 = 62.99, so the nearest 1/64 in is 63/64 in, only 0.000123 in over. On a ruler marked in sixteenths, the same 25 mm rounds to 1 in, 0.0157 in over.

A fraction of 1/64 in can still be off by up to half a sixty-fourth, about 0.0078 in (0.2 mm). The Rounded to each inch fraction table under a result shows every precision with its difference, so you can see which marks are close enough.

Miles, nautical miles and kilometers: what’s the difference?

A mile (statute mile) is 5,280 ft, exactly 1,609.344 m. A nautical mile, used at sea and in aviation, is exactly 1,852 m, so 1 nautical mile ≈ 1.15078 mi ≈ 6,076.12 ft, and 1 mi ≈ 0.868976 nautical miles. Knots are nautical miles per hour, which is why the two must not be mixed.

The US survey foot

Until the end of 2022, US geodetic surveying could use a slightly longer foot, 1200/3937 m, which is about 2 parts per million more than the international foot of 0.3048 m. NIST and NOAA declared it obsolete from January 1, 2023. The difference is tiny on a tape, about 3.2 mm over a mile, but not on map coordinates: 2,000,000 survey feet is 4.000008 ft (about 1.22 m) longer than 2,000,000 international feet. Choose US survey feet (legacy) only when converting records that were made in it; the page then says so above the result.

Reading the result

• The headline is the length in the To unit, followed by the equation and whether it is exact or how it was rounded.
• The factor line shows the conversion in each direction. For centimeters to inches it reads 1 cm ≈ 0.393700787402 in, a rounded figure, and 1 in = 2.54 cm exactly, the definition.
• The tiles (for inch results) show the length to your chosen inch fraction and to the nearest whole inch, each with its error, and the same length in plain inches or in feet and inches, to 0.01 in.
• How this was calculated shows the work with your numbers: the factor used, the split into feet and inches, and the fraction rounding.
• The conversion table lists the length in every common unit, each marked Exact or Rounded. Select any unit name, say yards, to put it in the main result; Download CSV and Copy table export the full list.
• Rounded to each inch fraction compares the whole inch, 1/2, 1/4, 1/8, 1/16, 1/32 and 1/64 in, with the difference from the exact length in inches and millimeters.
• Save for comparison keeps up to three conversions side by side, with the change in meters from the first.

Common length conversion mistakes

• Reading 5.11 ft as 5 ft 11 in. A decimal foot is not inches: 5.11 ft is 5 ft 1.32 in, and 5 ft 6 in is 5.5 ft, not 5.6 ft. The converter points this out when you enter a short decimal in feet.
• Rounding 2.54 to 2.5. It makes every centimeter figure 1.6% too small: 70 in becomes 175 cm instead of 177.8 cm.
• Converting back from a rounded answer. 0.3937 in per cm is already rounded, so going back and forth drifts. Start again from the original number, or use the exact direction (× 2.54).
• Mixing up miles and nautical miles. A nautical mile is about 15% longer, so a distance in nautical miles or knots read as statute miles comes out 13% short.
• Treating survey feet as feet. On large coordinates the 2-parts-per-million difference adds up to feet, as in the example above.

Assumptions and limitations

• The factors are the exact international definitions; the converter doesn’t use rounded tables. Results are rounded only for display, to the significant figures you choose.
• A converted value can’t be more precise than the measurement. A height of 178 cm measured to the nearest centimeter lies between 177.5 and 178.5 cm, which is 5 ft 9.88 in to 5 ft 10.28 in, so quoting 5 ft 10 1/16 in suggests more certainty than the tape gave.
• Inch fractions round halves up, and feet and inches keep two decimals of an inch.
• Lengths must be 0 or more and at most 10¹⁵ in the unit typed. Feet and inches go up to 750 trillion feet, which keeps them exact to 0.01 in; choose feet or miles for anything longer. Negative lengths, such as a change in height, are not accepted.
• Units not in the list (mils, fathoms, chains, furlongs) can be converted through inches or feet, as the questions below show.