Length Converter

Two systems, one planet

The world measures length in two systems and nobody's switching. The metric system — meters, kilometers, centimeters, millimeters — is used by everyone except three countries (the US, Liberia, and Myanmar). The imperial system — inches, feet, yards, miles — is used in the US and unofficially in a few places that nominally went metric decades ago but kept the old units around for pints, road signs, and people's heights.

So if you live in the US and order a desk from a European furniture site, the dimensions are in centimeters. If you're British and looking at an American spec sheet, the wall thickness is in inches. If you're a software engineer trying to remember whether 5'10" is closer to 178 cm or 168 cm, you need a converter. That's what the Length Converter does: type a value, pick the units, get the answer.

The conversions themselves aren't approximations. Since 1959, the international yard and pound have been defined in terms of metric units, so 1 inch is exactly 2.54 cm and 1 foot is exactly 0.3048 m. The numbers below come from those exact definitions, not from rounding.

How to use the Length Converter

Enter a number in any unit, pick the unit you're converting from, and pick the unit you want the answer in. The result updates as you type.

1. Type your value into the input box.
2. Pick your source unit from the dropdown — meters, feet, inches, miles, and so on.
3. Pick your target unit.
4. Read the converted value below.

The tool covers eight common units: millimeters, centimeters, meters, kilometers, inches, feet, yards, and miles. That's enough to handle anything from a screw thread to a hike length. Calculation happens in the browser — your numbers don't go anywhere.

The exact conversions worth memorizing

The word "exactly" matters here. These aren't measurements rounded to four decimal places — they're definitions. When the international yard and pound agreement was signed in 1959, the participating countries (US, UK, Canada, Australia, New Zealand, South Africa) agreed that one yard would be defined as 0.9144 meters. The inch and foot were derived from that. Before 1959 there was an "imperial inch" and a slightly different "US survey inch," and the agreement merged them.

What this means in practice: when you see "1 inch ≈ 2.54 cm," the ≈ is wrong. The two values are equal by definition. The metric system is the reference; imperial units are now defined in terms of it.

Worked example: how tall is 5'10" in centimeters?

This is the conversion English-speaking people google more than any other length conversion. The math is straightforward once you know the definitions.

5'10" means 5 feet and 10 inches. Convert each to centimeters, then add:

• 5 feet × 30.48 cm/ft = 152.4 cm
• 10 inches × 2.54 cm/in = 25.4 cm
• Total: 152.4 + 25.4 = 177.8 cm

Or, if you prefer to keep it in inches first: 5 × 12 + 10 = 70 inches, and 70 × 2.54 = 177.8 cm. Same answer, fewer steps. Either way, 5'10" is 177.8 cm — or 1.778 m — exactly.

For the reverse — say someone says they're 180 cm and you want to know what that is in imperial — divide by 2.54 to get inches (180 / 2.54 = 70.866 inches), then divide by 12 to get feet (70.866 / 12 = 5.905). The whole part is 5 feet; the decimal part times 12 gives the remaining inches (0.905 × 12 ≈ 10.86). So 180 cm is about 5 feet 10.9 inches — pretty close to 5'11".

Common length conversion table

Three of these are worth memorizing if you cross between systems often. To convert a height in inches to centimeters, multiply by 2.54. To convert a distance in miles to kilometers, multiply by 1.6 (close enough; the exact value is 1.609344). To convert kilometers to miles, multiply by 0.6 (close enough; the exact is 0.621). A 10 km run is about 6.2 miles. A marathon is 42.2 km, or 26.2 miles. A 5K is 3.1 miles.

Quick height reference

Beyond everyday units — the very small and the very large

The metric system extends in both directions on a power-of-ten scale, which is why scientists use it: the same prefixes work whether you're measuring an atom or a galaxy. The everyday units (mm, cm, m, km) are a tiny slice of a much wider range.

Below a millimeter:

• Micrometer (μm) = 0.001 mm = 1/1,000,000 of a meter. A human hair is roughly 50–100 μm wide. Bacteria are 0.5–5 μm.
• Nanometer (nm) = 0.001 μm = 1/1,000,000,000 of a meter. Visible light has wavelengths of 380–740 nm. Modern transistors are around 3–5 nm wide.
• Angstrom (Å) = 0.1 nm. Used in chemistry and crystallography; the typical bond length between two atoms.

Above a kilometer:

• Astronomical unit (AU) = the average distance from Earth to the Sun = about 149.6 million km. Used for measuring distances within the solar system.
• Light-year = the distance light travels in one year in vacuum = about 9.46 trillion km. The nearest star (Proxima Centauri) is 4.2 light-years away.
• Parsec = about 3.26 light-years. Used by astronomers because it falls out naturally from parallax measurements.

The Length Converter focuses on the everyday range — millimeters to miles. For scientific work, most lab software handles the very-small and very-large units natively.

Where converter mistakes actually happen

Conversion math is easy. Where errors creep in is at the edges. A few patterns to watch for.

Mixed-unit heights and lengths. When someone writes "5'10"," they mean 5 feet plus 10 inches, not 5.10 feet. The decimal-foot equivalent of 5'10" is 5.833 feet (10/12 = 0.833). Treating 5'10" as 5.10 feet gives you 5.10 × 30.48 = 155.4 cm — about 22 cm short of the real value.

Statute miles vs nautical miles. The familiar US/UK "mile" is the statute mile (1.609344 km). The nautical mile, used in aviation and shipping, is different — about 1.852 km, or roughly one minute of latitude. If you're reading aviation distances or marine charts, check which mile is meant.

US survey foot vs international foot. The US officially deprecated the survey foot at the end of 2022 in favor of the international foot. The two differ by about 2 parts per million — irrelevant for everyday work, but it matters for land surveys and old USGS maps. Most converters use the international foot.

Decimal feet for elevations. Elevations on US maps are often given in feet, sometimes to tenths or hundredths. 1234.5 feet is 1234 feet and half a foot — meaning 6 inches, not 0.5 inches. Converting decimal feet to cm before splitting into feet-and-inches avoids this.

Related conversions

• Weight Converter — kilograms, pounds, ounces, stone. The other big system mismatch.
• Temperature Converter — Celsius, Fahrenheit, Kelvin. The conversion math is different (it involves addition, not just multiplication), which is why people get it wrong.
• Feet to Inches — single-purpose tool for the most common imperial-to-imperial conversion. Faster if you only need that one.
• MM to Inches — for hardware specs, drill bits, and metric-marked tools.
• Inches to Feet — for converting total-inch measurements back into the familiar 5'10" format.

Frequently asked questions

Is 1 inch exactly 2.54 cm or is that rounded?

Exactly 2.54 cm, by definition. The international yard and pound agreement of 1959 defined one yard as exactly 0.9144 meters. The inch (1/36 of a yard) and foot (1/3 of a yard) follow from that. There's no rounding; 2.54 cm per inch is as exact as 100 cm per meter.

How tall is 5'10" in centimeters?

177.8 cm exactly. The math: 5 feet × 30.48 cm/foot = 152.4 cm, plus 10 inches × 2.54 cm/inch = 25.4 cm, giving 177.8 cm total. Or in one step: 70 inches × 2.54 = 177.8 cm.

Why is the US still on imperial?

Inertia, mostly. The US tried to switch in the 1970s — the Metric Conversion Act was signed in 1975, road signs got planned for kilometers, public information campaigns ran. Then the political will faded. Federal agencies use metric internally (NASA, the military), science uses metric exclusively, and a lot of US industry (cars, electronics, pharma) is metric-first. But consumer-facing measurements — recipes, road signs, gas pumps, body weight, height — stayed imperial because the cost of switching was high and the public didn't care enough.

What's the difference between a statute mile and a nautical mile?

A statute mile is 1.609344 km — the "mile" used for road distances in the US and UK. A nautical mile is 1.852 km — the mile used in aviation and at sea, defined as one minute of latitude (1/60 of a degree). One knot is one nautical mile per hour. If your converter says "miles," it almost always means statute miles; check the labels if you're working with aviation or marine data.

What's a meter actually defined as today?

The distance light travels in vacuum in 1/299,792,458 of a second. That definition was adopted in 1983 — it ties the meter to the speed of light, which is a fundamental constant of physics. Before that, the meter was defined by a physical prototype bar in Paris. The current definition lets any well-equipped lab in the world reproduce a meter from first principles.

How accurate is "1 mile = 1.6 km" as a rule of thumb?

Off by about 0.6%. The exact conversion is 1.609344 km per mile, so 100 miles is 160.9 km — not 160. For estimating gas mileage, run distance, or a road trip, the 1.6 approximation is fine. For anything billable, surveying, or technical, use the exact value.

How do I convert running pace between min/mile and min/km?

This one trips people up because it's a rate, not a length. To convert min/mile to min/km, multiply by 0.621 (one km is 0.621 miles, so each km takes 0.621 of a min/mile pace). To convert min/km to min/mile, multiply by 1.609. A 9 min/mile pace is 9 × 0.621 = 5.59 min/km, or about 5 minutes 35 seconds per km.

What's the smallest length unit the converter handles?

Millimeters. For micrometers, nanometers, and angstroms — used in microscopy, electronics, and chemistry — a scientific calculator with full SI prefix support is a better fit. The Length Converter focuses on the everyday range where most people need conversions: from screw threads (a few mm) to road distances (a few hundred miles).