Last updated: April 2026 · Reviewed by Calculatormatics Editorial Team

Length Converter

A free length converter covering ten units across both the metric and imperial systems: millimeter, centimeter, meter, kilometer, inch, foot, yard, mile, nautical mile, and light year. Enter a value in any unit and the calculator instantly shows the equivalent in every other. All conversions use exact factors — 1 inch = 2.54 cm exactly, 1 foot = 0.3048 m exactly, 1 mile = 1.609344 km exactly — so results are precise to as many decimal places as needed, not approximations. The metric units share a base-10 structure that makes mental conversion trivial (move the decimal point), while imperial units use less regular ratios (12 inches = 1 foot, 3 feet = 1 yard, 1,760 yards = 1 mile). The calculator works by converting your input to meters first (the SI base), then converting to the target unit, which guarantees consistent results across any unit pair. Useful for height conversions (5'10" → 178 cm), running and cycling distances (10K → 6.21 miles), construction measurements, sewing and tailoring, and aviation/marine navigation where nautical miles still rule. Enter a value below.

How to Convert Length Units

Length conversion is the process of expressing the same physical distance using a different unit of measurement. There are two major systems in everyday use: the metric system (SI) and the imperial/US customary system. Metric units use powers of 10, making conversion a matter of moving a decimal point. Imperial units use less regular ratios and require explicit multiplication or division factors.

All conversions in this tool work by first converting your input value to meters (the SI base unit for length), then converting from meters to the target unit. This two-step "base unit" method guarantees consistent results regardless of which pair of units you select.

Length Conversion Formulas

The formula structure is the same for every pair of units:

Result = Input × (From factor / To factor)

where each factor represents how many meters equal one of that unit.

Common Length Conversions

The most frequent conversions involve the cm-to-inch relationship (human height, screen sizes, clothing) and the km-to-mile relationship (road distances, speed limits). Here are the key reference values to memorize:

Metric vs. Imperial: Which System to Use?

The metric system is used by virtually every country in the world for science, engineering, and most everyday measurements. The United States is the main holdout for everyday use, still using miles, feet, and inches for most consumer contexts. The UK uses a hybrid: kilometers for EU-facing contexts, miles for road distances, and centimeters/meters for most other measurements.

For scientific work, always use SI units. For construction in the US, feet and inches remain standard. For international trade and engineering, metric is required by ISO standards. If you need to calculate room area from length measurements, our Square Footage Calculator handles multiple rooms at once. For geometric area computations like circles and triangles, see the Area Calculator.

Worked Example: Convert 100 cm to Inches

Using the formula Result = Input × (From factor / To factor) with the meter-based factor table above:

Step 1: Identify factors
  cm factor = 0.01 m per cm
  in factor = 0.0254 m per inch

Step 2: Apply the conversion formula
  Result = 100 × (0.01 / 0.0254)
         = 100 × 0.3937
         = 39.37 inches

So 100 cm = 39.37 inches. The same two-step pattern works for any length pair — convert to meters, then from meters to the target unit.

Frequently Asked Questions