Length Unit Converter
A length unit converter translates measurements between metric and imperial units, millimeters, centimeters, meters, kilometers, inches, feet, yards, and miles. For example, 10 mm = 0.3937 inches (divide millimeters by 25.4).
Pick any two length units, see the exact conversion factor and every step of the calculation.
How to convert length units
Enter a value, choose the from and to units, and press Calculate. The converter routes through meters, the SI base unit, showing the factor at each leg, so you see why 10 mm is 0.3937 inches instead of taking it on faith. Eight units are supported: millimeters, centimeters, meters, kilometers, inches, feet, yards, and miles.
The classic use case: 10 mm to inches (0.3937 in, just over 3/8″), the conversion woodworkers and 3D-printer owners look up constantly.
The converter handles everyday values and extreme ones alike. Every result is computed locally in your browser: nothing you type leaves your device.
Unit conversion factors (mm to inches and more)
Every conversion is two multiplications through a common unit. To go from unit A to unit B: value × (meters per A) ÷ (meters per B). The meter values are exact definitions, an inch is defined as 2.54 cm, a foot as 0.3048 m, a mile as 1609.344 m, so the only rounding is in the displayed answer, never in the method.
The step panel shows the combined factor too (“1 mm = 0.03937 in”), which is what you would memorize for repeated conversions.
Pro tips
Memorize five anchors: 25.4 mm/in, 2.54 cm/in, 0.3048 m/ft, 1.609 km/mi, 0.9144 m/yd. From these five you can derive any length conversion. For area: square the length factor (1 m² = 10.764 ft²). For volume: cube it (1 m³ = 35.315 ft³).
Worked example
Example: 10 mm to inches
Step 1, To meters: 10 mm × 0.001 = 0.01 m.
Step 2, To inches: 0.01 m ÷ 0.0254 = 0.3937 in.
Combined factor: 1 mm = 0.03937 in, so any millimeter value × 0.03937 gives inches directly.
Check: 25.4 mm = 1 inch exactly, and 10 × 0.03937 = 0.3937. ✓
More examples: 5 km → miles = 3.1069 mi; 6 ft → cm = 182.88 cm; 100 yd → m = 91.44 m.
Common mistakes
- Eyeballing mm→inches as “about 25”: it is 25.4 mm per inch, using 25 introduces a 1.6% error that compounds over long measurements.
- Converting area or volume with length factors: square the factor for area (1 ft² = 144 in², not 12) and cube it for volume.
- Rounding mid-calculation: keep full precision through both steps and round only the final answer, the calculator does this automatically.
- US survey foot vs international foot: for everyday work they are identical to 7 decimal places; only surveyors need care.
When to use it
Construction and DIY (metric plans, imperial tools), 3D printing and CNC (mm to inches), running and cycling (km to miles), science classes, and travel. For angles rather than lengths, use the angle converter (radians ↔ degrees).
Full walkthrough: a metric bookshelf plan
The plan says 1800 mm tall, 900 mm wide. Convert: 1800 mm = 1.8 m = 1.8 div 0.0254 = 70.87 in. Width: 900 mm = 35.43 in. The step panel shows the meter intermediate and the combined factor.
Key takeaway: every length conversion is multiply-then-divide through meters. Learn the five anchor factors and you will never look one up again.
Frequently asked questions
0.3937 inches, just over 3/8 of an inch. The exact factor is 1 mm = 0.0393701 in (25.4 mm per inch by definition). Enter 10 above with mm → inches to see the steps.
Divide millimeters by 25.4, since one inch is defined as exactly 25.4 mm. So inches = mm ÷ 25.4.
Multiply miles by 1.609344. One mile is defined as exactly 1609.344 meters. The converter shows both steps via meters.
The meter is the SI base unit, and every other unit has an exact defined value in meters. Routing through it means one consistent method instead of memorizing dozens of pairwise factors.
Exactly 2.54 cm, by international definition since 1959. So inches = cm ÷ 2.54, and cm = inches × 2.54.
5,280 feet exactly. A mile is 1609.344 meters and a foot is 0.3048 meters; 1609.344 ÷ 0.3048 = 5280.