Unit Converter
Convert between different units of measurement.
Length
Mass
Volume
Temperature
Quick Examples
Common Conversions
| Type | Conversion |
|---|---|
| Length | 1 mile = 1.609 km |
| Length | 1 inch = 2.54 cm |
| Mass | 1 lb = 0.454 kg |
| Volume | 1 gallon = 3.785 L |
| Temperature | °C = (°F - 32) × 5/9 |
What Is a Unit Converter?
A unit converter changes how a quantity is expressed without changing the quantity itself — a length, a mass, or a duration is the same physical thing whether measured in inches or centimeters. One hard rule: you can only convert within the same kind of quantity. Length converts to length, never to weight.
The One Formula Behind Almost Every Conversion
Every category except temperature works the same way. Each unit stores its size in the category's base unit (meter for length, kilogram for mass, liter for volume, second for time, m/s for speed). Converting is two steps: go from the source unit to the base unit, then from the base unit to the target unit.
Example: 5 feet to inches. 1 ft = 0.3048 m, 1 in = 0.0254 m, so the factor is 0.3048 ÷ 0.0254 = 12, and 5 × 12 = 60 inches. Convert through the base unit and every pair of units in a category agrees with every other — no separate factor to memorize per pair.
Temperature: The Special Case
Temperature is the one category that is not a simple multiplication, because Celsius and Fahrenheit start from different zero points. Use formulas, not factors:
Watch out for temperature differences: a rise of 10 °C is a rise of 18 °F (only the × 9/5 part applies), not 50 °F. And −40 is the same on both scales — the classic round-trip check.
Categories at a Glance
| Category | Base Unit | Example |
|---|---|---|
| Length | meter | 1 in = 2.54 cm (exact) |
| Mass | kilogram | 1 lb = 0.453592 kg (exact) |
| Volume | liter | 1 US gal = 3.78541 L |
| Temperature | kelvin | formulas above — not a factor |
| Area | square meter | 1 m² = 10.7639 ft² (factor squared) |
| Speed | m/s | 1 m/s = 3.6 km/h |
| Time | second | 1 day = 86,400 s |
| Data | byte | 1 GB = 1,000 MB; 1 GiB = 1,024 MiB |
Step-by-Step: Converting 150 lb to kg
- Step 1 — Find the factor: 1 lb = 0.45359237 kg (exact, by the 1959 international agreement).
- Step 2 — Multiply: 150 × 0.45359237 = 68.0389.
- Step 3 — Round to the input's precision: 68.04 kg — or about 68 kg for luggage and body weight.
- Step 4 — Sanity check: a kilogram is about 2.2 lb, so 150 ÷ 2.2 ≈ 68. The shortcut agrees.
Worked Examples
Example 1: Shelf length in centimeters
A 30-inch shelf: 30 × 2.54 = 76.2 cm. Report 76 cm unless your tape measure justifies more precision.
Example 2: Body temperature
98.6 °F = (98.6 − 32) × 5/9 = 66.6 × 5/9 = 37 °C exactly.
Example 3: Road race distance
5 km = 5 × 0.621371 = 3.10686 miles — so a 5K race is about 3.11 miles.
Common Mistakes
- Converting area or volume with a length factor: 1 m = 3.28084 ft, but 1 m² = 10.7639 ft² — square the factor, or understate the area threefold.
- Mixing US and UK gallons, pints, and tons: a US gallon is 3.785 L, a UK gallon 4.546 L; a US ton is 2,000 lb, a UK ton 2,240 lb, a metric tonne 1,000 kg.
- Treating temperature differences as temperatures: adds a phantom 32 °F.
- Converting weight to volume: cups to grams needs the ingredient's density — no converter can do it without that.
- Reading 1.5 hours as 1 h 50 min: it is 1 h 30 min.
Exact vs Approximate Factors
Some factors are exact by international agreement — the inch is defined as exactly 2.54 cm and the pound as exactly 0.45359237 kg (1959 agreement) — so length and mass conversions never lose precision beyond your rounding. Others are measured, not defined: the calorie, the standard atmosphere, and any "average" time unit are experimental or conventional values that can be revised.
Practical rule: keep one more digit than you need through the calculation, then round the final answer to the input's precision. A 5-inch measurement (one significant figure) converts to about 13 cm — writing 12.70000 cm pretends to know more than the ruler did.
Quick Mental Checks
For everyday estimates, these one-line approximations get within a few percent — good enough to catch a wrong-unit error before it matters:
| Approximation | True Value | Error |
|---|---|---|
| 1 in ≈ 2.5 cm | 2.54 cm | ~2% |
| 1 ft ≈ 30 cm | 30.48 cm | ~2% |
| 1 m ≈ 3.3 ft | 3.2808 ft | <1% |
| 1 km ≈ 0.6 mi | 0.6214 mi | ~3% |
| 1 kg ≈ 2.2 lb | 2.2046 lb | <1% |
| 1 L ≈ 0.26 gal | 0.2642 gal | ~2% |
| 1 oz ≈ 28 g | 28.35 g | ~1% |
If your precise conversion lands far from the approximation, recheck which units you entered — most conversion disasters are wrong-unit entries, not wrong factors.
Derived Units: When Categories Combine
Some units are built from two categories at once. Speed is length ÷ time (m/s), area is length × length (m²), density is mass ÷ volume (kg/m³), and force is mass × acceleration (newtons). Converting a derived unit means converting each part: 1 g/cm³ = 1,000 kg/m³, because the gram-to-kilogram factor (1/1,000) meets the cm³-to-m³ factor (1/1,000,000) and the two combine to 1,000.
This is also why converting area and volume with a length factor fails — the factor must be squared or cubed along with the unit.
Frequently Asked Questions
How many kilometers are in a mile? Exactly 1.609344 km; one kilometer is 0.621371 miles.
How many pounds are in a kilogram? About 2.20462 lb; one pound is exactly 0.45359237 kg.
Is a US gallon the same as a UK gallon? No — 3.785 L vs 4.546 L. Pints and fluid ounces differ too.
Why can't I convert grams to milliliters here? Weight and volume are different quantities; converting between them needs the substance's density.
How precise should my answer be? Match the input: a measurement of "5 in" (one significant figure) converts to about 13 cm, not 12.70000 cm.