A unit converter changes a measurement from one unit to another, such as kilometres to miles, pounds to kilograms or Fahrenheit to Celsius. Formula: Non-temperature: result = amount × factor(from) / factor(to). Temperature: convert to C, then to target (F: C×1.8+32; K: C+273.15). This tool covers length, weight, area and temperature, and uses the exact metric definitions of US customary units: 1 inch is exactly 2.54 cm and 1 pound is exactly 0.45359237 kg.
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How to use the unit converter
- Choose a category: length, weight, temperature or area.
- Enter the amount.
- Choose the unit you are converting from and the unit you want.
- Read the result, shown to up to four decimal places.
What is the unit converter formula?
For length, weight and area, the unit converter multiplies by the size of the starting unit and divides by the size of the target unit: result = amount × factor(from) ÷ factor(to). The base units are the metre, the kilogram and the square metre. Temperature cannot be scaled this way because the scales have different zero points, so the tool converts through Celsius:
- °C = (°F − 32) ÷ 1.8
- °F = °C × 1.8 + 32
- K = °C + 273.15
Unit converter worked examples
- 100 km to miles: 100 × 1,000 ÷ 1,609.344 ≈ 62.1371 mi.
- 150 lb to kg: 150 × 0.45359237 ≈ 68.0389 kg.
- 100 °F to Celsius: (100 − 32) ÷ 1.8 ≈ 37.7778 °C.
- 2,000 sq ft to square metres: 2,000 × 0.09290304 ≈ 185.8061 m².
Exact conversion factors
Since the 1959 international yard and pound agreement, the main US customary units have exact metric definitions. NIST lists these values:
| Unit | Exact value |
|---|---|
| 1 inch | 0.0254 m (2.54 cm) |
| 1 foot | 0.3048 m |
| 1 yard | 0.9144 m |
| 1 mile | 1,609.344 m |
| 1 pound (avoirdupois) | 0.45359237 kg |
| 1 ounce (avoirdupois) | 0.028349523125 kg |
| 1 square foot | 0.09290304 m² |
| 1 acre (international) | 4,046.8564224 m² |
| 1 hectare | 10,000 m² |
The converter uses these exact values, so converting an amount and then converting the result back returns the original number, apart from rounding in the four-decimal display. That keeps repeated conversions consistent across a project.
Common temperatures
| Celsius | Fahrenheit | Kelvin |
|---|---|---|
| −40 °C | −40 °F | 233.15 K |
| 0 °C | 32 °F | 273.15 K |
| 20 °C | 68 °F | 293.15 K |
| 37 °C | 98.6 °F | 310.15 K |
| 100 °C | 212 °F | 373.15 K |
Celsius and Fahrenheit meet at −40. With Temperature selected, the amount box accepts negative values down to absolute zero (−273.15 °C, −459.67 °F or 0 K); lower values show a dash. Length, weight and area accept amounts of zero or more. You can also use the temperature converter.
Rounding converted values
A converted number often carries more digits than the original measurement deserves. If a sign says a trail is 5.0 miles long, the unit converter gives 8.04672 km, but the original was measured to the nearest tenth of a mile, so 8.0 km is the honest result. Keep the same number of significant figures as the least precise input.
Some rules of thumb:
- Keep full precision during a calculation and round only the final answer. Rounding at each step adds error.
- Exact factors, such as 2.54 cm per inch, do not limit precision. Only measured values do.
- For recipes, building and travel, two or three significant figures is plenty. For lab work, match the instrument.
Use the unit converter for the arithmetic, then decide how many digits the result can honestly claim. A unit converter does not know how carefully you measured, so that judgment stays with you. For example, a room measured as 12 ft by 10 ft converts to 3.66 m by 3.05 m, and 3.7 m by 3.0 m is fine for buying flooring.
Tips for accurate conversions
- Pick the category first, then both units; the unit lists change with the category.
- Convert in one step instead of chaining several conversions, and round only the final answer.
- Square units scale by the square of the length factor: 1 ft = 0.3048 m, so 1 sq ft = 0.3048² = 0.09290304 m².
- The US survey foot (1200/3937 m) was retired by NIST at the end of 2022; this tool uses the international foot.
For education and everyday use. Check critical engineering, medical or legal measurements against the standard your project requires.
Related converters and calculators
Related tools:
- Length converter: more length units
- Weight converter: kg, lb, oz
- Area converter: sq ft, acres
- Temperature converter: °C, °F, K
- Dimensional weight calculator: parcel billing
Sources
NIST Office of Weights and Measures: Unit conversion; NIST SP 811: Guide for the Use of the International System of Units; NIST: U.S. survey foot.
Questions about this calculator
How many kilometres are in a mile?
Exactly 1.609344 km.
How do I convert pounds to kilograms?
Multiply by 0.45359237. For example, 150 lb ≈ 68.04 kg.
How do I convert Fahrenheit to Celsius?
Subtract 32, then divide by 1.8. 100 °F ≈ 37.78 °C.
How many square feet are in a square metre?
About 10.7639, because 1 sq ft is exactly 0.09290304 m².
Can I convert currency with this tool?
No. Use the currency converter for exchange rates.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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