Converting units of measurement is an everyday skill: kilometres to miles when travelling, grams to ounces in a recipe, or Celsius to Fahrenheit when reading the weather in another country. The rules are few and by learning the key factors you can solve almost any conversion in seconds.
The metric system: everything is powers of 10
The advantage of the international system is that prefixes work the same for metres, grams and litres:
| Prefix | Symbol | Factor |
|---|---|---|
| Kilo | k | × 1,000 |
| Hecto | h | × 100 |
| Deca | da | × 10 |
| (base) | — | × 1 |
| Deci | d | ÷ 10 |
| Centi | c | ÷ 100 |
| Milli | m | ÷ 1,000 |
Trick: to go from one prefix to another, count the jumps. From km to m it is 3 jumps (k→base): multiply by 1,000. From g to mg also 3 jumps: multiply by 1,000.
Length: metres, kilometres and more
Essential equivalences:
- 1 km = 1,000 m
- 1 m = 100 cm = 1,000 mm
- 1 m ≈ 1.0936 yards
- 1 km ≈ 0.6214 miles
- 1 inch = 2.54 cm
Example: How many metres are 12 km? → 12 × 1,000 = 12,000 m.
Mass: grams and kilograms
- 1 kg = 1,000 g
- 1 g = 1,000 mg
- 1 tonne = 1,000 kg
- 1 kg ≈ 2.2046 pounds
- 1 ounce = 28.3495 g
- 1 pound = 453.592 g
Example: 3.5 kg in grams → 3.5 × 1,000 = 3,500 g.
Volume: litres and millilitres
- 1 L = 1,000 mL
- 1 L = 1 dm³ = 0.001 m³
- 1 mL = 1 cm³
- 1 US gallon = 3.785 L
- 1 L ≈ 0.2642 US gallons
Useful tip: 1 litre of water weighs exactly 1 kg. That is the equivalence that links volume and mass in the metric system.
Temperature: the exception
Temperatures are not converted with simple factors because their scales start at different points. Memorise these three formulas:
| Conversion | Formula |
|---|---|
| °F → °C | °C = (°F − 32) × 5/9 |
| °C → °F | °F = (°C × 9/5) + 32 |
| °C → K | K = °C + 273.15 |
| K → °C | °C = K − 273.15 |
Example: 68 °F in °C → (68 − 32) × 5/9 = 36 × 5/9 = 20 °C.
Common conversion mistakes
- Confusing mass and weight: in cooking, grams and millilitres are not the same except for water.
- Forgetting that temperature is not linear: 20 °C is not “twice” 10 °C on an absolute scale.
- Getting the number of zeros right: from km to cm is 5 jumps (×100,000), not 3.
- Rounding too early: round at the end of the calculation, not at every step.
Your online unit converter
Our unit conversion calculator lets you instantly convert between more than 20 units of length, mass, temperature and volume, including the special-formula cases (temperature). Just choose category, value and source/target units: it does the rest for you.
Conclusion
Converting units is easy when you understand metric prefixes and learn the few “imperial” factors used often. The only real trap is temperature: always choose the correct formula. Use the calculator to verify, so you never make a mistake in a recipe or a construction quote.