How to Convert Units: Length, Weight and Temperature Made Simple
Unit conversion is one of those skills that quietly matters every day — following a recipe, reading a foreign weather forecast, buying fabric, or checking a measurement on a product. The underlying idea is always the same: multiply by a conversion factor that relates the two units.
The core idea
Every conversion is a multiplication by a ratio that equals one. Since 1 inch = 2.54 cm, to convert inches to centimetres you multiply by 2.54; to go back, you divide by 2.54. Once you have the right factor, every conversion is the same operation.
Handy length conversions
- 1 inch = 2.54 cm
- 1 foot = 30.48 cm = 0.3048 m
- 1 mile = 1.609 km
- 1 metre = 3.281 feet
Handy weight conversions
- 1 kilogram = 2.205 pounds
- 1 pound = 0.4536 kg = 453.6 g
- 1 ounce = 28.35 g
- 1 stone = 6.35 kg
Temperature is different
Most conversions are simple multiplications, but temperature involves a shift as well as a scale because the zero points differ. To convert Celsius to Fahrenheit: F = C × 9/5 + 32. To go back: C = (F − 32) × 5/9. So 100°C is 212°F (boiling water) and 0°C is 32°F (freezing). Forgetting the +32 step is the most common temperature mistake.
Avoiding errors
- Always check the direction — multiplying when you should divide flips the answer.
- Watch the units in word problems; mixing metric and imperial mid-calculation causes errors.
- For temperature, never forget the offset, not just the ratio.
- When precision matters, use a calculator rather than rounding factors in your head.
The trick that stops you converting the wrong way
The most common blunder isn't using the wrong factor — it's using the right factor backwards. Here's a simple sanity check: think about which unit is bigger. A centimetre is smaller than an inch, so a length in centimetres should give a smaller number once it's in inches (25 cm is only about 10 inches). If your answer goes the wrong direction — bigger when it should be smaller — you've multiplied where you should have divided. That five-second 'does this number even make sense?' check catches almost every slip before it matters.
Why recipes are a conversion minefield
Cooking conversions deserve extra care because volume and weight aren't interchangeable. A cup of flour and a cup of water weigh completely different amounts, so you can't just swap grams for millilitres — it depends on the ingredient. On top of that, US, UK and metric cups are all slightly different sizes. When a recipe really matters, baking especially, weighing in grams is far more reliable than measuring by volume — which is exactly why serious bakers keep a kitchen scale instead of a cup set.
Frequently asked questions
Why is temperature different from other conversions?
Because the scales don't share a zero point. Most units start at the same zero, so you simply scale them. Celsius and Fahrenheit begin in different places, so you scale and shift — that's the '+32' step everyone forgets.
How do I convert feet and inches to centimetres?
Turn the feet into inches first (multiply by 12), add the spare inches, then multiply the total by 2.54. So 5 ft 9 in is 69 inches, which is 69 × 2.54 ≈ 175 cm.
Is a metric tonne the same as a US ton?
No, and it catches people out. A metric tonne is 1,000 kg (about 2,205 lb), while a US 'short ton' is 2,000 lb (about 907 kg). Always check which 'ton' someone means.
The bottom line
Conversion is just multiplying by the right factor — except temperature, which also shifts. Keep a few common factors in mind and use our converters when accuracy counts, and you will never be caught out by unfamiliar units again.