Unit conversion is one of those skills that seems trivial until it goes wrong. Recipes, travel, fitness tracking, DIY, shipping, and international shopping all require moving between metric and imperial, and the two systems have no elegant relationship to each other.
The stakes are occasionally higher than you would expect. NASA lost the Mars Climate Orbiter in 1999 because one team worked in pound-force seconds and another in newton seconds. Conversion errors are not merely academic.
This guide covers the conversions people actually need for length, weight, and temperature, with exact factors, working shortcuts, and the specific traps worth knowing about.
Key Takeaways
- 1 inch is exactly 2.54 cm – this is a definition, not an approximation.
- 1 kg is about 2.205 lb; 1 lb is about 0.454 kg.
- Temperature conversion needs an offset as well as a factor, unlike length and weight.
- US and imperial gallons are different sizes – a common and costly confusion.
- Always convert before calculating, and keep units written alongside your numbers.
Length Conversions
The foundation of the whole length system is a single defined value: 1 inch = 2.54 cm exactly. Every other imperial-to-metric length conversion derives from it.
| From | To | Multiply By |
|---|---|---|
| Inches | Centimetres | 2.54 |
| Centimetres | Inches | 0.3937 |
| Feet | Metres | 0.3048 |
| Metres | Feet | 3.2808 |
| Yards | Metres | 0.9144 |
| Miles | Kilometres | 1.609344 |
| Kilometres | Miles | 0.6214 |
Example: height of 5 feet 9 inches in centimetres
- Convert to inches: (5 x 12) + 9 = 69 inches
- Multiply: 69 x 2.54 = 175.26 cm
Example: a marathon in kilometres
- 26.2 miles x 1.609344 = 42.16 km
A Useful Mental Shortcut
To convert kilometres to miles roughly, multiply by 0.6 – or take the number, halve it, and add a tenth. 80 km becomes 40 + 8 = 48 miles, against a true value of 49.7. Close enough for reading road signs.
Weight and Mass Conversions
Strictly, kilograms measure mass and pounds measure force, but in everyday use they are treated as interchangeable measures of weight.
The stone is worth knowing if you deal with UK body weight, where it remains in common use. One stone is 14 pounds, so 11 stone 4 pounds is (11 x 14) + 4 = 158 lb, or about 71.7 kg.
For a quick mental estimate, doubling kilograms and adding 10% gets close: 70 kg becomes 140 + 14 = 154 lb, which happens to be almost exact.
| From | To | Multiply By |
|---|---|---|
| Kilograms | Pounds | 2.20462 |
| Pounds | Kilograms | 0.453592 |
| Grams | Ounces | 0.035274 |
| Ounces | Grams | 28.3495 |
| Stone | Kilograms | 6.35029 |
| Kilograms | Stone | 0.157473 |
| Metric tonnes | Pounds | 2204.62 |
Example: 70 kg in pounds
- 70 x 2.20462 = 154.32 lb
Example: 154 lb in kilograms
- 154 x 0.453592 = 69.85 kg
Temperature Conversions
Temperature is the odd one out. Length and weight scales share a zero point, so conversion is pure multiplication. Temperature scales do not, so an offset is required.
Celsius to Fahrenheit: F = (C x 9/5) + 32
Fahrenheit to Celsius: C = (F – 32) x 5/9
The order of operations matters. Going to Celsius, subtract 32 first and then multiply. Going to Fahrenheit, multiply first and then add 32. Reversing the sequence produces a wrong answer every time.
Kelvin is simpler: K = C + 273.15, with no multiplication because Kelvin and Celsius use the same degree size.
Example: 37 C in Fahrenheit
- (37 x 9/5) + 32
- = 66.6 + 32 = 98.6 F
Example: 100 F in Celsius
- (100 – 32) x 5/9
- = 68 x 5/9 = 37.78 C
A Curiosity Worth Knowing
-40 C and -40 F are the same temperature. It is the single point where the two scales cross, and it is a handy way to check that you have the formula the right way round.
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Reference Temperatures
For a rough sense of Fahrenheit if you think in Celsius, these anchor points help more than the formula:
A serviceable mental approximation: double the Celsius figure and add 30. For 20 C that gives 70 against a true 68; for 30 C it gives 90 against 86. Accurate enough for deciding what to wear, not for cooking.
| Celsius | Fahrenheit | Reference |
|---|---|---|
| -40 | -40 | The crossover point |
| 0 | 32 | Water freezes |
| 10 | 50 | Cool day |
| 20 | 68 | Room temperature |
| 30 | 86 | Hot day |
| 37 | 98.6 | Body temperature |
| 100 | 212 | Water boils |
| 180 | 356 | Moderate oven |
Volume – Where the Real Trap Is
Volume conversion contains a genuine hazard: the US gallon and the imperial gallon are different sizes, and both are simply called ‘gallon’.
An imperial gallon is about 20% larger than a US gallon. If you are converting fuel economy, following a recipe from another country, or ordering liquid by volume, establishing which gallon is meant is essential.
Note the fluid ounce reverses the pattern – the imperial fluid ounce is slightly smaller than the US one, even though the imperial pint and gallon are larger. This is because an imperial pint is 20 fluid ounces while a US pint is 16.
| Unit | In Litres |
|---|---|
| US gallon | 3.78541 |
| Imperial (UK) gallon | 4.54609 |
| US pint | 0.473176 |
| Imperial pint | 0.568261 |
| US fluid ounce | 0.0295735 |
| Imperial fluid ounce | 0.0284131 |
How to Avoid Conversion Errors
Most conversion mistakes come from a small set of habits. These prevent nearly all of them:
- Write the units next to every number. Carrying units through a calculation makes an error visible – if they do not cancel correctly, something is wrong.
- Convert first, calculate second. Doing arithmetic in mixed units and converting at the end is where errors hide.
- Sanity check the direction. Converting km to miles should give a smaller number; kg to lb should give a larger one. If the result moved the wrong way, you multiplied when you should have divided.
- Watch squared and cubed units. Area and volume conversion factors are the length factor squared or cubed, not the same factor.
- Confirm which gallon or which ton. Both have multiple definitions in active use.
- Keep reasonable precision. Rounding 2.54 to 2.5 introduces a 1.6% error, which compounds through a calculation.
The first habit is the most valuable. Treating units as part of the number rather than a label at the end catches the majority of errors before they matter.
Conclusion
Length and weight conversions are single multiplications built on defined factors – 2.54 cm to the inch, 0.453592 kg to the pound. Temperature is different because the scales have different zero points, so it needs an offset as well as a ratio, applied in the right order.
The errors that cause real problems are rarely arithmetic. They come from converting after calculating rather than before, from squared and cubed units, and from ambiguous names like ‘gallon’ and ‘ton’. Write your units down, check that the answer moved in the direction you expected, and most conversion problems disappear.
Try the Calculator
Convert instantly rather than remembering factors – enter a value and get the equivalent.
Frequently Asked Questions
How do I convert inches to centimetres?
Multiply by 2.54, which is an exact definition rather than an approximation. To go from centimetres to inches, divide by 2.54 or multiply by 0.3937. A height of 5 feet 9 inches is 69 inches, or 175.26 cm.
What is the formula for Celsius to Fahrenheit?
Multiply the Celsius figure by 9/5 and then add 32. So 37 C becomes (37 x 1.8) + 32 = 98.6 F. Going the other way, subtract 32 first and then multiply by 5/9. The order of operations matters.
How many pounds are in a kilogram?
One kilogram is about 2.20462 pounds. For a quick mental estimate, double the kilograms and add 10 percent – 70 kg becomes 140 plus 14, giving 154 lb, which is almost exactly right.
Are US and UK gallons the same?
No. A US gallon is 3.78541 litres while an imperial gallon is 4.54609 litres, making the imperial gallon about 20 percent larger. Pints differ too, since an imperial pint is 20 fluid ounces and a US pint is 16.
Why is -40 the same in Celsius and Fahrenheit?
Because the two scales converge at that point. The Fahrenheit scale rises 1.8 degrees for every 1 degree Celsius but starts 32 degrees higher at water’s freezing point, and those two differences cancel exactly at -40.