Five conversions, five different ways to be wrong
Unit conversion looks like the easiest thing on the internet, which is exactly why nobody double-checks it. Then a suitcase is 2 kg over the limit, a shelf does not fit a 30 cm gap, or a recipe comes out wrong because someone converted ounces of chocolate using a fluid-ounce figure.
These are the five that cause real mistakes, with the numbers you should actually use.
1. Kilograms and pounds: use 2.2046, not 2
One kilogram is 2.20462 pounds. The mental shortcut of "about 2" is off by 10%, which is harmless for a rough guess and ruinous for an airline baggage limit.
- 23 kg — the typical international checked-bag limit — is 50.7 lb. Round-down shortcuts make it look like 46 lb, and you arrive at the airport confident and wrong.
- Going the other way: 1 lb = 0.4536 kg. A 45 lb suitcase is 20.4 kg.
- UK body weight often comes in stones: 1 stone = 14 lb = 6.35 kg.
2. Centimetres and inches: 2.54, not 2.5
One inch is exactly 2.54 cm — this one is exact by definition, not an approximation. The popular "multiply by 2.5" shortcut drifts by about 1.6%: fine for a rough idea of a screen size, wrong for furniture.
- 30 cm = 11.81 in. If you assumed 12 in, you are 5 mm optimistic about a gap.
- 80 cm = 31.5 in, the number that decides whether a 32-inch TV fits a shelf.
- 1 m = 39.37 in, not 40 — which is why "a metre is about a yard" is close but a metre of fabric is 3.4 inches longer than a yard.
3. Feet and inches are not a decimal
This is the single most common error. Someone 5 feet 11 inches tall is not 5.11 feet. The inches are a separate base-12 sub-unit:
- 5'11" = (5 × 12) + 11 = 71 inches = 180.3 cm.
- 5.11 feet would be 61.3 inches — about 155 cm, a full 25 cm short.
- Useful anchors: 6'0" = 182.9 cm, 5'6" = 167.6 cm, 5'9" = 175.3 cm.
If you only have centimetres and need feet and inches, divide by 2.54 to get total inches, then split: whole part ÷ 12 for feet, remainder for inches. 175 cm → 68.9 in → 5'9".
4. The ounce trap: weight and volume share a name
An ounce of weight and a fluid ounce of volume are unrelated units that happen to be called the same thing, and recipe sites mix them constantly.
- 1 oz (weight) = 28.35 g. That is what "8 oz of chocolate" means.
- 1 US fl oz = 29.57 ml of liquid volume. That is what "8 fl oz of milk" means.
- They are close enough in number (28 vs 30) to look interchangeable and completely different in meaning. You cannot weigh milk in ounces and get the same number as its fluid ounces — water is the one case where 1 fl oz ≈ 1 oz because its density is 1.
- UK and US fluid ounces also differ: 1 UK fl oz = 28.41 ml, about 4% smaller than the US one. Recipe sites rarely say which they mean.
5. Cups are volume, so they depend on the ingredient
A cup is not a weight. One US cup is 236.6 ml, and one metric cup (used in Australia, Canada, and much of Europe) is 250 ml — a 5% gap before you even get to the ingredient. A cup of flour is about 120 g, a cup of sugar about 200 g, a cup of water 237 g. This is why converting a recipe by volume to weight needs a per-ingredient table, not a single factor.
What this converter does, and what it does not
Unit Converter handles two categories: length and weight. Pick the category, enter the value, choose the source and target unit, and read the result — all in the browser, with nothing sent anywhere.
It deliberately does not try to be everything. Being clear about the edges saves you from trusting it where it cannot help:
- No temperature. Celsius and Fahrenheit are offset scales, not simple multiples, so they need their own formula: °F = (°C × 9/5) + 32, and °C = (°F − 32) × 5/9.
- No volume, area, or speed. Use the tables above for fluid ounces, or multiply miles by 1.609 for kilometres.
- Results use standard international factors, so they are as precise as the definition allows; extremely large or tiny values follow ordinary floating-point behaviour, which is why 0.1 + 0.2 problems show up in the last digit rather than the first.
How to use it
- Choose length or weight.
- Enter the value you have.
- Pick the source unit and the unit you want.
- Read the result — it updates as you type, so you can compare a few candidates without re-entering anything.
FAQ
Is 1 kg exactly 2.2 lb? No, it is 2.20462 lb. The 2.2 figure is a rounded convenience — the error is 0.2%, small but visible on a 23 kg baggage limit.
Why does the tool not convert temperature? Because temperature is an affine scale, not a proportional one: 0 °C is not 0 °F, so a single multiplier cannot work. The formula is in the section above.
Can I convert pounds to kilograms and back without losing precision? Within normal rounding, yes. Repeating a conversion a dozen times will accumulate small errors, so convert once from the original value rather than chaining results.
Is my input stored? No. The calculation runs in your browser tab; there is no server receiving the value or the units.