Length, weight, and capacity are the three families of everyday measurement: length measures distance, weight measures mass, and capacity measures volume. Each converts by multiplying by a single fixed factor — 1 inch = 2.54 cm, 1 lb = 0.45359237 kg, 1 US gallon = 3.785411784 L, all exact by definition. The traps are unit variants, not arithmetic.
That last sentence is the part most guides skip. Once you have the right factor, converting is one multiplication. Nearly every wrong answer comes from grabbing a factor that belongs to a different country's version of the same-sounding unit — or from mixing the three families together, which is the one thing a factor cannot fix.
What is the difference between length, weight, and capacity?
They answer three different questions about an object, and their units never interchange.
- Length answers how far or how long — measured in millimetres, centimetres, metres, and kilometres, or in inches, feet, yards, and miles.
- Weight (strictly, mass in everyday use) answers how heavy — grams, kilograms, and tonnes, or ounces, pounds, and stones.
- Capacity answers how much fits inside — millilitres and litres, or fluid ounces, cups, pints, and gallons.
You can convert freely within a family. You cannot convert between families without extra information: turning litres into kilograms requires the substance's density, and turning centimetres into litres requires all three dimensions of a shape. That boundary is the single most useful thing to hold on to, and it is the root of the classic cups-to-grams confusion covered in our guide to how unit conversions really work.
Which conversion factors should you actually memorise?
These are the ones that cover most everyday conversions. The "exact?" column matters: an exact factor is a legal definition and never changes, while a rounded one is that definition flipped over and trimmed.
| Family | From | To | Multiply by | Exact? |
|---|---|---|---|---|
| Length | inches | cm | 2.54 | exact by definition |
| Length | feet | cm | 30.48 | exact by definition |
| Length | yards | m | 0.9144 | exact by definition |
| Length | miles | km | 1.609344 | exact by definition |
| Length | m | ft | 3.28084 | rounded |
| Length | km | miles | 0.621371 | rounded |
| Weight | ounces (oz) | g | 28.349523125 | exact by definition |
| Weight | pounds (lb) | kg | 0.45359237 | exact by definition |
| Weight | stone | kg | 6.35029318 | exact (1 stone = 14 lb) |
| Weight | kg | lb | 2.20462 | rounded |
| Capacity | US fluid ounces | mL | 29.5735295625 | exact by definition |
| Capacity | imperial fluid ounces | mL | 28.4130625 | exact by definition |
| Capacity | US cups | mL | 236.5882365 | exact by definition |
| Capacity | US gallons | L | 3.785411784 | exact by definition |
| Capacity | imperial gallons | L | 4.54609 | exact by definition |
| Capacity | L | US gallons | 0.264172 | rounded |
How do you convert length step by step?
Pick the factor, multiply once, round at the very end.
Example 1 — a height in feet to centimetres. How tall is 6 ft in cm?
- 6 × 30.48 = 182.88 cm exactly (that is 1.8288 m — no rounding needed at all)
Example 2 — a marathon in kilometres. A marathon is 26.2 miles (rounded from 26.219):
- 26.2 × 1.609344 = 42.1648128
- Rounded to two decimal places: ≈ 42.16 km
Example 3 — metres back to feet and inches. How long is 1 m in feet?
- 1 ÷ 0.3048 = 3.2808399 ft
- The whole part is 3 ft; the remainder 0.2808399 ft × 12 = 3.37 in
- So 1 m ≈ 3 ft 3.37 in, rounded to two decimal places
Notice example 3 divides rather than multiplies. Converting the opposite way always means dividing by the factor — which is where rounded factors sneak in.
How do you convert weight step by step?
Identical mechanics, different constants.
Example 4 — pounds to kilograms. A 165 lb bag:
- 165 × 0.45359237 = 74.84274105
- ≈ 74.84 kg, rounded to two decimal places
Example 5 — kilograms to pounds. A 70 kg person:
- 70 ÷ 0.45359237 = 154.3235835
- ≈ 154.32 lb, rounded to two decimal places
Example 6 — grams to ounces. A 500 g pack:
- 500 ÷ 28.349523125 = 17.6369810
- ≈ 17.64 oz, rounded to two decimal places
One caution unique to weight: "ton" is three different things. A US short ton is 2,000 lb ≈ 907.185 kg, a UK long ton is 2,240 lb ≈ 1,016.047 kg, and a metric tonne is exactly 1,000 kg. Always name which one before converting.
How do you convert capacity without hitting the US vs imperial trap?
Capacity is where the same word most often means two sizes. A US gallon is 3.785411784 L; an imperial gallon is 4.54609 L — a gap of about 20%. Fluid ounces differ too, and confusingly in the opposite direction: a US fluid ounce (29.5735 mL) is slightly larger than an imperial one (28.4131 mL), because an imperial gallon contains 160 fl oz while a US gallon contains only 128.
Example 7 — US gallons to litres. Filling 2.5 US gal:
- 2.5 × 3.785411784 = 9.46352946
- ≈ 9.46 L, rounded to two decimal places
Example 8 — millilitres to US cups. A 750 mL bottle:
- 750 ÷ 236.5882365 = 3.1700646
- ≈ 3.17 US cups, rounded to two decimal places
Example 9 — litres to US gallons. A 5 L container:
- 5 ÷ 3.785411784 = 1.3208603
- ≈ 1.32 US gal, rounded to two decimal places
Cups carry their own variant problem: a US customary cup is 236.5882365 mL, the US "legal" cup on nutrition labels is 240 mL, and a metric cup is 250 mL. A recipe that just says "1 cup" is ambiguous until you know its origin.
What is the classic mistake when mixing the three families?
Treating a capacity unit as a weight unit. The old saying "a pint's a pound the world around" is a rough rule for water only, and it is not even true in both systems:
- A US pint is 473.176473 mL. Water at room temperature is close to 1 g per mL, so a US pint of water weighs roughly 473 g ≈ 1.04 lb — near enough to a pound.
- An imperial pint is 568.26125 mL, so an imperial pint of water weighs roughly 568 g ≈ 1.25 lb. The British version of the saying is "a pint of water weighs a pound and a quarter" — and that one checks out.
Change the substance and both versions collapse. A pint of honey is far heavier than a pint of oil, because you are no longer converting units at all — you are applying a density. Whenever a "conversion" crosses from capacity to weight, ask what substance it assumes. More errors of this kind, and how to catch them, are collected in our measurement mistakes guide.
FAQ
Is capacity the same as volume?
Effectively yes in everyday use, with a shade of difference. Volume is how much space something occupies; capacity is how much a container can hold. Both are measured in the same units — millilitres, litres, fluid ounces, gallons — so the conversion factors are identical either way.
Why is 1 inch exactly 2.54 cm rather than approximately?
Because it is a definition, not a measurement. The 1959 international agreement fixed the yard at exactly 0.9144 m, which makes 1 ft exactly 30.48 cm and 1 in exactly 2.54 cm. The same agreement fixed the pound at exactly 0.45359237 kg. Those digits never drift.
Is a US fluid ounce bigger or smaller than an imperial one?
Bigger: 29.5735295625 mL versus 28.4130625 mL, about 4.1% larger. It surprises people because the US gallon is 20% smaller than the imperial gallon — the difference is that an imperial gallon is divided into 160 fluid ounces while a US gallon is divided into 128.
Can I convert litres to kilograms directly?
Only for water, and only approximately. Water is about 1 kg per litre, so 5 L of water is roughly 5 kg. For anything else you need that substance's density: 1 L of cooking oil is around 0.92 kg, 1 L of honey around 1.4 kg. There is no universal litres-to-kilograms factor.
How many decimal places should I keep?
Carry every digit through the working and round only the final answer — two decimal places suits most everyday uses. Rounding an intermediate step corrupts everything after it, especially if you then convert a second time. The same discipline applies to percentages and other daily maths in our everyday calculations guide.
Convert all three in one click — with the factor shown
Knowing which factor applies is the part that protects you from a wrong answer; typing the multiplication a dozen times a day is just labour. The free length, weight, and volume converters on Wapuula Tools handle all three families — including the US and imperial variants that trip people up — and every result displays the exact factor and formula behind it, so you can verify the working rather than trust a black box. Try the free unit converters on Wapuula Tools and check the maths on the page.
