Fill in a US medical form, follow a gym program written in pounds, or squint at the back of a baby photo from 1968, and you run into the same two conversions: height between centimetres and feet-and-inches, and body weight between kilograms, stone and pounds. Both are simple once the working is laid out, and both hide a trap that catches people every single day. This page shows the arithmetic honestly so you can check any figure yourself.
Why feet and inches is an awkward unit
A height in centimetres is one number. A height in feet and inches is two numbers pretending to be one: whole feet, then a remainder in inches, with twelve inches to the foot. Mathematicians call this a mixed-radix system, and it has the same structure as hours and minutes. You cannot convert into it with a single multiplication, and that is exactly where errors creep in.
The correct method takes two steps. First, convert the centimetres to total inches by dividing by 2.54. That constant is exact, not an approximation: the inch has been defined as precisely 2.54 cm since 1959. Second, divide the total inches by 12 to get whole feet, and keep whatever is left over as inches. Never detour through decimal feet.
Worked example one: 170 cm. Divide by 2.54 and you get 66.93 total inches. Twelve goes into that five times, using up 60 inches and leaving 6.93, so 170 cm is 5 ft 6.9 in. Round the remainder and you have the familiar 5 ft 7 in.
Worked example two: 180 cm. Divide by 2.54 to get 70.87 total inches. Five whole feet account for 60 of them, leaving 10.87, so 180 cm is 5 ft 10.9 in. Call it 5 ft 11 in on a form.
The rounding mistake that shifts everyone an inch
The classic error is converting straight to decimal feet and misreading the decimal. Take 180 cm again. In decimal feet it is 180 ÷ 30.48 = 5.91 ft. Plenty of people glance at 5.91 and write down 5 ft 9 in, which is nearly two inches short of the truth. Others round the whole thing up to 6 ft, quietly gifting themselves 2.9 cm. The decimal part of a feet figure is not inches. It is tenths of a foot, and each tenth is worth 1.2 inches. Multiply the decimal part by 12 (0.91 × 12 = 10.9) and the correct 5 ft 10.9 in appears.
The chart below applies the two-step method across the common adult range. Total inches are rounded to two decimal places and the inch remainder to one, always computed from the unrounded total, which is why a value can look a whisker different from mental arithmetic done on already-rounded figures.
| Centimetres | Total inches | Feet & inches |
|---|---|---|
| 150 cm | 59.06 in | 4 ft 11.1 in |
| 155 cm | 61.02 in | 5 ft 1.0 in |
| 160 cm | 62.99 in | 5 ft 3.0 in |
| 165 cm | 64.96 in | 5 ft 5.0 in |
| 170 cm | 66.93 in | 5 ft 6.9 in |
| 175 cm | 68.90 in | 5 ft 8.9 in |
| 180 cm | 70.87 in | 5 ft 10.9 in |
| 185 cm | 72.83 in | 6 ft 0.8 in |
| 190 cm | 74.80 in | 6 ft 2.8 in |
| 195 cm | 76.77 in | 6 ft 4.8 in |
Kilograms, pounds and stone
Body weight starts out easier because the first hop is one honest multiplication. The pound is defined as exactly 0.45359237 kg, so kilograms become pounds when you divide by that constant, or multiply by its reciprocal 2.20462 if that sits better in your head. No remainders yet.
Then the stone arrives and brings the mixed-radix problem straight back. A stone is 14 pounds, and it is still the everyday unit for body weight across the UK and Ireland. Older Australians who grew up before metrication in the 1970s reach for it too. Converting works exactly like feet and inches: get total pounds first, then divide by 14 and keep the remainder.
Worked example: 85 kg. Divide by 0.45359237 to get 187.39 lb. Fourteen goes into that thirteen times, using up 182 lb and leaving 5.39, so 85 kg is 13 st 5.4 lb. Round the pounds before dividing by 14 and the answer can drift by half a pound, so save the rounding for the very end. Going back the other way is a single line: (13 × 14 + 5.4) × 0.45359237 = 85.0 kg.
When precision matters, and when it honestly does not
Medication dosing is the blunt case. Paediatric doses and plenty of adult ones are calculated per kilogram of body weight, and a rough conversion has no place anywhere near them. Never estimate here. If a weight was recorded in pounds or stone, convert it with the exact factor, keep a decimal place, and have the prescriber confirm the figure. A converter can do the arithmetic; the checking belongs to a clinician.
Gym plates sit at the opposite end. A program built around a "45 lb plate" is describing a 20.4 kg plate, and the 20 kg plate in an Australian gym weighs 44.1 lb. For training purposes those are the same lump of iron. Rounding your bodyweight to the nearest kilogram, or your barbell total to the nearest plate, costs a training log nothing. Match the precision to the decision.
Travel paperwork lands in between. Foreign licence and visa forms want the nearest whole inch and pound, so write 5 ft 7 in and move on rather than agonising over the 0.9.
Growth charts, birth weights and BMI
Old family records are half the reason this page exists. Australian and British birth weights from before the 1970s were written in pounds and ounces, with 16 ounces to the pound: yet another mixed radix. A 3.5 kg newborn works out to 7.72 lb, and the leftover 0.72 lb × 16 gives 11.5 oz, so the hospital card would read 7 lb 11.5 oz. Modern growth charts in Australia and the UK are fully metric, so a quick conversion is the only bridge between your nan's handwriting and today's clinic printout.
BMI is metric by definition: weight in kilograms divided by height in metres, squared. For 85 kg at 1.80 m the sum is 85 ÷ (1.80 × 1.80) = 26.2. If your figures live in stone and feet, convert both to metric first; the imperial variant with its 703 correction factor exists only to imitate the metric formula. And that is where this page stops. What any BMI number means for your health is a conversation for your GP, not for a unit converter.
When you want the machine to do the long division, the cm to inches converter handles height in both directions, the universal tool has a preset for kilograms to stone, and the FAQ covers the definitions behind every factor used above.