Height, Weight and BMI Across Unit Systems
Body measurements are among the most personal numbers we use, and they are recorded in a surprising variety of units. An American gives height in feet and inches and weight in pounds; a Briton often uses feet and inches for height but stones and pounds for weight; most of the world uses centimetres and kilograms; and doctors everywhere increasingly work in metric.
This guide shows how to convert each measurement accurately and how to calculate Body Mass Index (BMI) in either system without mixing units.
Height: feet and inches to centimetres
Heights in feet and inches have to be turned into a single number before converting. Multiply the feet by 12, add the inches, then multiply the total by 2.54 — the exact number of centimetres in an inch. For example, 5 ft 10 in is 70 inches, and 70 × 2.54 = 177.8 cm.
Going the other way, divide centimetres by 2.54 to get total inches, then divide by 12: the whole number is the feet and the remainder is the inches. 170 cm ÷ 2.54 = 66.9 inches, which is 5 feet and 6.9 inches — usually rounded to 5 ft 7 in.
| Feet & inches | Total inches | Centimetres |
|---|---|---|
| 5 ft 0 in | 60 | 152.4 cm |
| 5 ft 4 in | 64 | 162.6 cm |
| 5 ft 7 in | 67 | 170.2 cm |
| 5 ft 10 in | 70 | 177.8 cm |
| 6 ft 0 in | 72 | 182.9 cm |
| 6 ft 3 in | 75 | 190.5 cm |
Weight: pounds, stones and kilograms
One kilogram is about 2.2046 pounds, and one pound is exactly 0.45359237 kilograms. To convert pounds to kilograms, divide by 2.2046; to convert kilograms to pounds, multiply by 2.2046. A quick mental version: halve the pounds and subtract 10% of the result (180 lb → 90 → 81 kg; the exact answer is 81.6 kg).
In the UK and Ireland, body weight is often given in stones and pounds. A stone is 14 pounds. To turn “11 st 6 lb” into pounds, multiply 11 by 14 and add 6, giving 160 lb, which is about 72.6 kg. To go from kilograms to stones, convert to pounds first, then divide by 14: 70 kg is 154.3 lb, which is 11 stone and about 0.3 lb.
| Kilograms | Pounds | Stones & pounds |
|---|---|---|
| 50 kg | 110.2 lb | 7 st 12 lb |
| 60 kg | 132.3 lb | 9 st 6 lb |
| 70 kg | 154.3 lb | 11 st 0 lb |
| 80 kg | 176.4 lb | 12 st 8 lb |
| 90 kg | 198.4 lb | 14 st 2 lb |
| 100 kg | 220.5 lb | 15 st 10 lb |
Calculating BMI in metric
Body Mass Index is defined using metric units: weight in kilograms divided by the square of height in metres. Someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9.
The most common mistake is using centimetres instead of metres. Height must be in metres, so 175 cm becomes 1.75 before squaring.
Calculating BMI in imperial units
If you have weight in pounds and height in inches, you can calculate BMI directly by multiplying by a conversion constant of 703: BMI = 703 × pounds ÷ (inches × inches). The 703 factor absorbs the conversions from pounds to kilograms and inches to metres. For someone weighing 154 lb who is 69 inches tall: 703 × 154 ÷ 4,761 ≈ 22.7.
Never mix systems — dividing pounds by metres squared, for instance, gives a number more than twice the correct BMI.
What the BMI categories mean, and their limits
For adults, the World Health Organization uses these BMI ranges: below 18.5 is underweight, 18.5 to 24.9 is the healthy range, 25 to 29.9 is overweight, and 30 or above is classed as obesity. These are screening categories for populations, not a diagnosis for an individual.
BMI does not distinguish between muscle and fat, so muscular athletes can register as “overweight” while being very lean. It also does not show where fat is carried, which is why clinicians often add waist measurements. For children and teenagers, BMI is interpreted using age- and sex-specific percentile charts rather than the adult categories. Anyone with concerns about their weight should talk to a healthcare professional.
Babies and medicine: why units matter
Newborn weights are a familiar example of mixed units. In the US a baby is announced as “7 pounds 11 ounces”, while most of the world would say about 3.5 kg. To convert, multiply the kilograms by 2.2046 to get pounds; the decimal remainder multiplied by 16 gives the ounces.
In healthcare, accuracy matters even more. Many medicine doses, especially for children, are calculated in milligrams per kilogram of body weight. A weight recorded in pounds but treated as kilograms would more than double the dose, which is why patient-safety organisations encourage hospitals to record weight in kilograms only.