How to Calculate BMI
The exact formula used by the WHO, the NHS, and the CDC — in metric, imperial, and Excel. Three steps, two worked examples, and a free calculator at the top of the page.
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The BMI formula
The official WHO/NHS Body Mass Index formula is the same one Adolphe Quetelet published in 1832 — your weight divided by the square of your height.
BMI = weight (kg) ÷ height (m)²
For imperial units the formula is mathematically identical — it just adds a 703 multiplier to convert pounds-and-inches into the same scale as kilograms-and-metres:
BMI = (weight in lbs ÷ height in inches²) × 703
Why is height squared? Because healthy adult weight scales roughly with height-squared, not height itself. Squaring it makes BMI roughly height-independent — a 1.5 m and a 2.0 m adult with the same BMI have similar weight-for-height proportions.
Three steps to work out BMI by hand
Note your weight in kg and height in metres
Use a recent weight (morning, after the bathroom, before breakfast for best consistency). For height, use a tape measure against a wall — even 2 cm matters because BMI uses height squared.
Square your height
Multiply your height in metres by itself. A 1.70 m adult: 1.70 × 1.70 = 2.89. (Imperial: square your height in inches. 5 ft 7 in = 67 inches, 67 × 67 = 4,489.)
Divide weight by height-squared
In metric: 70 kg ÷ 2.89 = BMI 24.2. In imperial: (147 lbs ÷ 4,489) × 703 = BMI 23.0. Compare against the WHO/NHS categories — 18.5 to 24.9 is the healthy range.
Worked examples
Example 1 — Adult, 70 kg, 170 cm
Convert height to metres: 170 cm = 1.70 m. Square it: 1.70 × 1.70 = 2.89. Divide weight by squared height: 70 ÷ 2.89 = BMI 24.2. That falls in the WHO/NHS healthy range of 18.5–24.9.
Example 2 — Adult, 154 lbs, 5 ft 7 in
Convert height to inches: 5 ft 7 in = 67 in. Square it: 67 × 67 = 4,489. Apply the imperial formula: (154 ÷ 4,489) × 703 = BMI 24.1 — practically identical to Example 1 (the small difference is rounding).
Example 3 — UK adult, 11 stone, 5 ft 7 in
11 stone = 154 lbs (1 stone = 14 lbs). Use the imperial formula above: BMI 24.1. You can also do it in metric: 11 stone = 69.85 kg, 5 ft 7 in = 1.702 m, 69.85 ÷ 2.897 = BMI 24.1.
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Calculate BMI in Excel or Google Sheets
If you’d rather use a spreadsheet — track your BMI over time, calculate it for a list of people, or build a clinical tool — here are the formulas.
Metric (kg + m)
=A1/(B1^2)Where A1 = weight in kg, B1 = height in metres.
Imperial (lbs + inches)
=A1/(B1^2)*703Where A1 = weight in lbs, B1 = height in inches.
With auto-classification (assumes BMI in C1)
=IF(C1<18.5,"Underweight",IF(C1<25,"Normal",IF(C1<30,"Overweight","Obese")))Edge cases and special groups
Children and teens (2–19)
Calculate BMI the same way, but interpret it against CDC age-and-sex percentile charts — not the adult 18.5–24.9 cut-offs.
Child BMI calculatorPregnant women
BMI during pregnancy is misleading. Use your pre-pregnancy weight and the IOM 2009 / NICE PH27 weight-gain ranges instead.
Pregnancy BMI calculatorSouth Asian, Chinese, Black & Middle Eastern adults
NICE PH53 and WHO Asia-Pacific recommend lower cut-offs: overweight starts at BMI 23, obese at 27.5.
NHS BMI with ethnicity bandsMuscular athletes
BMI cannot distinguish muscle from fat. A rugby player at BMI 28 with 12% body fat is metabolically very different from a sedentary adult at the same BMI.
BMI for athletes & bodybuildersAdults over 65
A slightly higher healthy range (23–27) may be more protective in older age. Frailty and bone-density loss outrank obesity risks past 70.
Healthy weight ranges by ageReverse calculation
If you have a target BMI and want the goal weight, use our reverse calculator: target weight (kg) = target BMI × height (m)².
Reverse BMI calculatorHow to calculate BMI — common questions
Quick answers to the most common questions
BMI is calculated by dividing weight in kilograms by height in metres squared: BMI = kg ÷ m². For imperial units use: BMI = (lbs ÷ in²) × 703. The result classifies you against the WHO/NHS healthy range of 18.5–24.9.
The official WHO BMI formula is BMI = weight (kg) / height (m)². The same formula is used by the NHS, the CDC, and almost every clinical guideline worldwide. The imperial version is mathematically identical — it just adds a 703 multiplier to convert pounds-and-inches into the same kg-and-metres scale.
Three steps: (1) square your height in metres (e.g. 1.70 × 1.70 = 2.89). (2) divide your weight in kilograms by that number (e.g. 70 ÷ 2.89 = 24.2). (3) compare the result to the WHO/NHS categories — under 18.5 is underweight, 18.5–24.9 is healthy, 25–29.9 is overweight, and 30+ is obese.
Convert your weight to kilograms (1 stone = 6.35 kg, so 11 stone = 69.85 kg) and your height to metres (1 foot = 0.3048 m, so 5 ft 7 in = 1.702 m). Then apply BMI = kg ÷ m². Or use our calculator which accepts stones and feet/inches natively.
Put your weight (kg) in cell A1 and your height (m) in cell B1. Then in cell C1 enter the formula =A1/(B1^2). For imperial, put weight (lbs) in A1, height (inches) in B1, and use =A1/(B1^2)*703. To classify the result, wrap it in: =IF(C1<18.5,"Underweight",IF(C1<25,"Normal",IF(C1<30,"Overweight","Obese"))).
For ages 2–19 the BMI number itself is calculated the same way, but it is interpreted against CDC age-and-sex BMI-for-age percentile charts — not the adult 18.5–24.9 cut-offs. A child is healthy weight if their BMI falls between the 5th and 84th percentile for their exact age and sex. Use our child BMI calculator for percentile look-ups.
No. The BMI formula is identical for men, women, ladies, guys, and any adult over 18 — height (m) squared into the denominator, weight (kg) on top. What can differ is the interpretation: women carry more essential body fat than men at the same BMI, and ethnicity-specific cut-offs apply for South Asian, Chinese, Black and Middle Eastern adults (NICE PH53 / WHO Asia-Pacific).
The BMI calculation itself does not change with age — only the interpretation does. Adults over 65 may have a slightly higher healthy range (BMI 23–27 rather than 18.5–24.9) because higher BMI is mildly protective against frailty and bone-density loss in older age. For children, age is a key input via the percentile system.
BMI is a screening tool, not a diagnostic test. It is accurate as a population-level signal — a higher BMI correlates with higher disease risk on average — but it cannot distinguish muscle from fat in an individual. A muscular athlete may have a BMI of 27 with very low body fat, while a sedentary person at BMI 23 may still have unhealthy abdominal fat. Always combine BMI with waist circumference and body fat percentage.
Adolphe Quetelet (the Belgian statistician who devised it in 1832) found that healthy adult weight scales roughly with height squared, not height itself. Squaring the height makes BMI roughly independent of height — a 1.5 m and a 2.0 m adult with the same BMI have similar weight-for-height proportions. The formula is technically the "Quetelet index" and was renamed Body Mass Index in 1972.
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