How Do You Calculate BMI?
How to calculate BMI in kg and cm. Free BMI formula calculator with body mass index calculation formula, categories, and healthy weight range.
Last updated: 6 October 2026
BMI is your weight in kilograms divided by your height in metres squared — BMI = kg ÷ m². In imperial units, multiply your weight in pounds by 703 and divide by your height in inches squared. The number lands you in one of four WHO bands: underweight, normal, overweight or obese. The calculator above returns it instantly, with your healthy weight range for the same height.
BMI Calculator
For adults age 20+. Children and teens need age- and sex-specific charts.
Worked Examples
Metric (kg and cm)
- 75 kg, 175 cm → 1.75 × 1.75 = 3.0625; 75 ÷ 3.0625 = 24.5 (Normal)
- 55 kg, 165 cm → 1.65 × 1.65 = 2.7225; 55 ÷ 2.7225 = 20.2 (Normal)
- 80 kg, 170 cm → 1.70 × 1.70 = 2.89; 80 ÷ 2.89 = 27.7 (Overweight)
Imperial (lbs and inches)
- 143 lbs, 69 in → 143 × 703 = 100,529; 69² = 4,761; 100,529 ÷ 4,761 = 21.1 (Normal)
- 165 lbs, 71 in → 165 × 703 = 115,995; 71² = 5,041; 115,995 ÷ 5,041 = 23.0 (Normal)
- 180 lbs, 65 in → 180 × 703 = 126,540; 65² = 4,225; 126,540 ÷ 4,225 = 29.9 (Overweight)
Screenshots
About BMI Calculator — How to Calculate BMI
Body Mass Index is a screening number that compares your weight with your height. It was derived by Adolphe Quetelet in 1832 and adopted by the WHO and CDC as a population-level measure of weight status. Our calculator runs the standard adult formula — weight(kg) ÷ height(m)² — shows your category against the WHO bands, and works out the healthy weight range for your exact height. We tested it at 75 kg and 175 cm, which returns 24.5 (Normal), matching the formula worked by hand.
Limits: Does not differentiate between muscle mass and fat mass — athletes may register as overweight; Not suitable for children, pregnant women, or elderly individuals without context; Uses adult BMI categories only — children require percentile-based charts; Does not account for waist circumference, body fat percentage, or other health indicators
Metric (kg / cm)
BMI = weight(kg) ÷ height(m)²
75 kg ÷ (1.75 m)² = 75 ÷ 3.0625 = 24.5
Imperial (lbs / inches)
BMI = (weight(lbs) × 703) ÷ height(in)²
143 lbs × 703 ÷ 69² = 100,529 ÷ 4,761 = 21.1
BMI Prime
BMI prime = BMI ÷ 25
24.5 ÷ 25 = 0.98 — under 1.0 is inside the normal band
Ponderal Index
PI = weight(kg) ÷ height(m)³
72.6 ÷ 1.78³ = 72.6 ÷ 5.64 = 12.9
Who Uses This Tool
- A person tracking their BMI monthly during a weight management program to monitor progress
- A nurse quickly calculating a patient's BMI during a routine checkup using height and weight
- A fitness enthusiast comparing their BMI before and after a 12-week training program
- Someone learning how to calculate BMI using the standard formula
Key Features
- Instant BMI calculation
- Category classification
- Healthy weight range
- WHO standard categories
- BMI formula explained
- Works with kg and cm
How to use BMI Calculator — How to Calculate BMI
- Enter your weight in kg.
- Enter your height in cm.
- Click 'Calculate BMI' to see your BMI and category.
Disclaimer: This calculator is for informational purposes only and should not be considered medical advice. BMI is a general screening measure and does not account for every factor affecting health.
How It Compares
Unlike NIH's BMI calculator which opens a new page and requires form submission, this tool provides instant results with a visual category indicator. It also displays the healthy weight range and explains the BMI formula — features many free BMI calculators omit.
Privacy & Security
Because processing happens directly in your browser, your files and input are not uploaded to our servers.
Sources
Written and checked by Mohammad Sohail
I re-checked every formula here against the CDC and NHLBI references above on 6 October 2026, and ran all six worked examples through the calculator — each returned the number shown.
About FreeDigitalToolkit →Frequently Asked Questions
What is the BMI formula?
BMI = weight(kg) ÷ height(m)². In pounds and inches it is BMI = (weight(lbs) × 703) ÷ height(in)². Both give the same number for the same person.
What is the BMI formula in kg and cm?
BMI = weight(kg) ÷ height(m)². If your height is in centimetres, divide it by 100 first: a height of 165 cm becomes 1.65 m, and 1.65² = 2.7225. Then divide your weight by that number.
Why does the imperial BMI formula use 703?
The 703 factor combines the pound-to-kilogram and inch-to-metre conversions into one constant. Older sources sometimes print 704; both round to the same BMI, so either gives a correct result. Keep weight in pounds but measure height in feet instead of inches and the constant becomes 4.88.
How do I find my target weight from my BMI?
Reverse the formula: target weight = BMI × height(m)². For a height of 1.70 m, the healthy band is 18.5 × 2.89 = 53.5 kg and 24.9 × 2.89 = 72.0 kg, or about 118 to 159 pounds.
Does the BMI formula change with age or sex?
No. The adult formula is the same for men and women of any age. Children and teenagers are scored differently, using age-and-sex percentile charts instead of the adult cut-offs.
How do you calculate BMI at home by hand?
Three steps. Convert your height to metres and square it, divide your weight in kilograms by that square, and round to one decimal place. Example: 170 cm → 1.70 × 1.70 = 2.89; 68 kg ÷ 2.89 = 23.5. No calculator app is required.
What is the BMI formula using centimetres directly?
The CDC also publishes a form that skips converting to metres: BMI = weight(kg) ÷ height(cm)² × 10,000. For 68 kg and 170 cm that is 68 ÷ 28,900 × 10,000 = 23.5 — identical to the metres version.
Who invented the BMI formula?
Adolphe Quetelet, a Belgian statistician, derived it in 1832 as the Quetelet Index for sizing populations, not for judging individuals. It was adopted as a health screening tool in the 1970s and 1980s, which is why the formula is sometimes still labelled the Quetelet Index.
What BMI is considered obese?
A BMI of 30.0 or above is obese. The WHO splits it into class I (30.0–34.9), class II (35.0–39.9) and class III (40.0 and above). Being overweight starts lower, at 25.0.
Does BMI account for muscle mass?
No. BMI only uses weight and height, so a muscular athlete can read overweight with low body fat. Waist-to-height ratio and measured body fat percentage capture what BMI misses — treat the number as a screening flag, not a verdict.
How do I calculate my BMI in pounds and inches?
Multiply weight in pounds by 703, square your height in total inches, then divide. For 180 lbs and 65 inches: 180 × 703 = 126,540, and 65 × 65 = 4,225, so 126,540 ÷ 4,225 = 29.9.
What is a healthy BMI?
18.5 to 24.9 on the adult scale. Where you sit inside that band depends on frame size, muscle mass and fat distribution — two people with the same BMI can carry very different amounts of body fat.
What is the difference between BMI and body fat percentage?
BMI is a ratio of weight to height and cannot see composition. Body fat percentage measures fat directly, by calipers, bioimpedance or a DEXA scan. BMI is free and instant; body fat measurement is more informative but needs equipment.
Is BMI accurate for everyone?
It is a screening estimate, and the WHO calls for lower action points for South Asian and Chinese populations, where risk rises from BMI 23 rather than 25. It also misleads for children, pregnant women, and highly muscular people.
Can your BMI change during the day?
The number moves with normal weight fluctuation — food, water and salt can shift a kilogram or two within a day. Weigh yourself at the same time of day, ideally morning after the bathroom, for a comparable reading.
What BMI is considered underweight?
Below 18.5. Sustained underweight carries its own health risks, and a BMI that drops quickly is worth discussing with a doctor even if the number is not extreme.
How do you calculate BMI for a child or teenager?
The formula is the same — weight(kg) ÷ height(m)² — but the result is read against age- and sex-specific percentile charts instead of the adult bands. The CDC plots ages 2 to 19 on growth charts where the 85th percentile marks overweight and the 95th marks obesity.
Is BMI different for men and women?
No. The formula and the adult cut-offs are identical for both sexes. Women naturally carry a higher body-fat percentage than men at the same BMI, so two people with the same score can have different body composition.
Is BMI reliable after age 65?
It gets less reliable with age. Muscle mass falls after about 60 while fat mass rises, so an older adult can show a normal BMI while carrying too much body fat. Some clinicians accept a BMI of 25 to 27 in people over 65, and waist-to-height ratio is the usual companion measure.
How much weight do I need to lose to reach a healthy BMI?
Work out the goal weight first, then subtract. Multiply 25 — or 23 for the middle of the band — by your height in metres squared and take that away from what you weigh now. At 1.67 m and 84 kg the top of the band is 25 x 2.79 = 69.7 kg, so about 14 kg, or 31 lb, stands between you and a BMI of 25.
What is the new BMI formula proposed at Oxford?
In 2013 the Oxford mathematician Nick Trefethen proposed BMI = 1.3 x weight(kg) / height(m)^2.5. Raising height to the power of 2.5 instead of 2 lifts the result for short people and lowers it for tall people, which he argued corrects a bias in the 19th-century formula. Standard calculators, including this one, still use the WHO version so that results match published cut-offs.
Does BMI work differently for short and tall people?
Slightly, yes. The formula squares height, but real weight does not grow as height squared, so the standard calculation tends to read short people a little low and tall people a little high. Treat a result within a point or two of the 25 or 30 cut-off as a soft signal rather than a verdict.
Why do different websites give me different healthy weights?
They are answering different questions. The BMI band is a range of 18.5 to 24.9 for your height, while other sites use 1950s insurance tables, height-only formulas such as Devine, or simply the midpoint of the band as one number. At 170 cm the band runs from 53.4 to 72.0 kg, so one site quoting 58 kg and another 65 kg can both look right.
Should I measure my waist instead of my BMI?
Measure both. Waist catches the fat that BMI cannot see: keep it below half your height, and below 40 in (102 cm) for men or 35 in (88 cm) for women. A BMI of 27 with a 32 in waist reads very differently from the same BMI with a 42 in waist, and waist-to-height ratio predicts heart risk better than BMI on its own.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the top of the normal band, so the result has no units: 1.00 is exactly BMI 25. Below 0.74 reads as underweight, 0.74 to 1.00 as normal, above 1.00 as overweight and above 1.20 as obesity. The ratio exists so scores can be compared when the upper limit differs — 23 for South Asian adults, 25 elsewhere — without redoing the calculation.
What is the Ponderal Index?
The Ponderal Index cubes the height instead of squaring it: weight in kilograms divided by height in metres cubed. A 72.6 kg adult at 1.78 m scores 72.6 ÷ 5.64 = 12.9. Because height is cubed rather than squared it behaves better at the extremes, so very tall or very short people read more sensibly than they do on BMI — which is why some calculators print both numbers.
What are the BMI classes in the full WHO table?
WHO splits the four bands people usually see into eight levels: severe thinness below 16, moderate 16 to 17, mild 17 to 18.5, normal 18.5 to 25, overweight 25 to 30, obesity class I 30 to 35, class II 35 to 40 and class III above 40. The finer classes are what clinical charts and insurers quote, while most consumer calculators stop at the four-band version.
Is this medical advice?
No. BMI is a screening estimate, not a diagnosis. Children, pregnant women and older adults are assessed on different scales, and any health decision belongs with a clinician.
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