Body Mass Index (BMI) Calculator
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Your BMI
24.2
Normal
About the Body Mass Index (BMI) Calculator
Body mass index is weight divided by height squared. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was studying the distribution of body types across populations — not diagnosing individuals. It became a clinical screening tool in the 1970s largely because it is cheap, requires no equipment beyond a scale and a tape measure, and correlates reasonably well with body fat across large groups.
Understanding that origin explains both its usefulness and its limits. Across a population of ten thousand people, BMI predicts rates of cardiovascular disease, type 2 diabetes and all-cause mortality well enough to be worth collecting. For any single person standing in front of you, it is a crude proxy that knows nothing about muscle, bone density, fat distribution, age or ancestry.
The practical position most clinicians take: BMI is a reasonable first screen that should trigger further measurement, never a conclusion on its own. A result outside the normal range is a prompt to look at waist circumference, body composition, blood pressure and metabolic markers — all of which carry more information about actual health risk.
Formula
BMI = weight (kg) / height (m)² | BMI = 703 × weight (lb) / height (in)²
- weight
- Body mass in kilograms, or pounds for the imperial form
- height
- Height in metres, or inches for the imperial form
- 703
- Unit conversion constant that makes the imperial form produce metric-scale results
Squaring height is the part that draws criticism. Body mass scales roughly with volume, which is a cubic quantity, so dividing by height squared rather than cubed systematically flatters tall people and penalises short ones. Quetelet chose the square because it happened to fit the population data he had, not because it followed from any biological principle.
The 703 constant in the imperial version comes from unit conversion: 1 kg = 2.20462 lb and 1 m = 39.3701 in, and 39.3701² / 2.20462 ≈ 703. It carries no meaning beyond making both formulas land on the same scale.
Height error is amplified. Because height is squared, a 2 cm measurement mistake shifts BMI by roughly 0.6 units — enough to move a borderline result across a category line. Measure without shoes, first thing in the morning if you want consistency, since spinal compression costs most people about 1 cm over a day.
Worked Examples
Metric: 82 kg at 1.78 m
An adult weighing 82 kilograms standing 1.78 metres tall.
Result: BMI 25.9 — just inside the "overweight" range
This person is 3 kg above the top of the normal range (79.2 kg would give exactly 25.0). At this margin, body composition matters more than the number: a lean, muscular person at 25.9 and a sedentary person at 25.9 have very different risk profiles.
Imperial: 180 lb at 5 ft 10 in
The same calculation in US units. Convert the height to inches first.
Result: BMI 25.8 — overweight by one tenth of a unit
Note how narrow the margin is. Losing four pounds would place this person in the normal range. Categories are hard lines drawn across a continuous variable, and a result within about 1.0 of a boundary should be treated as ambiguous.
Where BMI clearly fails: a strength athlete
A rugby forward weighing 95 kg at 1.80 m with 12% measured body fat.
Result: BMI 29.3 — nominally "overweight" at 12% body fat
Muscle is roughly 18% denser than fat, so muscular people are pushed up the scale by tissue that lowers rather than raises health risk. This is the textbook case where BMI should be disregarded in favour of a body composition measurement.
How to Use the Result
The WHO categories for adults
These thresholds apply to adults aged 20 and over and are the standard used by the World Health Organization and most national health services:
- —Below 18.5 — underweight. Associated with nutritional deficiency, reduced bone density and, in older adults, higher mortality than the overweight range.
- —18.5 to 24.9 — normal weight.
- —25.0 to 29.9 — overweight. Elevated risk, though the association with mortality in this band is weaker than commonly assumed.
- —30.0 to 34.9 — obesity class I.
- —35.0 to 39.9 — obesity class II.
- —40.0 and above — obesity class III, sometimes called severe or morbid obesity.
Where BMI systematically misreads people
The measure has known, well-documented failure modes. It will overstate risk for:
- —Athletes and anyone with substantial muscle mass, for the density reason above.
- —Very tall people, because squaring height rather than cubing it does not fully correct for scale.
- —Pregnant and breastfeeding women, for whom the measure is not applicable at all.
And where it understates risk
The opposite error is less discussed and arguably more dangerous, because a reassuring number discourages further investigation:
- —Older adults, who lose muscle and bone with age. A stable BMI at 70 can conceal a substantial gain in fat mass alongside sarcopenia.
- —People of South Asian, Chinese and some other Asian ancestries, who develop metabolic disease at lower BMI values. The WHO recommends action thresholds of 23 for overweight and 27.5 for obesity in these populations rather than 25 and 30.
- —Anyone with "normal weight obesity" — a BMI in range but high body fat and low muscle, which carries cardiovascular risk comparable to conventional obesity.
- —People with predominantly visceral fat. Fat stored around the organs is considerably more metabolically harmful than subcutaneous fat, and BMI cannot distinguish them.
Better measures when BMI is ambiguous
If your result sits near a boundary or falls into one of the categories above, three inexpensive measurements add real information:
Waist circumference, measured at the navel, flags visceral fat directly. Risk thresholds are commonly set at 102 cm (40 in) for men and 88 cm (35 in) for women, lower for Asian populations. Waist-to-height ratio is simpler still: keeping your waist below half your height is a well-validated rule that works across ages and ethnicities without needing category tables.
Body fat percentage from DEXA, hydrostatic weighing or a reasonable bioimpedance scale gives the composition BMI cannot see. Healthy ranges are roughly 10–20% for men and 18–28% for women, varying with age.
Edge Cases and Common Mistakes
Children and teenagers use percentiles, not fixed cutoffs
Body composition changes rapidly during growth, so a BMI of 19 means something completely different at age 8 than at age 18. For anyone under 20, BMI is plotted against age- and sex-specific growth charts and reported as a percentile. Overweight is defined as the 85th percentile and obesity as the 95th. Applying the adult table to a child produces meaningless results.
Amputation and limb differences require adjustment
BMI assumes an intact body. After a limb amputation, weight falls but height does not, producing an artificially low result. Clinical practice is to adjust estimated body weight by the proportion the missing segment represents — roughly 16% for a full leg, 5% for a full arm — before calculating.
The measure is unreliable at the extremes
Below about 1.5 m and above about 2.0 m, the height-squared assumption breaks down noticeably. Very short people are pushed toward higher BMI values and very tall people toward lower ones for identical body composition. Some researchers use the Ponderal index, which divides by height cubed, for these populations.
Frequently Asked Questions
Is BMI accurate for individuals?
It is accurate as a rough screen and unreliable as a diagnosis. Studies comparing BMI to DEXA body composition scans find it misclassifies roughly a quarter of people — most often labelling muscular individuals as overweight and older or sedentary individuals with normal BMI as healthy when their body fat is elevated.
Should BMI thresholds differ by sex?
The standard categories do not, even though women carry more essential body fat than men at the same BMI. Population data supports using the same cutoffs for screening purposes, but it means a man and a woman at BMI 24 typically have quite different body fat percentages.
What is a healthy BMI for older adults?
Evidence consistently shows the lowest mortality for people over 65 falls in the 23 to 30 range — higher than the standard normal band. Being underweight in older age is associated with worse outcomes than being mildly overweight, largely because low weight often reflects muscle loss or underlying illness.
Why do Asian populations use lower thresholds?
At any given BMI, people of South Asian and East Asian ancestry tend to carry more visceral fat and develop insulin resistance and type 2 diabetes earlier. The WHO therefore recommends 23 as the overweight action point and 27.5 for obesity in these groups, roughly two units below the standard cutoffs.
How much weight change moves BMI by one unit?
One BMI unit equals height² kilograms. At 1.70 m that is 2.89 kg; at 1.85 m it is 3.42 kg. In imperial terms, roughly 6 to 8 pounds per unit for most adults. This is worth knowing because it shows how small a change separates adjacent categories.
Does BMI account for age?
Not for adults. The same table applies from 20 to 100, despite the fact that body composition changes substantially over that span. This is one of the measure's clearest weaknesses, and it is why waist circumference or a body composition measurement is more informative for anyone past middle age.