How to Calculate BMI
Last updated: August 2026
Body mass index is your weight divided by your height squared. One division, one multiplication — the whole reason the number has survived nearly two hundred years. You can do it on a phone keypad in ten seconds.
Both formulas are below, worked through with real numbers, plus the part most calculators skip: where the result misleads. Once you have your figure, the BMI chart tells you which category it falls into.
The metric formula: kilograms and meters
BMI = weight in kilograms ÷ (height in meters × height in meters)
The only trap is the unit: meters, not centimeters. 175 cm is 1.75 m — use 175 and you get a BMI of 0.0025.
Worked example
78 kg at 175 cm:
- Convert: 175 cm = 1.75 m
- Square: 1.75 × 1.75 = 3.0625
- Divide: 78 ÷ 3.0625 = 25.5
That is just inside overweight, which begins at 25.0 — about 1.5 kg from the healthy band. Never treat a result within a point of a boundary as firm: normal daily weight fluctuation can move you across the line and back within a week.
The imperial formula: pounds and inches
BMI = (weight in pounds × 703) ÷ (height in inches × height in inches)
The 703 converts pounds per square inch into kilograms per square meter — 39.37 inches per meter, squared, divided by 2.2046 pounds per kilogram — so both formulas give the same answer.
Worked example
172 lb at 5 ft 9 in:
- Height in inches: (5 × 12) + 9 = 69 in
- Square: 69 × 69 = 4,761
- Weight × 703: 172 × 703 = 120,916
- Divide: 120,916 ÷ 4,761 = 25.4
Same person as the metric example, same answer. The classic mistake: leaving feet and inches separate and squaring only the inches.
Quick reference: what your result means
For adults aged 20 and over, men and women alike:
| BMI | Category | Example at 175 cm / 5 ft 9 in |
|---|---|---|
| Below 18.5 | Underweight | Under 57 kg / 125 lb |
| 18.5 – 24.9 | Healthy weight | 57 – 76 kg / 125 – 168 lb |
| 25.0 – 29.9 | Overweight | 77 – 91 kg / 169 – 201 lb |
| 30.0 – 34.9 | Obesity class 1 | 92 – 107 kg / 202 – 236 lb |
| 35.0 – 39.9 | Obesity class 2 | 107 – 122 kg / 237 – 269 lb |
| 40.0 and above | Obesity class 3 | 123 kg / 270 lb and above |
If you are of South Asian, Chinese, or other Asian ancestry, several health bodies apply lower thresholds — overweight from about 23, obesity from about 27.5 — because metabolic risk rises earlier. The full picture, including a height-by-height table, is in the BMI chart guide.
Where the number goes wrong
The arithmetic is simple, so wrong BMIs come from the inputs.
- Guessed height. Most adults are a centimeter or two shorter than their license says, and height shrinks with age. At 175 cm, 1 cm moves BMI by about 0.3 — enough to cross a boundary. Measure barefoot against a wall, once, and reuse the figure.
- Inconsistent weigh-ins. You can be 1 to 2 kg heavier in the evening. Weigh after waking, before eating.
- Mixed units, squared weight, over-rounding. Kilograms with inches produce nonsense, and rounding height to the nearest 5 cm can shift the result a full point. Two calculators disagreeing is almost always one of these.
Then there is what the formula cannot see. Muscle is denser than fat, so a well-trained rower can post a BMI of 28 to 30 at single-digit body fat — if you lift seriously, treat BMI as close to meaningless and use body fat percentage instead. The reverse holds: someone sedentary at BMI 23 can carry enough abdominal fat to raise metabolic risk. Risk begins at different BMIs in different populations, the same BMI at 60 holds more fat than it did at 30, women carry more essential fat than men, BMI is not valid in pregnancy, and children use percentile charts. The cheapest fix is a tape measure: waist-to-height ratio takes thirty seconds and sees the fat distribution BMI cannot.
Doing it by hand vs letting an app do it
Calculating BMI once is trivial. Calculating it for every weigh-in for a year, then seeing what the trend did — that is where doing it by hand quietly falls apart.
The free Weight & BMI Feeltracker app removes the step: enter your height once and BMI is calculated for every entry, charted with trend lines next to your weight and a goal you set. It records body fat percentage too, syncs with Apple Health and across your devices via iCloud with no account or email, and exports to PDF, XLSX, CSV, or JSON for appointments. Optional AI insights summarize your history.
Whichever route you take, the formula above is the whole of it. The value is in checking occasionally, not in three decimal places.
Frequently asked questions
What is the BMI formula?
In metric: weight in kg ÷ (height in m)². In imperial: (weight in lb × 703) ÷ (height in in)². Both give the same result — the 703 simply converts pounds and inches into the metric equivalent.
Why is 703 used in the imperial BMI formula?
It is the unit conversion baked into the equation: one meter is 39.37 inches, one kilogram is 2.2046 pounds, and 39.37² ÷ 2.2046 comes out at roughly 703.
Is BMI calculated differently for men and women?
No — the formula and the adult categories are identical, even though women carry more essential body fat at the same BMI. For a sex-specific measure, use body fat percentage, which has separate ranges.
How accurate is BMI for athletes?
Poor. Muscle weighs more than fat for the same volume, so trained athletes routinely land in the overweight or obese categories while being extremely lean. For anyone doing serious resistance training, waist measurement and body fat estimates are far more informative.
Do I need to recalculate BMI every time I weigh myself?
Not manually. Your height rarely changes, so BMI moves in lockstep with weight. Checking the figure monthly is plenty, and an app recalculates it automatically anyway.
