FitDexBMI calculator
Body mass index is a one-line calculation that divides your weight by your height squared. It has been in use for ninety years because it's cheap, fast and works at population scale. Describing a single person, it falls short. Below you'll find both the number and where it falls short.
Body mass index
Healthy weight
- Your healthy weight range53.5 – 72.0 kg
- Body fat percentage30.6%
This number comes from the very same formula the FitDex app uses. Install the app and you won't be handed a different target.
How is it calculated?
The calculation is one line: weight in kilograms divided by height in metres times height in metres. For 170 cm and 70 kg: 70 ÷ (1.70 × 1.70) = 24.2. Height goes in as metres — dividing by centimetres makes the number a hundred times too small, and that's the most common mistake.
The result is graded against the World Health Organization thresholds: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30–34.9 obesity class I, 35–39.9 class II, 40 and above class III.
Your healthy weight range comes out of the same equation, reversed: 18.5 × height² to 24.9 × height². At 170 cm that's between 53.5 and 72.0 kilos. The range being that wide is not sloppiness — two people of the same height really can differ that much in frame and muscle.
The body fat estimate alongside it uses the Deurenberg equation: 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex is 1 for men and 0 for women. That equation takes BMI as an input, so it isn't an independent measurement — but because it accounts for age and sex it tells you more than raw BMI does.
How to read your result
BMI cannot tell fat from muscle. Weight is weight; it never asks where it came from. So someone who lifts regularly can read 27 and be perfectly healthy, while someone who never moves can read 23 and carry a high waist circumference. The literature calls that second case "normal weight obesity", and it is precisely what BMI cannot see.
Three groups the number lies about: athletes with high muscle mass (they look heavier than they are), older adults (BMI drops as muscle is lost even while fat has risen) and people who are very short or very tall (squaring the height distorts at the extremes). If you're in one of those, look past the raw number to the body fat estimate beside it and to your waist measurement.
The practical use is this. Treat BMI as a direction, not an alarm. If it's over 25, don't read it as "I must do something" but as "let me measure my waist and look at my body fat". Above 88 cm for women or 102 cm for men, metabolic risk rises independently of BMI; below that, a BMI of 26–27 is not on its own a diagnosis.
The CHANGE over time means more than the absolute value. Going from 24 to 27 in six months at the same height tells a different story than having been 27 for six months. A single reading is a photograph. The information is in the series.
Frequently asked questions
My BMI is over 25 — should I start a diet right away?
No — check two more things first. One: your waist. Run the tape at navel height, after breathing out, snug but not tight. Under 88 cm for women or 102 cm for men, a BMI of 26–27 is not on its own an emergency. Two: the last twelve months. If your weight is stable and your energy is fine, you're looking at a balance, not a trend. If both check out, the answer isn't a diet — it's more movement and more protein, which is usually what brings BMI down anyway.
Do the same thresholds apply to children and during pregnancy?
No, on both counts. In children and teenagers BMI is read against age- and sex-specific percentile curves; a value of 22 means something entirely different at twelve. That's why this calculator doesn't accept ages under 13. It isn't interpreted in pregnancy either: most of the gained weight is baby, placenta and fluid, and BMI counts all of it as fat. What's tracked in pregnancy is not BMI but recommended total weight gain based on PRE-pregnancy BMI, and that is monitored by a clinician.
Are the thresholds different for people of Asian descent?
Yes, and it's a detail that often gets skipped. After observing that type 2 diabetes and cardiovascular risk rise at lower BMI values in South and East Asian populations, the World Health Organization proposed 23 as the point of increased risk and 27.5 as the obesity threshold for those populations. The reason is where the fat sits: at the same BMI, visceral fat can be higher. The thresholds on this page are the general-population ones; if that description fits you, reading the number two to three units lower is closer to the truth.
What should I look at instead of BMI?
If you want a single number, waist-to-height does better than BMI: divide waist by height and aim to stay under 0.5. It's just as easy to measure and it also tells you where the fat sits. Second comes the body fat estimate on this page. Third — and most valuable — is the series: weighing yourself weekly under the same conditions and watching the curve says far more than any single day's figure. FitDex keeps all three on one screen, so you can see how weight, BMI and body fat move over time.
Related tools
- Waist-to-hip ratio calculatorEnter your waist, hip and height to see three separate risk measures at once and your target waist. In the sample body the ratio is 0.94, the target waist 89 cm and the gap 9 cm.
- Body fat calculatorEstimate your body fat percentage and lean mass from height, weight, age and sex. We don't ask for a tape measure — and we explain why, along with the margin of error.
- Ideal weight calculatorWork out the healthy weight range for your height. We don't hand you a single "ideal weight" number — because there isn't one. You get the range, where you sit in it, and which end fits you.
- Calorie calculatorEnter your height, weight, age and activity level to see your daily calorie needs, basal metabolic rate and macro split in seconds. The same formula as the FitDex app.
This tool is for information only and is not a substitute for medical diagnosis or treatment. Talk to a healthcare professional before making a lasting change to how you eat.