BMI Calculator
Result
BMI
- BMI Prime
- 0.91
- Healthy weight, low end
- 56.7 kg
- Healthy weight, high end
- 76.6 kg
Body mass index (BMI) is your weight in kilograms divided by your height in metres squared. It is the screening measure the World Health Organization uses to sort adults into underweight, normal, overweight and obese bands, and it is the number most doctors reach for first. This BMI calculator also reports your BMI Prime — how your BMI compares to the upper limit of the normal range — and the healthy weight range that would put you inside that normal band at your height.
WHO BMI classification for adults
| Category | BMI |
|---|---|
| Underweight | below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese | 30.0 and above |
Cut-offs apply to adults aged 20 and over. They are not used for children or during pregnancy.
Formula
BMI = weight (kg) / height (m)²
- weight
- Body weight, in kilograms (kg)
- height
- Standing height, in metres (m)
Use it as a first screen for weight-related risk in adults. Reach for a body-fat measurement instead when you train seriously, and for waist circumference or a waist-to-height ratio when you want to know where the fat sits — BMI is blind to both.
Worked examples
A 175 cm adult weighing 70 kg
- Convert height to metres: 175 cm = 1.75 m
- Square it: 1.75 × 1.75 = 3.0625
- Divide weight by that: 70 ÷ 3.0625 = 22.86
- BMI Prime: 22.86 ÷ 25 = 0.91 — you sit at 91% of the normal upper limit
At 22.9 you are inside the normal band (18.5–24.9), with roughly 6.6 kg of headroom before you cross into overweight.
A 175 cm adult weighing 75 kg
- Height squared is still 3.0625
- 75 ÷ 3.0625 = 24.49
- 24.49 is below the 25.0 overweight threshold, so this still reads as normal
24.5 is the last stretch of the normal band. Half a kilogram more puts you at 25.0 and into overweight.
Limitations
BMI does not measure body fat, and it cannot tell muscle from fat. A trained athlete can read as overweight while carrying very little fat, and an older adult who has lost muscle can read as normal while carrying a high fat percentage. It is also not validated for children, for people under 18, or for pregnancy — those need age- and sex-specific percentile charts instead. Treat the band as a screening prompt, not a diagnosis.
Frequently asked questions
- What is a healthy BMI?
- For adults the World Health Organization puts the normal band at 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 and above is obese. These thresholds come from population-level risk of type 2 diabetes and cardiovascular disease, not from an individual's own health.
- What is BMI Prime?
- BMI Prime is your BMI divided by 25, the upper limit of the normal band. A value of 1.00 means you sit exactly on that limit; 0.91 means you are at 91% of it. It is a quick way to see how much weight you would need to lose or gain to reach the normal range.
- Does BMI work the same for athletes and muscular people?
- No. Muscle is denser than fat, so a heavily muscled person can land in the overweight band with a low body fat percentage. If you train seriously, a body fat measurement or a waist-to-height ratio will tell you more than BMI does.
- Should I use BMI for a child?
- No. Children and teenagers are still growing, so their BMI is read against age- and sex-specific percentile charts rather than fixed adult cut-offs. A BMI that is normal for an adult can be high for a 10-year-old.
- How much weight do I need to lose to reach the normal band?
- Subtract the healthy maximum this BMI calculator gives you from your current weight. Because BMI scales with height squared, the answer is different for every height — at 175 cm the normal band tops out near 76.6 kg, while at 160 cm it tops out near 64 kg.
References
- Obesity: preventing and managing the global epidemic (WHO Technical Report Series 894) — World Health Organization
- Body Mass Index (BMI) — healthy weight — US Centers for Disease Control and Prevention