What this calculator does
Body mass index divides weight by the square of height. It was designed in the nineteenth century as a way of describing populations, and it remains useful for that: it is quick, needs no equipment, and correlates reasonably well with body fat across large groups.
For an individual it is a rough screening figure rather than a diagnosis. BMI cannot distinguish muscle from fat, does not account for where fat sits on the body, and reads differently across ages and ethnic backgrounds. A muscular athlete and a sedentary person can share a BMI while having very different health profiles.
The formula
Weight in kilograms is divided by height in metres squared. Squaring the height is what makes the index roughly independent of body size, though it under-corrects for very tall people and over-corrects for very short ones. BMI prime simply expresses the result relative to 25, the top of the healthy band.
| Term | Meaning |
|---|---|
| BMI | Weight in kilograms ÷ height in metres squared. |
| BMI prime | BMI ÷ 25. A value of 1.0 sits exactly at the top of the healthy range. |
| Healthy range | The weights that give a BMI between 18.5 and 24.9 at your height. |
The inputs explained
| Field | What to enter |
|---|---|
| Weight (kg) | Your weight in kilograms. If you use pounds, divide by 2.205. |
| Height (cm) | Your height in centimetres. If you use feet and inches, the height and weight converter will translate it. |
When to use it
A quick general screening
BMI is a starting point, not a conclusion. If it falls outside the healthy band, it is a reason to look at other measures, waist circumference, blood pressure, activity level, rather than an answer on its own.
Seeing the healthy range for your height
The range shown is often more useful than the single number. It gives a span of perhaps ten kilograms rather than one target, which is a more realistic way to think about weight.
Tracking a change over time
A trend across months is more informative than any single reading. Weight fluctuates daily with hydration and food in transit, so compare weekly averages rather than individual days.
Understanding where BMI misleads
Athletes with high muscle mass frequently read as overweight. Older adults who have lost muscle may read as healthy while carrying a high proportion of body fat. In both cases a body composition measure is more informative.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
BMI at different weights for a 175 cm adult
How BMI and its category change with weight at a fixed height of 175 cm.
| Weight | BMI | Category | BMI prime |
|---|---|---|---|
| 55 kg | 18.0 | Underweight | 0.72 (1.0 = top of healthy range) |
| 65 kg | 21.2 | Healthy weight | 0.85 (1.0 = top of healthy range) |
| 75 kg | 24.5 | Healthy weight | 0.98 (1.0 = top of healthy range) |
| 85 kg | 27.8 | Overweight | 1.11 (1.0 = top of healthy range) |
| 95 kg | 31.0 | Obese class I | 1.24 (1.0 = top of healthy range) |
| 110 kg | 35.9 | Obese class II | 1.44 (1.0 = top of healthy range) |
The healthy weight range at different heights
The same 75 kg body at a range of heights, showing how the healthy range scales.
| Height | BMI | Category | Healthy weight range for your height |
|---|---|---|---|
| 155 cm | 31.2 | Obese class I | 44.4 – 59.8 kg |
| 165 cm | 27.5 | Overweight | 50.4 – 67.8 kg |
| 175 cm | 24.5 | Healthy weight | 56.7 – 76.3 kg |
| 185 cm | 21.9 | Healthy weight | 63.3 – 85.2 kg |
| 195 cm | 19.7 | Healthy weight | 70.3 – 94.7 kg |
Questions
What is a healthy BMI?
The World Health Organization defines 18.5 to 24.9 as the healthy range, 25 to 29.9 as overweight and 30 and above as obese. These thresholds were derived largely from European populations and some health authorities use lower cut-offs for people of South Asian, Chinese and other Asian descent.
Is BMI accurate for athletes?
Often not. Muscle is denser than fat, so a well-trained athlete can register as overweight or obese while carrying very little fat. Body fat percentage or waist measurements are better suited to that case.
Does BMI work for children?
Not in this form. Children and teenagers are assessed against age-and-sex percentile charts rather than fixed thresholds, because healthy body composition changes throughout growth. Speak to a paediatrician or family doctor rather than relying on an adult BMI figure.
Why is my BMI different from another calculator?
The arithmetic is standardised, so any difference comes from rounding or from unit conversion. Check that height is in centimetres and weight in kilograms.
Should I aim for a specific BMI?
BMI is a screening tool, not a goal. Health outcomes depend on many factors, fitness, diet quality, sleep, blood markers, that BMI does not measure. A doctor can interpret it alongside those.
For a measure that accounts for body composition, see body fat percentage or waist-to-hip ratio. For energy needs, use the TDEE calculator.