Quick answer: BMI = weight in kilograms divided by height in metres squared. For a person who weighs 70 kg and is 1.75 m tall, BMI = 70 ÷ (1.75 × 1.75) = 22.9, which is in the healthy range of 18.5 to 24.9.
Body Mass Index, or BMI, is a simple number that relates your weight to your height. Doctors and health organisations use it as a first screening step, because it is quick, it needs only two measurements, and it gives a rough sense of whether a person's weight is likely to be raising their health risk. It is not a diagnosis, and it does not measure body fat directly, but it is a useful starting point if you understand what it can and cannot tell you.
Medical disclaimer: BMI is a screening tool, not a diagnosis. It does not measure body fat, muscle, bone density, or health on its own. If your BMI is outside the healthy range, or if you have a health condition, speak to a doctor or another qualified health professional before changing your diet, exercise, or treatment. The information in this article is general and educational only.
The BMI Formula: Metric and Imperial
There are two versions of the formula, one for metric units and one for imperial units. They give the same result if you convert the units correctly, so use whichever matches the measurements you have.
Metric formula
BMI = weight in kilograms ÷ (height in metres × height in metres)
Height must be in metres, not centimetres. If you know your height in centimetres, divide by 100 first. A height of 175 cm is 1.75 m. Squaring the height means multiplying it by itself, so 1.75 × 1.75 = 3.0625.
Imperial formula
BMI = 703 × weight in pounds ÷ (height in inches × height in inches)
The number 703 is a conversion factor that makes the imperial calculation give the same answer as the metric one. Height must be in inches, not feet and inches. A height of 5 feet 9 inches is 69 inches, because 5 × 12 = 60, plus 9 = 69. Squaring 69 gives 4,761.
If you prefer not to do the arithmetic by hand, use the BMI calculator. Enter your weight and height and it will work out your BMI and show which category it falls in.
Worked Example: 70 Kg and 1.75 m
This is the example most people look up, because 70 kg and 1.75 m are close to average adult measurements in many countries. Work through it step by step so you can repeat it with your own numbers.
Step 1: Write down the weight and height. Weight = 70 kg. Height = 1.75 m. Step 2: Square the height. 1.75 × 1.75 = 3.0625. Step 3: Divide the weight by the squared height. 70 ÷ 3.0625 = 22.857. Step 4: Round to one decimal place. 22.857 rounds to 22.9.
So BMI = 22.9. That is inside the healthy range, which is 18.5 to 24.9, and it is near the middle of that range.
Here is the same person in imperial units, as a check. Seventy kilograms is 154.32 lbs, and 1.75 m is 68.90 inches, which is about 5 feet 9 inches. Using 154 lbs and 69 inches: BMI = 703 × 154 ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7. The small difference from 22.9 comes from rounding the weight and height before the calculation. If you use the unrounded values, both formulas give the same answer. That is a useful check: if the metric and imperial answers are far apart, one of the conversions or one of the squarings has gone wrong.
BMI Categories: What the Numbers Mean
The World Health Organization uses the same categories for adults in most countries. They are shown in the table below.
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese, class I |
| 35.0 – 39.9 | Obese, class II |
| 40 and above | Obese, class III |
Healthy Weight by Height
It is often more useful to turn the question round: instead of asking what your BMI is, ask what weight range would put you in the healthy category at your height. The healthy range is 18.5 to 24.9, so multiply those two numbers by your height squared.
For a height of 1.75 m, height squared is 3.0625. The lower end is 18.5 × 3.0625 = 56.7 kg, and the upper end is 24.9 × 3.0625 = 76.3 kg. So a person who is 1.75 m tall is in the healthy range between about 57 kg and 76 kg.
| Height | Healthy weight range |
|---|---|
| 1.60 m (5 ft 3 in) | 47.4 – 63.7 kg |
| 1.70 m (5 ft 7 in) | 53.5 – 72.0 kg |
| 1.75 m (5 ft 9 in) | 56.7 – 76.3 kg |
| 1.80 m (5 ft 11 in) | 59.9 – 80.7 kg |
| 1.90 m (6 ft 3 in) | 66.8 – 89.9 kg |
Check the 1.60 m row as an example: 1.60 × 1.60 = 2.56, 18.5 × 2.56 = 47.36 kg, and 24.9 × 2.56 = 63.74 kg. The table rounds those to one decimal place. If you are near the top or bottom of your range, a small change in weight moves you across a boundary, which is another reason to treat the categories as guides rather than sharp dividing lines.
If you want to see what else your weight and height imply, the BMR calculator estimates how many calories your body uses at rest, using the same basic measurements in a different formula.
Limitations: What BMI Does Not Tell You
BMI has real limits, and knowing them is part of using it well. The most important limit is that it does not distinguish muscle from fat. A rugby player or a weightlifter can have a BMI over 30, which the table calls obese, while having very little body fat. A person with little muscle and a lot of body fat can have a BMI in the healthy range while still carrying a health risk. BMI measures weight relative to height, not body composition.
A second limit is that it does not say where fat is stored. Fat around the waist is more closely linked to health risk than fat on the hips or thighs, but two people with the same BMI can have very different waist measurements. Waist circumference, waist-to-height ratio, and body-fat percentage are better measures for that question, and a doctor can measure them directly.
A third limit is that the same BMI can mean different things in different populations. The categories were developed largely from data on European populations, and some health authorities use lower cut-offs for Asian populations, where health risk can rise at a lower BMI. Age, sex, and pregnancy also affect how BMI should be read. None of this makes BMI useless, but it does mean it should be used alongside other information, not on its own.
How to Measure Yourself Correctly
Weigh yourself in the morning, before eating, on a hard floor, and measure height barefoot against a wall. Small errors matter: two centimetres of height changes a BMI of 22.9 to about 22.4 at 70 kg. Measure both values on the same day, under the same conditions, when tracking change over time.
Try it: Work out your BMI and see your category with the BMI Calculator — enter your weight and height, and it does the division for you.
Frequently Asked Questions
How do I calculate BMI? Divide your weight in kilograms by your height in metres squared. For 70 kg and 1.75 m, that is 70 ÷ (1.75 × 1.75) = 22.9. In imperial units, use BMI = 703 × weight in pounds ÷ (height in inches × height in inches).
What is a good BMI? A BMI of 18.5 to 24.9 is classed as healthy weight for most adults. Below 18.5 is underweight, 25.0 to 29.9 is overweight, and 30 and above is obese. These are screening categories, not a diagnosis on their own.
What is 70 kg and 1.75 m in BMI? It is 22.9. The calculation is 70 ÷ 3.0625 = 22.857, rounded to 22.9, which is in the middle of the healthy range. At 1.75 m, the healthy weight range is about 56.7 kg to 76.3 kg.
Is BMI accurate? BMI is useful as a screening tool, but it does not measure body fat directly and does not distinguish muscle from fat. Athletes can have a high BMI with little body fat, and the same BMI can mean different risks in different people, so it should be used alongside other measures.
