How do I work out my BMI?
For adults: weight in kg, divided by height in metres squared. Same sum for women and men. Your family background can move the lines: a BMI of 28 is overweight for some, obesity for others.
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How do I calculate my BMI?
The NHS works out adult BMI as your weight in kilograms divided by your height in metres squared. Squared just means times itself.
You need two numbers: your height and your weight. Here’s the NHS’s own example.
Work out your BMI in three steps
- 1
Square your height
Times your height in metres by itself. 1.70 x 1.70 = 2.89.
- 2
Divide your weight by it
Your weight in kg goes on top. 70 ÷ 2.89 = 24.22.
- 3
Round to one decimal place
So 24.22 becomes a BMI of 24.2.
Source: NHS
Now do it with your own numbers. Or use the NHS BMI calculator, which also asks about your ethnic background.
Got your number? Find it on the scale further down.
How do I work out my BMI in stone and feet?
Change both numbers to metric first. One stone is 6.35 kg, and one pound is about 0.45 kg.
For height, find your row below. The last column has done step one for you.
| Height | In metres | Squared |
|---|---|---|
| 4ft 11in | 1.50 m | 2.25 |
| 5ft 1in | 1.55 m | 2.40 |
| 5ft 3in | 1.60 m | 2.56 |
| 5ft 5in | 1.65 m | 2.72 |
| 5ft 7in | 1.70 m | 2.89 |
| 5ft 9in | 1.75 m | 3.06 |
| 5ft 11in | 1.80 m | 3.24 |
| 6ft 1in | 1.85 m | 3.42 |
| 6ft 3in | 1.91 m | 3.65 |
Source: NHS method
Say you weigh 12 stone and you’re 5ft 5in. 12 x 6.35 = 76.2 kg. Then 76.2 ÷ 2.72 = 28.0.
Between two rows? Measure your height in cm, divide by 100, and square that.
What does my BMI result mean?
For most adults, NICE puts a healthy weight at a BMI of 18.5 to 24.9. Overweight is 25 to 29.9. Obesity is 30 or more.
Below 18.5 counts as underweight in the NHS health survey for England.
- Below 18.5 kg/m²Underweight.
- 18.5 to just under 25 kg/m²Healthy weight.
- 25 to just under 30 kg/m²Overweight.
- 30 kg/m² or aboveObesity.
Sources: NICE NG246; NHS; Health Survey for England
So the 28 from the stone example is overweight for most adults. The NHS says a higher BMI raises the chance of type 2 diabetes and heart disease.
For a number-by-number read, see what each BMI number means.
Is BMI different for women, men and age?
Not for women and men. NICE gives one set of adult BMI lines. The NHS adult calculator asks your height and weight, not your sex.
Your family background is the thing that can change the lines. People from some backgrounds have a higher chance of health problems at a lower BMI.
So that BMI of 28 is overweight for most adults. For these backgrounds, it’s obesity.
| Who | What changes | What to do |
|---|---|---|
| Women and men aged 18 and over | Nothing. Same sum, same lines. | Use the three steps above. |
| South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background | Overweight from 23, obesity from 27.5. | Read your result against these lines. |
| Aged 65 and over | NICE says read BMI with caution, alongside other health conditions. A slightly higher BMI may be protective when older. | Ask your GP how to read yours. |
| Aged 2 to 17 | The NHS child tool uses date of birth and sex, against UK growth charts. | Use the NHS child BMI calculator. Under 2: ask your GP or health visitor. |
| Pregnant | The NHS adult calculator is not for you (nor if you have a condition that affects your height). | If you’re worried about your weight, ask a pharmacist or your GP surgery. |
Sources: NICE NG246; NHS adult BMI; NHS child BMI
Find your row, then go back to the scale with the right lines in mind.
What should I do with my BMI result?
First, know its blind spot. BMI can’t tell muscle from fat. With a lot of muscle, you may show as overweight or obese despite having low body fat.
So check your waist too. Here’s how to measure your waist. For a rough fat figure, try the tape measure body fat sum.
Your BMI next steps
- Work out your BMI: square your height in metres, then divide your weight in kg by it. Or use the NHS BMI calculator.
- If your background is South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean, read your result against the lower lines: overweight from 23, obesity from 27.5.
- If your BMI is under 35, try to keep your waist to less than half your height.
- Overweight or obese for you (25 or more, or 23 or more for the backgrounds above)? Losing 5 to 10% of your body weight can improve your health.
- Aged 65 or over? Ask your GP how to read your BMI.
Sources: NHS; NICE NG246
Want your healthy range in kg for your height? Use this healthy weight chart.
Below 18.5? The NHS says being underweight could mean you’re missing out on vitamins and minerals. Try these healthy weight gain foods.
When to speak to a GP
The NHS says to see a GP if:
- Your BMI is overweight or obese and you want help to lose weight, or obesity runs in your family.
- Your BMI is overweight or obese and your waist is more than half your height.
- Your BMI is overweight or obese and you have pre-diabetes (blood sugar higher than normal, but not yet diabetes), diabetes, high blood pressure, joint pain, high cholesterol, sleep apnoea (breathing that stops and starts as you sleep) or heart and circulation disease.
- You’ve lost weight suddenly, or without changing your diet or exercise routine.
- Your BMI is overweight or obese and you have, or think you may have, an eating disorder, such as binge eating. That means eating large amounts in one go and feeling out of control.
- You use control of food to cope with feelings and other situations.
Have, or think you may have, an eating disorder? The NHS says not to use its adult BMI calculator. If you’re worried about your weight, contact a pharmacist or your GP surgery.