In short: body mass index (BMI) is your weight in kilograms divided by your height in meters squared. In pounds and inches it is weight × 703 ÷ height². For example, 154 lb at 5’9” (69 in): 154 × 703 ÷ (69 × 69) ≈ 22.7. WHO defines a BMI of 25 or more as overweight and 30 or more as obesity in adults1. But BMI cannot see where fat sits, and it cannot tell muscle from fat. That is why the second number is your waist: the NHS advises keeping it under half your height3.
To work out your own BMI, healthy weight range, and waist-to-height ratio, use the BMI calculator.
How to calculate BMI
- Multiply your height in inches by itself: 5’6” is 66 in, and 66 × 66 = 4,356.
- Multiply your weight in pounds by 703: 165 lb × 703 = 115,995.
- Divide the second number by the first: 115,995 ÷ 4,356 ≈ 26.6.
In metric it is weight (kg) ÷ height² (m²), the formula WHO uses1. It does not work the same way for children and teenagers, whose BMI is compared with age-based charts1.
What the numbers mean
| BMI | Category for most adults |
|---|---|
| under 18.5 | underweight |
| 18.5–24.9 | healthy weight4 |
| 25–29.9 | overweight1,2 |
| 30 or more | obesity1,2 |
For people of South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean background, the NHS uses lower cutoffs: 23–27.4 is overweight and 27.5 or more is obesity2. Health risks start to rise at a lower BMI in these groups.
BMI chart by height
We worked out from the formula which weights match each BMI cutoff. Weights are rounded to the whole pound so they stay inside each range.
| Height | BMI 18.5–24.9 | BMI 25 from | BMI 30 from | Half your height (keep your waist under it) |
|---|---|---|---|---|
| 5’0” (152 cm) | 95–127 lb | 129 lb | 154 lb | 30 in |
| 5’2” (157 cm) | 102–136 lb | 137 lb | 165 lb | 31 in |
| 5’4” (163 cm) | 108–145 lb | 146 lb | 175 lb | 32 in |
| 5’6” (168 cm) | 115–154 lb | 155 lb | 186 lb | 33 in |
| 5’8” (173 cm) | 122–163 lb | 165 lb | 198 lb | 34 in |
| 5’10” (178 cm) | 129–173 lb | 175 lb | 210 lb | 35 in |
| 6’0” (183 cm) | 137–183 lb | 185 lb | 222 lb | 36 in |
| 6’2” (188 cm) | 145–193 lb | 195 lb | 234 lb | 37 in |
| 6’4” (193 cm) | 152–204 lb | 206 lb | 247 lb | 38 in |
What BMI cannot see
- Muscle versus fat. The NHS says it plainly: a BMI calculator cannot tell muscle from fat. Someone with a lot of muscle can land in the overweight range with little body fat2.
- Where the fat sits. The same BMI can come with fat on the hips or with fat around the organs in the belly. The second is worse for health, and the waist shows it. More in exercise and visceral fat.
- Pregnancy and childhood. The usual adult cutoffs do not apply.
Your waist: the second number
The NHS rule is simple: keep your waist under half your height3. At 5’8” (68 in), that means under 34 inches. The NHS suggests this measure for adults with a BMI under 352.
The NHLBI (US National Heart, Lung, and Blood Institute) also gives fixed cutoffs: risk goes up with a waist over 35 inches for women and over 40 inches for men4.
How to measure, per the NHLBI: stand, wrap a tape measure around your middle just above your hip bones, and read it right after you breathe out4. Do not suck in your stomach or pull the tape tight.
What to do with the result
A number is a prompt, not a diagnosis. The NHS suggests seeing a doctor if your BMI is in the overweight or obese range and at least one of these applies: your waist is more than half your height; you have prediabetes or diabetes, high blood pressure, or joint pain; or obesity runs in your family2. The same page notes that losing even 5–10% of your body weight can improve your health2.
Where to start without a crash diet is covered in losing weight without crash diets and calorie deficit in plain language. The simplest movement to start with is walking.
Common questions
I have a lot of muscle and my BMI is over 25. Does that mean obesity?
Not necessarily. BMI cannot tell muscle from fat2. Your waist says more here: if it is under half your height, there is usually less reason to worry.
How often should I recalculate?
Body weight moves by a pound or more from day to day, so a BMI from a single morning says little. Use your weekly average: why the scale jumps. Measuring your waist once a month, under the same conditions, is enough.
Sources
- WHO fact sheet: obesity and overweight. The BMI formula and the adult cutoffs of 25 and 30.
- NHS: overweight and obesity in adults. BMI cutoffs, lower cutoffs by ethnic background, the limits of the calculator, and when to see a GP.
- NHS: waist-to-height ratio calculator. Keep your waist under half your height.
- NHLBI: aim for a healthy weight. BMI 18.5–24.9, waist cutoffs, and how to measure your waist.
What readers think
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