Health & Fitness
BMI Calculator
Work out your body mass index and see the healthy weight range for your height, in metric or imperial.
BMI Calculator
About the BMI Calculator
Body mass index compares your weight to your height. Divide weight in kilograms by height in metres squared and you get a single number that public health bodies use to sort adults into weight categories. This calculator does that in metric or imperial, and shows the healthy weight range for your exact height.
BMI is also called the Quetelet index, after the Belgian statistician Adolphe Quetelet, who described it in the 1830s.
The number is genuinely useful at population scale, which is what it was designed for. It is a rough guide for any one individual, and the page below is honest about where it breaks down — muscular builds, older adults, and different ethnic backgrounds all sit awkwardly with a single set of thresholds.
BMI applies to adults aged 20 and over. Children and teenagers are assessed on percentiles for their age and sex instead, which the child BMI calculator handles.
How to use the BMI Calculator
- Choose metric or imperial units.
- Enter your height — in centimetres, or feet plus inches.
- Enter your weight in kilograms or pounds.
- Select Calculate BMI to see your index, classification and healthy weight range.
- The coloured scale shows where your reading falls between 15 and 40.
The BMI formula
Metric : BMI = weight (kg) ÷ height (m)²
Imperial : BMI = 703 × weight (lb) ÷ height (in)²
The 703 is just the unit conversion — both give the same answer.
Worked example
Someone 1.75 m tall weighing 78 kg.
Height squared: 1.75 × 1.75 = 3.0625.
78 ÷ 3.0625 = 25.5
BMI 25.5 — just inside the overweight band
The healthy range for that height is 56.7 kg to 76.3 kg, so this person sits about 1.7 kg above it. Square the height first — dividing by 1.75 instead of 3.0625 gives 44.6, which is nonsense.
The WHO classification bands
| BMI | Category |
|---|---|
| Below 16.0 | Severe underweight |
| 16.0 – 18.4 | 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 |
| 40.0 and above | Obese, class III |
These thresholds were derived largely from European-ancestry populations. The WHO notes that health risk rises at lower BMI values in many Asian populations, and several countries use adjusted cut-offs — often around 23 for overweight and 27.5 for obesity. If that applies to you, read the standard bands with that in mind and ask your doctor which apply.
Healthy weight range by height
Rather than a single target, BMI gives a range — anything from 18.5 to 24.9. Here is what that means in kilograms:
| Height | Healthy range (kg) | Healthy range (lb) |
|---|---|---|
| 1.55 m (5'1") | 44.4 – 59.8 | 98 – 132 |
| 1.60 m (5'3") | 47.4 – 63.7 | 104 – 141 |
| 1.65 m (5'5") | 50.4 – 67.8 | 111 – 149 |
| 1.70 m (5'7") | 53.5 – 72.0 | 118 – 159 |
| 1.75 m (5'9") | 56.7 – 76.3 | 125 – 168 |
| 1.80 m (5'11") | 59.9 – 80.7 | 132 – 178 |
| 1.85 m (6'1") | 63.3 – 85.2 | 140 – 188 |
| 1.90 m (6'3") | 66.8 – 89.9 | 147 – 198 |
The ranges are wide on purpose. A 1.75 m person can sit anywhere across a 20 kg span and still be classified as a healthy weight, which reflects how much natural variation there is in build.
Where BMI stops working
BMI measures weight against height. It has no way of knowing what that weight is made of, and that is the root of every limitation:
- Muscle weighs more than fat by volume. A trained athlete can register as overweight or obese while carrying very little body fat.
- It ignores fat distribution. Abdominal fat carries more metabolic risk than fat on the hips, and BMI cannot tell them apart. Waist circumference adds information BMI misses.
- Older adults lose muscle. Someone may hold a steady BMI while body composition shifts unfavourably underneath it.
- It was built on one population. Risk thresholds genuinely differ across ethnic groups.
- Pregnancy makes it meaningless. Standard BMI does not apply. For pregnant women, doctors use the BMI from before pregnancy to set a healthy weight-gain range — roughly 25–35 lb for a healthy-weight BMI and 15–25 lb for an overweight BMI.
For a measure that estimates composition rather than mass, use the body fat calculator. For a target weight, the ideal weight calculator shows four accepted formulas side by side.
Using BMI sensibly
Treat it as one reading among several, in the same way you would treat a single blood pressure measurement. It is most useful for spotting a trend over months, and least useful as a verdict on a single day.
If your BMI sits outside the healthy band, the practical next step is not a diet plan from a calculator. It is a conversation with a doctor who can look at your waist measurement, blood pressure, blood sugar, family history and activity levels together — all of which say more about health than BMI does alone.
This is a screening tool, not a diagnosis. It cannot see your muscle mass, bone density, medical history or family background, and it does not replace an examination. If you are concerned about your weight or health, speak to a doctor who can assess you properly.
BMI chart for women (ladies) and men
For adults aged 20 and over, the BMI bands are the same for women and men. There is no separate "ladies' BMI chart". What differs is body make-up: at the same BMI, women usually carry more body fat than men. So a BMI chart for women and a chart for men look identical:
| BMI | Category | At 5'5" (165 cm) | At 5'10" (178 cm) |
|---|---|---|---|
| Below 18.5 | Underweight | under 111 lb (50 kg) | under 129 lb (59 kg) |
| 18.5–24.9 | Healthy weight | 111–149 lb | 129–173 lb |
| 25–29.9 | Overweight | 150–179 lb | 174–208 lb |
| 30 and above | Obese | 180 lb and up | 209 lb and up |
Want a sense of body fat, not just weight? The body fat calculator uses a tape measure and treats men and women differently.
BMI formula in pounds and inches
Using imperial units? Multiply by 703 to convert:
BMI = weight (lb) ÷ height (in)² × 703
Example: 160 lb at 5'7" (67 inches). 67² = 4,489. 160 ÷ 4,489 × 703 = 25.1 — just into the overweight band.
NHS BMI ranges and ethnicity
The NHS uses the same formula and the same WHO bands. But UK guidance from NICE adds lower thresholds for some groups. People of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background have a higher health risk at a lower BMI.
- Overweight starts at a BMI of 23 instead of 25.
- Obesity starts at 27.5 instead of 30.
If this applies to you, read your result against these lower lines.
BMI for women over 50 and older adults
The adult bands do not change with age. Still, as people get older they tend to lose muscle and gain fat, so the same BMI can hide a different body. Some research suggests a BMI slightly above 25 is not linked to higher risk in older adults. Waist size and strength are useful extra checks — talk to your doctor about what is right for you.
Lean body mass calculator and FFMI
BMI cannot tell muscle from fat. Lean body mass (LBM) can. It is everything except fat: muscle, bone, organs and water. The Boer formula estimates it from height and weight:
Men : LBM = 0.407 × weight (kg) + 0.267 × height (cm) − 19.2
Women: LBM = 0.252 × weight (kg) + 0.473 × height (cm) − 48.3
FFMI = lean body mass (kg) ÷ height (m)²
For a man of 78 kg and 175 cm, LBM is about 59.3 kg and the result of a fat free mass index calculator (FFMI) is 59.3 ÷ 3.0625 = 19.4. An FFMI of 18–20 is typical for men who do not train; 21–23 suggests a lot of muscle. That is why a muscular athlete can have a “high” BMI and still be lean.
The lean body mass formula is only an estimate. For a figure based on your own measurements, use the body fat calculator, which also works out fat mass and lean mass.
Army height and weight calculator vs BMI
The US Army does not use a plain army BMI calculator. It screens soldiers with a height and weight table that sets a maximum weight for each height, age and sex. Soldiers above it take a tape test to estimate body fat. An athlete with a BMI over 25 can still pass, because the tape test looks at waist size, not just weight.
The body fat calculator includes the Army one-site tape formula. Standards change, so always check the current Army regulation before a weigh-in.
Other ways to use this calculator
The same BMI formula serves many purposes.
- BMI calculator for women and men — adult bands are the same for both sexes; see the chart above.
- NHS-style BMI calculator — the UK uses the same formula and bands, with lower thresholds for some ethnic groups.
- BMI calculator in pounds and inches — switch to Imperial units.
- BMI calculator for kids — under 20s need a percentile instead — use the child BMI calculator.
- Healthy weight calculator — the result shows the healthy weight range for your height.
- BMI metric imperial — switch units; the BMI formula in pounds multiplies by 703.
- WHO BMI classification — the bands table above shows the WHO categories.
- BMI chart women and BMI calculator men — adults of both sexes use the same chart.
- BMI calculator NHS — the NHS uses the same formula, with lower thresholds for some ethnic groups.
Frequently asked questions
For most adults, a BMI between 18.5 and 24.9 is classified as a healthy weight by the World Health Organization. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above falls into the obese categories. These thresholds came largely from European-ancestry populations, and several countries use lower cut-offs for Asian populations, so ask your doctor which apply to you.
Often not. BMI compares weight to height and cannot tell muscle from fat, so a trained athlete may register as overweight or even obese while carrying very little body fat. Rugby players and weightlifters are the classic examples. If you carry noticeably more muscle than average, a body fat estimate or a waist measurement will describe you far better than BMI does.
Divide your weight in kilograms by your height in metres squared. At 78 kg and 1.75 m: 1.75² is 3.0625, and 78 ÷ 3.0625 = 25.5. In imperial, multiply pounds by 703 and divide by height in inches squared. Square the height before dividing — that single step is where most hand calculations go wrong.
Because children are still growing, and the same BMI means different things at different ages. A BMI of 18 is healthy for a fifteen-year-old and high for a five-year-old. Children and teenagers are assessed against age-and-sex percentiles from a growth reference instead of fixed adult thresholds — our child BMI calculator does exactly that for ages 2 to 20.
No. Adults aged 20 and over use the same BMI bands whether they are women or men: 18.5 to 24.9 is a healthy weight. Women tend to have more body fat at the same BMI, which is why body fat measures treat the sexes separately.
Divide your weight in pounds by your height in inches squared, then multiply by 703. At 160 lb and 67 inches tall, the BMI is 160 ÷ 4,489 × 703 = 25.1.
The Boer formula for men is 0.407 times weight in kg, plus 0.267 times height in cm, minus 19.2. For women it is 0.252 times weight, plus 0.473 times height, minus 48.3. A 78 kg, 175 cm man has about 59.3 kg of lean body mass.
Sources and further reading
- World Health Organization — Obesity and overweight — the adult BMI classification bands used here
- WHO — Appropriate BMI for Asian populations (The Lancet, 2004) — why lower cut-offs apply in some populations
- NHLBI — Assessing your weight and health risk — BMI alongside waist circumference and risk factors
- NHS — BMI and healthy weight — practical guidance and limitations
- NICE — Overweight and obesity management — lower BMI thresholds for some ethnic groups