BMI Calculator – Check Your Body Mass Index in kg, cm, Stones or Pounds

This free BMI calculator works out your body mass index from your height and weight in seconds, and tells you what the number actually means. Switch between metric and imperial, add your age and sex for context, and see the healthy weight range for your exact height next to the full BMI chart. The weight categories this body mass index calculator uses follow the classification published by the World Health Organization and the US Centers for Disease Control and Prevention.

BMI is a screening tool, not a diagnosis. It gives you a fast, useful signal about whether your weight is in a healthy range for your height — and a sensible reason to book a conversation with your GP if it is not. What it cannot do is tell muscle from fat, and that limitation matters more for some people than others. We cover exactly who it misreads further down.

What This BMI Calculator Gives You

Enter your height and weight once and the calculator returns six results together. There is no separate BMI test to run and no need to look your BMI index up on a chart afterwards.

• Your BMI, to one decimal place, from the standard WHO formula.
• Your weight category against the BMI normal range: underweight, healthy weight, overweight, or obesity class I, II or III.
• Your healthy weight range in the units you entered, so the target is a weight rather than an abstract number.
• How far you are from that range, shown in kilograms, in stones and pounds, or in pounds.
• Your BMI Prime — your BMI divided by 25. Under 1.00 means you are inside the healthy range; 1.20 means 20 per cent above the upper limit.
• A position marker on the BMI scale, so you can see where you sit rather than only read a number.

Three unit systems are supported: centimetres with kilograms, feet and inches with stones and pounds, or feet and inches with pounds. Age and sex are optional — they change the guidance shown beside your result, not the BMI figure itself. You can also switch to the lower NICE thresholds where they apply.

How Do I Calculate My BMI? The Formula Explained

To calculate BMI, divide your weight in kilograms by your height in metres squared. That is the whole formula. A person who is 170 cm tall and weighs 68 kg has a BMI of 68 ÷ (1.70 × 1.70) = 23.5.

Metric BMI formula (kg and cm)

BMI = weight (kg) ÷ height (m)²

Convert centimetres to metres first by dividing by 100 — 170 cm becomes 1.70 m. This is the form used by the WHO Global Health Observatory.

Imperial BMI formula (pounds and inches)

BMI = [weight (lb) ÷ height (in)²] × 703

The 703 is a conversion constant that makes the imperial result match the metric one. Convert your height to total inches first: 5′ 7″ is (5 × 12) + 7 = 67 inches. The National Heart, Lung, and Blood Institute publishes the same formula.

Working in stones? Multiply stones by 14 to get pounds, then add the spare pounds. 11st 5lb is (11 × 14) + 5 = 159 lb. The calculator above does all of this for you, in whichever units you think in.

BMI Chart: Weight Categories and What Each Range Means

Adults aged 20 and over are read against a single BMI chart, regardless of age or sex. These are the categories used by the WHO, the CDC, the NHS and the HSE in Ireland.

BMI range	Category	What it indicates
Below 18.5	Underweight	Possible undernutrition, reduced bone density or an underlying condition worth investigating
18.5 – 24.9	Healthy weight	The range associated with the lowest weight-related health risk for most adults
25.0 – 29.9	Overweight	Raised risk of type 2 diabetes, high blood pressure and heart disease
30.0 – 34.9	Obesity class I	Moderate risk — worth a clinical conversation
35.0 – 39.9	Obesity class II	High risk
40.0 and above	Obesity class III	Very high risk, sometimes called severe obesity

BMI chart by height: your healthy weight range in kg, stones and pounds

A BMI number on its own is abstract. This table converts the healthy BMI band (18.5 to 24.9) into an actual target weight for each height, in every unit you might use.

Height	Healthy weight (kg)	Healthy weight (st/lb)	Healthy weight (lb)
145 cm (4′ 9″)	39 – 52 kg	6st 2lb – 8st 3lb	86 – 115 lb
150 cm (4′ 11″)	42 – 56 kg	6st 8lb – 8st 12lb	92 – 124 lb
155 cm (5′ 1″)	44 – 60 kg	7st 0lb – 9st 6lb	98 – 132 lb
160 cm (5′ 3″)	47 – 64 kg	7st 6lb – 10st 1lb	104 – 141 lb
165 cm (5′ 5″)	50 – 68 kg	7st 13lb – 10st 9lb	111 – 149 lb
170 cm (5′ 7″)	53 – 72 kg	8st 6lb – 11st 5lb	118 – 159 lb
175 cm (5′ 9″)	57 – 76 kg	8st 13lb – 12st 0lb	125 – 168 lb
180 cm (5′ 11″)	60 – 81 kg	9st 6lb – 12st 10lb	132 – 178 lb
185 cm (6′ 1″)	63 – 85 kg	10st 0lb – 13st 6lb	140 – 188 lb
190 cm (6′ 3″)	67 – 90 kg	10st 7lb – 14st 2lb	147 – 198 lb
195 cm (6′ 5″)	70 – 95 kg	11st 1lb – 14st 13lb	155 – 209 lb

Note how wide each band is. At 5′ 7″ the healthy range spans nearly three stone. That spread is deliberate — frame size, muscle mass and bone density all vary, and BMI is not precise enough to name one correct weight.

What Is a Healthy BMI?

For most adults a healthy BMI is between 18.5 and 24.9. That band is where population studies show the lowest rate of weight-related illness. But the threshold is not universal, and two adjustments matter.

Lower thresholds for some ethnic groups

People of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background develop type 2 diabetes and cardiovascular disease at a lower BMI than white European populations. NICE therefore sets the increased-risk threshold at BMI 23.0 and the high-risk threshold at 27.5 for these groups, rather than 25 and 30. The calculator above applies these lower cut-offs if you select that option.

Waist measurement matters as much as BMI

Where you store fat predicts risk better than how much you weigh. Fat around the abdomen is more metabolically harmful than fat on the hips and thighs. The NHS advises keeping your waist under 94 cm (37″) for men and 80 cm (31.5″) for women, whatever your BMI says. Measure halfway between the bottom of your ribs and the top of your hips, breathing out normally.

BMI Calculator for Women

The BMI formula and the 18.5–24.9 healthy range are identical for women and men. There is no separate female BMI calculation — anyone selling you one is selling you nothing. What differs is what the same number represents inside the body.

At an identical BMI, women typically carry a higher body fat percentage than men, because essential fat requirements are higher for female physiology. A woman and a man both at BMI 23 can differ by roughly ten percentage points of body fat. This is normal and healthy, not a problem to correct — but it does mean BMI slightly understates body fat in women and overstates it in men.

Three situations where BMI is unreliable for women specifically:

• Pregnancy. Standard BMI categories do not apply. Weight gain is expected and necessary, and your midwife tracks it against your pre-pregnancy BMI instead.
• Menopause. Falling oestrogen shifts fat storage from hips to abdomen. BMI can stay unchanged while cardio-metabolic risk rises, so waist measurement becomes the more informative number.
• Normal cycle fluctuation. Fluid retention can move the scale by one to two kilograms across a month. Weigh at the same point in your cycle if you are tracking change.

BMI Calculator for Men

Men use the same formula and the same thresholds. The dominant failure mode for men is the opposite of the one for women: BMI routinely misclassifies muscular men as overweight or obese, because it cannot distinguish lean mass from fat mass.

A 180 cm rugby player at 95 kg has a BMI of 29.3 — firmly "overweight" on paper, while carrying 12% body fat. The number is arithmetically correct and clinically meaningless. If you strength train seriously, treat BMI as background noise and use body fat percentage or waist-to-height ratio instead.

The reverse trap is quieter and more common. A sedentary man with low muscle mass can sit at BMI 24, inside the healthy range, while carrying 28% body fat around the abdomen — a pattern sometimes described as normal-weight obesity. A normal BMI is not by itself evidence of metabolic health.

What Is the Correct BMI for My Age?

For adults aged 20 and over, the healthy BMI range is 18.5–24.9 at every age. There is no official age-adjusted BMI table for adults, and charts claiming otherwise are inventing numbers. The CDC and WHO apply one adult standard. Age changes the interpretation, not the threshold.

Adults over 65

Here the evidence genuinely becomes less clear-cut. Muscle mass declines with age, so an older adult can hold a normal BMI while having lost significant lean tissue and gained fat — the number stays still while body composition deteriorates underneath it. Several large cohort studies have also found that a BMI slightly above 25 is associated with better survival in older adults, possibly because some reserve helps during illness. Being underweight carries clear risks in this group: frailty, falls and poor recovery. If you are over 65, unintentional weight loss is more worth raising with your GP than a BMI of 26.

Under 20

Adult categories do not apply at all. See the next section.

BMI for Children and Teenagers

For anyone aged 2 to 19, BMI is read as a percentile against growth charts for that exact age and sex, not against the adult table. Children's body composition changes constantly through growth and puberty, so a raw BMI figure means nothing without that comparison.

BMI-for-age percentile	Category
Below 5th percentile	Underweight
5th to below 85th	Healthy weight
85th to below 95th	Overweight
95th percentile and above	Obesity

Do not use the adult calculator on this page to assess a child. Ask your GP or practice nurse to plot the measurement on the correct growth chart, where it can be read alongside the child's own growth history.

BMI Calculator Ireland: Local Guidance and Thresholds

This BMI calculator works for Ireland without adjustment. Ireland uses the standard WHO classification, so the ranges on this page apply directly. The HSE provides weight management guidance and can refer you into structured support through your GP. If your BMI falls outside the healthy range, a GP visit is the right next step — they can check blood pressure, HbA1c and lipids, which tell you far more than BMI alone.

Irish adults using stones and pounds can switch the calculator to imperial units above; the healthy weight table earlier on this page also lists every height in st/lb.

Where BMI Gets It Wrong

BMI was devised in the 1830s by Adolphe Quetelet as a tool for describing populations, not for assessing individuals. It survives because it is cheap, instant and needs no equipment — not because it is accurate. It will mislead you if you fall into any of these groups:

• Muscular and athletic people — muscle is denser than fat, so BMI reads high despite low body fat.
• Older adults — age-related muscle loss means a normal BMI can hide a high fat percentage.
• Pregnant and breastfeeding women — the categories do not apply.
• Children and teenagers — percentile charts are required instead.
• Some ethnic groups — risk rises at a lower BMI, so standard thresholds understate it.
• Anyone with a very tall or very short frame — the height-squared term distorts at the extremes, overstating BMI in tall people and understating it in short people.

The single biggest gap is that BMI says nothing about where fat sits. Visceral fat packed around the liver and pancreas drives metabolic disease; subcutaneous fat on the hips is comparatively benign. Two people at BMI 28 can have very different risk profiles. A tape measure around your waist costs nothing and closes much of that gap.

What to Measure Alongside BMI

Use BMI as one reading among several rather than a verdict:

• Waist-to-height ratio — keep your waist under half your height. Simple, and a better predictor of cardio-metabolic risk than BMI.
• Body fat percentage — separates lean mass from fat mass, which is exactly what BMI cannot do.
• Basal metabolic rate — what your body burns at rest, and the starting point for any change in weight.
• Daily calorie needs — turns a target into a plan you can actually follow.
• Blood pressure, HbA1c and lipids — the measures that actually describe metabolic health. Your GP can run all three.

If your BMI is outside the healthy range

Book a GP appointment rather than starting a drastic plan from a web page. Modest, sustained change works better than aggressive change: losing 5–10% of body weight produces measurable improvements in blood pressure, blood glucose and cholesterol, and that is achievable without anything extreme. If your BMI is below 18.5, unintentional weight loss or difficulty gaining weight deserves investigation rather than a diet plan.

Medical Disclaimer

This BMI calculator and the information on this page are provided for general informational and educational purposes only. They are not medical advice, diagnosis or treatment, and are not a substitute for consultation with a qualified healthcare professional. BMI is a population-level screening tool: it does not account for muscle mass, bone density, fat distribution, ethnicity, pregnancy or individual medical history, and it is not valid for children, teenagers or pregnant women. Never disregard professional medical advice or delay seeking it because of something you have read here. Always consult your GP, a registered dietitian or another qualified clinician before making decisions about your weight, diet, medication or exercise. If you are concerned about your relationship with food or your body, speak to your GP.

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Frequently Asked Questions About BMI

1. How do I calculate my BMI?

Divide your weight in kilograms by your height in metres squared. For 68 kg at 1.70 m: 68 ÷ (1.70 × 1.70) = 23.5. In imperial units, divide your weight in pounds by your height in inches squared and multiply by 703. Both methods give the same answer. The calculator at the top of this page handles either, including stones and pounds. Formula per the CDC and the NHLBI.

2. How do I calculate my BMI ranges?

Work backwards from the formula. Multiply 18.5 by your height in metres squared for the bottom of your healthy range, and 24.9 by the same figure for the top. At 1.70 m that gives 53 kg to 72 kg. The healthy weight table earlier on this page lists these ranges for every height in kg, stones and pounds. Boundaries follow the WHO classification.

3. What is the correct BMI for my age?

For every adult aged 20 and over it is 18.5 to 24.9 — the threshold does not change with age. Charts offering age-specific adult BMI targets are not based on any official standard. Age does change interpretation: over 65, muscle loss can mask a high fat percentage behind a normal BMI, and being underweight carries greater risk than being slightly above 25. Under 20, BMI must be read as a percentile instead. See the CDC adult BMI categories.

4. How is BMI of a human body calculated and what does it actually measure?

BMI measures weight relative to height — nothing else. It does not measure body fat, muscle, bone density or fat distribution, and it never has. It was designed in the 1830s to describe populations, and it is used today because it needs only a scale and a tape measure. That makes it a reasonable first screen and a poor final answer. For body composition you need a different measurement, such as the US Navy body fat method. Background per the WHO.

5. What is a healthy BMI?

18.5 to 24.9 for most adults. If you are of South Asian, Chinese, other Asian or Middle Eastern background, NICE sets increased risk from 23.0 and high risk from 27.5, because metabolic risk rises at a lower BMI in these groups. Pair whichever threshold applies with your waist measurement — under 94 cm for men, under 80 cm for women per NHS guidance.

6. Does BMI differ for men and women?

The formula and the thresholds are identical. The interpretation differs: at the same BMI, women carry a higher body fat percentage than men as a normal feature of physiology, so BMI slightly understates fat in women and overstates it in muscular men. There is no separate female or male BMI calculation in any official guidance, including the CDC adult BMI tool.

7. Why is my BMI high even though I exercise regularly?

Because BMI counts muscle as weight and cannot tell it apart from fat. Muscle is roughly 18% denser, so building it raises BMI while lowering body fat — the number moves the wrong way as you get fitter. A 180 cm man at 95 kg scores 29.3 whether he is a sprinter or sedentary. If you train regularly, use body fat percentage or waist-to-height ratio instead. The NHS notes the same limitation for muscular adults.

8. Can I use a BMI calculator while pregnant?

No. Adult BMI categories do not apply during pregnancy, when weight gain is both expected and necessary. Your midwife or obstetric team tracks gain against your pre-pregnancy BMI and your individual circumstances instead. Do not use the ranges on this page to judge your weight while pregnant or in the early postpartum period. See NICE antenatal care guidance.

9. How often should I check my BMI?

Every few months is plenty for most people. BMI moves slowly, and checking daily tells you about fluid and food in your gut, not body composition. If you are actively changing your weight, monthly is enough to see a real trend. Weigh at the same time of day under the same conditions, and judge the trend rather than any single reading. Routine weight checks also form part of a standard GP review via the HSE.

10. What should I do if my BMI is too high or too low?

Treat it as a prompt for a GP appointment, not a diagnosis. Your GP can check blood pressure, HbA1c and cholesterol, which describe metabolic health far better than BMI does, and rule out conditions behind unexplained weight change. If weight loss is appropriate, losing 5–10% of body weight delivers measurable improvement in those markers without anything drastic. A BMI below 18.5, or weight you did not intend to lose, warrants investigation rather than a diet plan. Guidance per NICE CG189.

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Sources and References

• World Health Organization — Obesity and overweight fact sheet
• World Health Organization — Body mass index, Global Health Observatory
• Centers for Disease Control and Prevention — Adult BMI calculator and categories
• Centers for Disease Control and Prevention — BMI for children and teens
• National Heart, Lung, and Blood Institute — Calculate your body mass index
• NHS — BMI healthy weight calculator
• NICE — PH46: BMI thresholds for black, Asian and other minority ethnic groups
• NICE — CG189: Obesity identification, assessment and management
• HSE Ireland — Healthy weight

Content last reviewed: 14 October 2026. This page is reviewed against WHO, CDC, NHS, NICE and HSE guidance and updated when those classifications change.