BMI Calculator

Work out your Body Mass Index (BMI) in seconds. Enter your height and weight, pick metric or imperial units, and get your BMI, weight category, healthy weight range, BMI prime and ponderal index — all instantly, right in your browser.

What Is BMI?

Body Mass Index (BMI) is a simple number calculated from your height and weight. Doctors and health organisations around the world use it as a quick screening tool to estimate whether an adult is underweight, at a healthy weight, overweight, or living with obesity. It does not measure body fat directly, and it cannot tell muscle apart from fat — but as a first check, it is fast, free, and surprisingly useful.

The idea goes back to the 1830s, when Belgian statistician Adolphe Quetelet noticed that adult weight tends to grow with the square of height. The formula was later adopted by the World Health Organization, which set the category cut-offs still used today. If your BMI lands outside the healthy band, it is a prompt to look closer — not a verdict on your health.

How BMI Is Calculated

The formula is the same in every unit system; only the numbers you plug in change:

• Metric: BMI = weight (kg) ÷ height (m)²
• Imperial: BMI = 703 × weight (lb) ÷ height (in)²

Example: a person who weighs 70 kg and stands 175 cm tall has a BMI of 70 ÷ 1.75² = 22.9, which falls in the healthy range. The calculator above does this maths instantly, in either unit system, and also works out the weight range that would put you between BMI 18.5 and 24.9 at your exact height.

Two extra numbers in the result are worth knowing. BMI prime is your BMI divided by 25 (the top of the healthy range). A prime below 1.0 means you are inside the healthy band; 1.20 means you are 20% above it. The ponderal index divides weight by height cubed instead of squared, which makes it steadier for very short or very tall people, where ordinary BMI can mislead.

BMI Categories (WHO Classification)

Some populations use lower cut-offs: for many people of South or East Asian descent, health risks start rising at a BMI of 23 rather than 25. If that applies to you, treat 23–27.5 as the overweight band and discuss it with your doctor. The World Health Organization’s obesity fact sheet explains how these thresholds are used worldwide, and the US National Heart, Lung, and Blood Institute publishes its own BMI guidance for adults.

What BMI Gets Wrong

BMI has real limits, and honest calculators say so. It cannot distinguish muscle from fat, so a muscular athlete can score “overweight” with very little body fat. It also ignores where fat sits on the body — abdominal fat carries more risk than fat on the hips and thighs — and it does not adjust for age, sex, or ethnicity. Older adults can have a “normal” BMI while carrying an unhealthy share of body fat, because muscle is gradually replaced by fat with age.

That is why BMI works best as part of a small set of checks. Pair it with your body fat percentage (which estimates fat versus lean mass), your ideal weight range from the classic clinical formulas, and your daily calorie needs if you plan to change your weight. Together, these give a much fairer picture than any single number.

Tips for Using Your Result

• Use the range, not the point. A healthy weight is a band (BMI 18.5–24.9), not one magic number. Aim anywhere inside it.
• Weigh consistently. Weigh yourself at the same time of day, on the same scale, without shoes — daily swings of 1–2 kg are normal water fluctuation.
• Recheck monthly, not daily. Trends over weeks matter; single readings bounce around.
• Measure height properly. Stand barefoot against a wall. Even 2 cm of error shifts your BMI noticeably.
• Don’t use adult cut-offs for children. Kids and teens are assessed against age- and sex-specific percentile charts, not the fixed adult bands.

If your BMI is outside the healthy range and you want to act on it, your total daily energy expenditure tells you how many calories you actually burn, and your daily water target covers the hydration side of any new routine.

Healthy Weight Range by Height

Because BMI is a ratio, the healthy weight band moves with your height. The table below shows, for a range of adult heights, the weight range that keeps BMI between 18.5 and 24.9. Use it as a quick lookup: find your height, and you have a sensible target zone without doing any maths.

Weights are rounded to one decimal place. Remember that this band is for adults and that muscle, frame size, and ethnicity shift where inside (or just outside) the band is right for an individual. If you carry a lot of muscle, compare your result with your body fat percentage before drawing conclusions.

BMI in Children, Teens, and Older Adults

Adult BMI bands do not apply to children and teenagers, because healthy weight changes constantly as they grow. Young people are assessed with BMI-for-age percentile charts instead: a child between the 85th and 95th percentile for their age and sex is classed as overweight, and at or above the 95th percentile as obese. A 12-year-old and a 16-year-old can share the same BMI number and be in completely different categories, which is why the adult calculator on this page must never be used for children.

Older adults are a different story. Muscle loss with age means an older person can sit inside the healthy BMI band while carrying an unhealthy amount of fat, and several large studies have found that slightly higher BMIs (up to around 27) are linked with lower mortality in people over 65. This is sometimes called the obesity paradox, and it is debated, but the practical lesson is clear: for older adults, BMI should be read alongside strength, mobility, waist size, and body composition, not on its own.

BMI and Waist Circumference Together

Waist size fixes one of BMI’s biggest blind spots: where the fat sits. Abdominal fat, packed around the organs, drives far more health risk than fat on the hips and thighs, and two people with identical BMIs can carry very different waistlines. Health authorities commonly use these waist cut-offs for adults, measured at the level of the navel while breathing out naturally:

• Men: over 94 cm means increased risk, over 102 cm means substantially increased risk.
• Women: over 80 cm means increased risk, over 88 cm means substantially increased risk.

A simpler rule of thumb is the waist-to-height ratio: keep your waist under half your height. A 170 cm person should aim for a waist under 85 cm. If your BMI is healthy but your waist is over the cut-off, or your BMI is high but your waist is fine and you are muscular, the waist number is usually the more honest signal. Track both numbers monthly and you will rarely be misled.

Three Worked Examples

Example 1 — above the healthy range. A person weighing 90 kg at 1.70 m: BMI = 90 / (1.70 x 1.70) = 31.1, which is Obese Class I. To reach the top of the healthy band (24.9), they would need to weigh 24.9 x 2.89 = 72.0 kg, a loss of about 18 kg. That sounds like a lot, but even the first 5 to 10% of body weight lost (4.5 to 9 kg here) measurably improves blood pressure, blood sugar, and joint load.

Example 2 — inside the healthy range. A person weighing 62 kg at 1.78 m: BMI = 62 / 3.17 = 19.6, comfortably healthy. Their healthy band runs from 58.5 to 78.7 kg, so they have plenty of room; for them, the useful goals are fitness and strength rather than weight change.

Example 3 — where BMI misleads. A rugby player weighing 100 kg at 1.80 m: BMI = 100 / 3.24 = 30.9, technically obese. If a body fat check shows 14%, the player is clearly not carrying excess fat, and the BMI label is simply wrong for them. This is the classic case for pairing BMI with a body fat measurement before acting on the number.

How to Measure Height and Weight Correctly

A BMI result is only as good as the two numbers fed into it. Small measuring errors move the result more than most people expect: at 175 cm, misreading your weight by 3 kg shifts BMI by about 1.0, and misreading your height by 2 cm shifts it by about 0.5. That is enough to move someone across a category boundary, so it is worth taking two careful minutes to measure properly rather than guessing from memory.

Weigh yourself on a hard, flat floor, not on carpet, which can under-read by a kilogram or more. Step on the scale in light clothing, without shoes, and stand still with your weight evenly spread. Height is best measured barefoot, standing with your heels, buttocks, and shoulder blades touching a wall, looking straight ahead while someone else marks the top of your head with a flat object, such as a book, held level. Do this in the morning if you can: people are measurably taller first thing, because the discs in the spine compress slightly during the day.

1. Place the scale on a hard, level floor and check it reads zero before you step on.
2. Weigh yourself in the morning, after using the bathroom and before eating or drinking.
3. Wear light clothing, or the same clothing each time, and no shoes.
4. Stand barefoot against a wall, heels together, looking straight ahead.
5. Have someone mark the top of your head with a level object, then measure from the floor to the mark.
6. Record both numbers and enter them exactly — do not round your height up or your weight down.

Common Measuring Mistakes

• Guessing height from memory. Most adults overestimate their height by 1 to 3 cm, which flatters the BMI result.
• Weighing after a large meal. Food and drink can add 1 to 2 kg temporarily, pushing BMI up by nearly a full point.
• Measuring height in shoes. Trainers and heels add 2 to 4 cm, which makes BMI look lower than it really is.
• Using a scale on carpet. Soft flooring absorbs weight and gives a falsely low reading.
• Comparing readings from different scales. Home scales disagree by a kilogram or more, so stick to one scale for trends.

BMI for Different Populations

The standard WHO cut-offs were built mostly from studies of people of European descent, and they do not fit every population equally. For many people of South Asian, Chinese, Japanese, and other East and Southeast Asian descent, the health risks linked to excess weight — especially type 2 diabetes and heart disease — start rising at a lower BMI, because body fat tends to be higher and more centrally stored at any given BMI. Several health authorities therefore use lower action points for these populations, and the World Health Organization has acknowledged that lower thresholds are appropriate for Asian populations.

If this applies to you, read your result against the lower row: a BMI of 24 that looks healthy on the standard scale sits in the overweight band on the Asian action points. The practical advice is the same either way — eat well, move regularly, and watch your waist — but the trigger for taking action comes sooner. Discuss which thresholds your doctor uses, since guidance varies by country and organisation. Pregnant people, children, and elite athletes all need different reference points again, which is why a single calculator can never be the final word.

BMI Prime and Ponderal Index Explained

The calculator shows two extra numbers that most BMI tools leave out, and both are worth understanding. BMI prime is your BMI divided by 25, the top of the healthy range. It answers the question “how far am I from the healthy ceiling?” in one tidy number: a prime below 1.00 means you are inside the healthy band, 1.00 means you sit exactly on the boundary, and anything above it tells you by what fraction you exceed it. A prime of 1.20, for example, means your BMI is 20% above the healthy ceiling.

The ponderal index divides weight by height cubed instead of height squared. Because it scales with volume rather than area, it stays steadier for very short and very tall people, where ordinary BMI tends to overstate weight for the short and understate it for the tall. Neither number replaces BMI; they are second opinions printed alongside it.

• BMI prime is best for tracking progress toward the healthy band, because the goal is always the same: get the prime under 1.00.
• Ponderal index is most useful at the extremes of height, roughly below 150 cm or above 195 cm, where BMI is least reliable.
• Both numbers are screening aids, not diagnoses — a doctor reads them together with waist size, blood pressure, blood sugar, and family history.

BMI and Health Risks: What the Evidence Shows

The reason doctors care about BMI is not the number itself but what it predicts. In large population studies, risk of several conditions rises steadily once BMI passes the healthy band, and rises steeply once it passes 30. The relationship is a gradient, not a cliff: a BMI of 27 carries modestly raised risk, a BMI of 35 carries substantially raised risk, and the curve keeps climbing from there. Equally, being below the healthy band carries its own risks, including nutritional deficiency, bone loss, and reduced immune function.

• Type 2 diabetes: risk rises with BMI, and rises earlier in people who store fat around the abdomen.
• Heart disease and stroke: higher BMI is linked with higher blood pressure, higher cholesterol, and greater strain on the heart.
• Joint problems: extra weight loads knees, hips, and the lower back, accelerating wear and osteoarthritis.
• Sleep apnoea: excess weight around the neck and chest narrows the airway during sleep.
• Certain cancers: higher BMI is associated with raised risk of several cancers, including bowel and post-menopausal breast cancer.

What a High BMI Does Not Prove

A high BMI does not prove that a person is unhealthy. Fitness matters independently: a person with a BMI of 28 who exercises regularly, eats well, does not smoke, and has normal blood pressure and blood sugar is at lower risk than a sedentary person with a BMI of 24 and poor metabolic markers. This is sometimes called being “fat but fit”, and while it is not a free pass — risk still rises with weight over time — it is a reminder that BMI is one risk factor among several. The honest use of BMI is as a prompt to check the other factors, not as a verdict delivered on its own.

A Practical Four-Week Plan After Your Result

If your result sits outside the healthy band, the best response is a small, boring plan rather than a dramatic overhaul. Crash diets fail because they are unsustainable; a modest calorie deficit, daily walking, and better sleep work because they can be kept up for months. Start by finding your maintenance calories with the calorie calculator, then build the plan below around that number.

1. Week 1 — measure and record. Track everything you eat for seven days without changing anything. Weigh yourself daily and take your waist measurement.
2. Week 2 — set the target. Eat 250 to 500 kcal below maintenance, add a 30-minute daily walk, and set a daily water target to replace sugary drinks.
3. Week 3 — add resistance. Two short strength sessions a week protect muscle while fat comes off, which keeps your metabolism steadier.
4. Week 4 — review and adjust. Compare weekly average weights, not daily readings. If the trend is falling by 0.25 to 0.5 kg a week, keep going unchanged.

If the trend stalls for three full weeks despite honest tracking, recalculate your calorie target — a lighter body burns less, so old targets quietly stop working. And if your BMI is well outside the healthy range, or you have a medical condition, involve your doctor before starting: the plan above is general guidance, not a prescription.

BMI Compared With Other Measures

BMI is the quickest check, but it is not the only one, and the others each fix a different blind spot. The table below compares the measures available on this site, so you can see which combination suits your situation.

The practical combination for most adults is BMI plus waist size, checked monthly, with a body fat percentage estimate added when the two disagree or when you train seriously. If your BMI says overweight but your waist is fine and you lift weights, body fat percentage is the tie-breaker. If your BMI is healthy but your waist is over the cut-off, the waist number deserves more attention than the BMI.

Moving Between BMI Categories

It helps to see how much weight actually separates the categories, because the distances are often smaller than people fear. The table below shows, for a person 170 cm tall, the weight at each key BMI boundary. Moving from a BMI of 30 to a BMI of 25 at that height means losing about 14 kg — a large change, but one made of many small steps. Moving from 27.5 to 24.9 is about 7.5 kg. And the first 5% of body weight lost already brings measurable improvements in blood pressure and blood sugar, long before any category boundary is crossed.

Use your own numbers from the calculator above rather than this table if your height differs — the calculator works out your exact healthy range for your height. Set intermediate targets every 5% of body weight, celebrate each one, and remember that maintaining a loss counts as success just as much as losing more.

More BMI Guides

• How to Calculate BMI — the metric and imperial formulas, worked examples, and a BMI chart.
• What Is My BMI? — check your number, your category, and what to do next.
• BMI Calculator for Adults — cut-offs by age, men vs women, and what your number means after 40, 50, and 65.
• BMI Chart by Age and Weight — find your healthy range in the tables by age and weight.

Frequently Asked Questions

What is a good BMI?

For most adults, a BMI between 18.5 and 24.9 is considered healthy. That band is linked to the lowest risk of weight-related disease in large population studies, though individual health depends on much more than one number.

Is BMI accurate for athletes?

Not always. BMI cannot tell muscle from fat, so heavily muscled people often score as overweight or obese despite low body fat. Athletes should check body fat percentage alongside BMI.

Does BMI differ for men and women?

The formula is identical, but women naturally carry a higher body fat percentage at the same BMI. The categories are the same for both sexes; interpretation should account for body composition.

What is BMI prime?

BMI prime is your BMI divided by 25, the top of the healthy range. A prime of 1.00 means you sit exactly at that boundary; below 1.00 is inside the healthy band, above 1.00 is over it.

Can I use this BMI calculator for my child?

No — children and teenagers need age- and sex-specific BMI percentile charts, not the adult cut-offs. Ask your paediatrician for the right assessment.

How much weight do I need to lose to reach a healthy BMI?

Multiply 24.9 by your height in metres squared. At 1.70 m that is 24.9 x 2.89 = 72.0 kg. Subtract that from your current weight and you have the distance to the top of the healthy band. Aim for it in stages: the first 5 to 10% of your body weight already brings real health benefits.

Should I trust BMI if I lift weights?

Use it with caution. BMI cannot tell muscle from fat, so regular lifters often score overweight or obese with low body fat. Check your body fat percentage and waist size alongside BMI, and let those decide whether your weight is actually a problem.

Is a BMI of 25 really overweight?

By the WHO classification, yes: 25.0 is the first point of the overweight band. But the risk at 25.0 is only slightly above the risk at 24.9 — the boundary is a convention for population statistics, not a sudden change in your body. Treat 25 to 27 as a nudge to check your waist, activity, and eating habits, not as an emergency.

How often should BMI be checked?

Monthly is plenty for most adults. Daily BMI checks mostly track water fluctuation and scale noise, which causes needless worry. Weigh weekly under identical conditions if you prefer, and compare monthly averages for the real trend.

Does BMI work for very tall or very short people?

Less well than for average heights. BMI tends to overstate weight for shorter people and understate it for taller people, because it divides by height squared while bodies scale more like volume. The ponderal index shown in the result, which divides by height cubed, is steadier at the extremes — and waist size and body fat percentage matter even more there.

Disclaimer: This calculator is for general information only and is not medical advice. BMI is a screening measure, not a diagnosis. Talk to a healthcare professional about your weight and health.