BMI Calculator (Body Mass Index)

What is BMI?

BMI stands for Body Mass Index. It compares your weight with your height to estimate whether your weight falls in a healthy range.

Healthcare services use it because it is quick, cheap, needs no equipment beyond scales and a tape measure, and is backed by decades of population research. Those are real advantages — but they are advantages for screening large groups of people, which is not quite the same as describing any one person accurately.

BMI should be read alongside waist circumference, physical activity, diet quality, muscle mass and medical history. On its own it is a starting point, not a verdict.

How is BMI calculated?

BMI uses one formula:

BMI = weight (kg) ÷ height (m)²

For someone weighing 75 kg at 1.75 m tall:

75 ÷ (1.75 × 1.75) = 24.5

The calculator above converts automatically if you enter height in feet and inches or weight in stones and pounds, so you never need to do the conversion yourself.

BMI categories

These bands are used by the NHS, the WHO and the CDC to assess weight-related health risk in adults. They are population thresholds — the risk they describe rises gradually rather than switching on at 25.0.

What does your BMI mean?

Underweight (below 18.5)

Being underweight can carry its own risks, including nutrient deficiencies, reduced immune function, lower bone density, fertility problems and slower recovery from illness.

A low BMI is worth discussing with a healthcare professional rather than simply eating more, because the underlying cause matters — unintentional weight loss in particular should always be checked.

Healthy weight (18.5 – 24.9)

A BMI in this range generally suggests your weight is appropriate for your height. The useful goal from here is maintenance without preoccupation: a balanced diet, regular activity, strength work to hold onto muscle, decent sleep, and an occasional check rather than a daily weigh-in.

Mealiv can help you plan balanced meals and keep an eye on nutrition without turning it into restriction.

Overweight (25 – 29.9)

Being in this band does not automatically mean you are unhealthy, and plenty of people here are fitter than people in the band below. It does mean an increased average risk of type 2 diabetes, high blood pressure, heart disease, some cancers and sleep apnoea.

Where weight loss is appropriate, the evidence is encouraging about how little is needed: losing 5–10% of body weight produces meaningful improvements in blood pressure, blood glucose and cholesterol for many people — well before reaching a "healthy" BMI.

Obesity (30 and above)

A BMI of 30 or more is associated with higher risk of long-term conditions, and the risk rises across classes I, II and III.

Gradual, sustained changes consistently outperform rapid ones. That is not a moral point about effort — it is that fast weight loss is disproportionately likely to be regained, and repeated cycles are harder on both body and motivation than a slower change that holds. Support from a GP is worth seeking, particularly where other conditions are present.

BMI isn't perfect

This is the part most BMI calculators skip, and it is the part that decides whether your result means anything.

BMI does not measure body fat percentage, muscle mass, bone density, where fat is distributed, or fitness. It measures weight for height. Everything else is inference.

Athletes and muscular people

Muscle is denser than fat. Someone who trains seriously can land in the overweight or even obese band with low body fat and excellent metabolic health. For this group BMI is close to useless as an individual measure.

Older adults

Muscle mass declines with age while fat can increase, so an older adult may have a "healthy" BMI while carrying more body fat than the number suggests. Grip strength, walking speed and waist measurement tell you more.

Pregnancy

BMI should not be used during pregnancy. Weight gain is expected and the categories do not apply. Your midwife or GP will use booking weight and pregnancy-specific guidance instead.

Children and teenagers

Adult categories do not apply to under-18s. Children need age- and sex-specific BMI centiles, because healthy body composition changes substantially through growth. Use the NHS calculator for children or speak to your GP.

Where the fat sits

Two people can share a BMI and carry very different risk depending on whether fat is stored around the hips or around the abdomen. Visceral fat around the organs is the more strongly associated with metabolic risk, and BMI cannot see it at all. A waist measurement takes ten seconds and adds genuinely new information.

Ethnicity changes the thresholds

The standard BMI bands were derived largely from white European populations, and they do not transfer cleanly.

People of South Asian, Chinese, Black African, African-Caribbean and Middle Eastern family background tend to develop type 2 diabetes and cardiovascular disease at lower BMIs. The NHS therefore applies lower thresholds for these groups: a healthy range up to 23, increased risk from 23, and high risk from 27.5 — rather than 25 and 30.

That is a substantial difference. Someone with a BMI of 24 would be told they are a healthy weight on the standard scale and at increased risk on the adjusted one, and for a large share of the UK population the adjusted figure is the more clinically useful. You can switch the calculator above between the two.

Should you lose weight based on BMI alone?

Not on its own, no. BMI is a screening tool, and screening tools are designed to flag who might benefit from a closer look — not to make the decision.

A fuller picture includes waist circumference, blood pressure, cholesterol, blood glucose or HbA1c, physical fitness, muscle mass, and how your habits and weight have trended over years rather than weeks.

If your BMI has flagged something and you are not sure what it means for you specifically, that is a good reason to speak to your GP — particularly if you have a family history of type 2 diabetes or heart disease.

Better ways to track progress

BMI moves slowly and tells you little about what changed. If you are working on your health, these are more informative week to week:

• Your weight trend over weeks, rather than any single reading
• Waist measurement, which picks up abdominal fat that BMI misses
• Strength — whether the weights or the reps are going up
• Fitness, such as how you feel on a familiar hill or flight of stairs
• Energy levels through the afternoon
• Sleep quality and duration
• Diet quality: fibre, protein, fruit and vegetables, and how much is ultra-processed
• Consistency — how many days a week the plan actually happens

Mealiv tracks the nutrition side of that list and plots weight as a trend rather than a series of daily readings, which is what stops normal fluctuation looking like failure.

Healthy ways to improve your BMI

None of these are surprising. That is rather the point: the habits with the best evidence behind them are unglamorous and they work whatever your starting BMI.

• Eat more vegetables and fruit — variety as much as volume
• Include a protein source at each meal, which helps with fullness and preserves muscle
• Choose wholegrains where you can, for the fibre as much as anything
• Reduce ultra-processed food, which is easy to over-eat by design
• Move daily, including the unstructured kind — walking counts
• Strength train a couple of times a week, especially if losing weight
• Sleep seven to nine hours; short sleep reliably increases appetite
• Drink water rather than calories

If you want numbers to work to, the calorie calculator estimates your daily needs and the macro calculator turns that into protein, carbohydrate and fat targets.

Using Mealiv to reach a healthy weight

Knowing your BMI is the easy part. Changing it means changing what happens at four o'clock on a Wednesday, week after week.

Mealiv builds personalised meal plans, tracks calories and macros, analyses vitamins and minerals, imports recipes from any website, generates shopping lists from your plan, and shows progress over months rather than days. Whether the goal is loss, gain or maintenance, the mechanism is the same: make the healthy option the one that is already planned.

Important information

This calculator is for informational purposes only and does not constitute medical advice or a diagnosis. BMI is a screening tool and cannot assess your individual health. It should not be used during pregnancy or for anyone under 18. Consult your GP or another qualified healthcare professional before making significant changes to your diet or activity, particularly if you have an existing health condition or a history of disordered eating.