NHS BMI Calculator

Last updated: 16 October 2026 · BMI bands follow the standard adult classification used across NHS services.

By the NHS Bands Pay editorial team

The calculator

What BMI actually measures

Body mass index is a single number that relates your weight to your height. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet as a way to describe the build of populations, and it became the standard screening tool for weight-related health risk in the twentieth century because it needs nothing more than a set of scales and a tape measure. The idea is simple: weight alone tells you little, because a tall person naturally weighs more than a short person. Dividing by the square of height adjusts for that, producing a figure that can be compared between people of different statures.

What BMI does not do is look inside your body. It cannot tell muscle from fat, bone from water, or where on your body the weight sits. Two people can share the same BMI with completely different body composition: a rugby player and a sedentary office worker can land on the same number for opposite reasons. That is why every clinician treats BMI as a starting point, not a conclusion. A high reading prompts a conversation and usually a couple of extra checks, such as waist circumference or blood pressure, rather than a diagnosis on the spot.

For individuals, the most useful property of BMI is that it is repeatable. If you measure the same way, at roughly the same time of day, over weeks and months, the trend line tells you more than any single reading. A BMI drifting slowly upward over a year is information worth acting on, even if the absolute number still sits in the healthy band. This page is a staff-wellbeing tool on a pay information site: it exists so that the people who spend their working lives caring for others can check one basic number about themselves, privately, in under a minute.

The adult BMI categories

The bands below are the standard adult classification used across NHS services. They apply to adults aged 18 and over. Children and teenagers are assessed on a completely different basis, against age- and sex-specific growth charts, so the numbers here must not be used for under-18s.

A word on the boundaries, because they are sharper than biology. The difference between 24.9 and 25.0 is a rounding convention, not a cliff edge: health risk changes gradually, and a BMI of 25.1 in a fit, active person is a different matter from the same number in someone who never moves. The categories are flags for conversation, each one telling you and any clinician roughly where the conversation should start. Do not treat crossing a boundary as good or bad news in itself; treat it as information.

It is also worth knowing that these bands describe populations better than they describe individuals. They were drawn from large studies linking weight to health outcomes across whole groups, which is why they remain the standard triage tool in clinics. For one person on one day, they are a rough guide. The calculator above gives you the number; the sections below help you read it with the context it needs.

How to calculate BMI by hand

The calculator does the arithmetic for you, but the formula is short enough to do on paper, and knowing it helps you sanity-check any result. There are two versions, one for metric units and one for imperial.

Metric: BMI = weight in kilograms divided by height in metres squared. In symbols: BMI = kg / m². Take your height in metres and multiply it by itself, then divide your weight in kilograms by that number.

Worked example: a person weighing 70 kg who is 1.75 m tall. First square the height: 1.75 × 1.75 = 3.0625. Then divide: 70 / 3.0625 = 22.86. That sits in the 18.5 to 24.9 band, so the category is healthy weight. Enter the same figures into the calculator above and you will get exactly the same answer.

Imperial: BMI = (weight in pounds divided by height in inches squared) multiplied by 703. Convert your height to inches and your weight to pounds first, square the inches, divide the pounds by that, then multiply by 703. The 703 is simply the constant that converts imperial units back to the metric-based scale, so the bands stay the same.

Worked example: a person weighing 12 stone who is 5 ft 9 in tall. Convert: 12 stone = 168 lb; 5 ft 9 in = 69 inches. Square the height: 69 × 69 = 4,761. Divide: 168 / 4,761 = 0.0353 (rounded). Multiply: 0.0353 × 703 = 24.81. Again a healthy-weight result, right at the upper end of the band. This is also a good illustration of why the healthy range matters as much as the single number: this person is healthy today, but has little headroom before the overweight band begins.

Two practical tips for accurate readings. Weigh yourself first thing in the morning, after using the bathroom and before eating, wearing light clothing or none. Measure your height standing straight against a wall, without shoes, looking straight ahead, with the tape or stadiometer level. Small errors in height shift the result more than small errors in weight, because height is squared, so the height measurement is the one worth doing carefully.

Your healthy weight range

A single BMI number is less useful than the weight range that keeps you in the healthy band. That range comes from rearranging the formula: healthy weight in kilograms runs from 18.5 × height² to 24.9 × height². The calculator above works it out for your exact height; the table below covers the common range so you can see it at a glance.

Notice how wide each band is. At 1.75 m, the healthy span runs from about 57 kg to about 76 kg: nearly 20 kg of latitude. That is deliberate. The healthy band is not a target weight, it is a zone, and where you sit comfortably within it depends on your build, muscle mass and general fitness. Somewhere in the middle suits most people; the edges are where the caveats in the next section start to matter.

If your current weight sits above the top of your range, the gap between where you are and the top of the range is a more honest goal than any single "ideal weight". A person at 1.75 m weighing 90 kg needs to lose about 14 kg to re-enter the healthy band: a real but finite number, and a far better planning figure than an abstract BMI of 29.4. Work with the range, not just the rating.

When BMI misleads

BMI is a blunt instrument, and honest guidance says so upfront. The main situations where the number needs a second opinion:

Muscular builds. Muscle is denser than fat, so people who train with weights, do heavy manual work or play strength sports can register as overweight or even obese while carrying very little body fat. Porters, estates staff and anyone whose job is physically demanding should read their BMI alongside a mirror test and, ideally, a waist measurement rather than taking the category at face value.

Pregnancy. BMI is not designed for use during pregnancy, when weight gain is expected and healthy. Midwives track weight against pregnancy-specific guidance, and the standard adult bands do not apply. The same caution applies in the early postnatal period while weight is still settling.

Older adults. Body composition changes with age: muscle mass tends to fall and fat distribution shifts, which can make the standard bands slightly less informative past about 65. A slightly higher BMI can be protective in older age, which is another reason the number is a prompt for a conversation with a clinician rather than a self-diagnosis.

Ethnicity. People of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds can face higher health risks at lower BMIs than the standard bands suggest, because of differences in typical body-fat distribution. Clinicians may use lower thresholds for these groups when assessing risk. If this applies to you, mention it when you discuss your result.

Waist circumference. Where fat sits matters independently of how much there is. Fat carried around the abdomen is more strongly linked to health risk than fat carried on the hips and thighs. A practical extra check, used widely in clinics: measure your waist halfway between your lowest rib and the top of your hip bone, and compare it with your height. For most adults, keeping the waist below half the height is the commonly used guide, whatever the BMI says. If your BMI is healthy but your waist is high, that combination still deserves attention.

None of this makes BMI useless. It makes it a first filter: cheap, instant and good enough to sort most adults into the right broad group. The limitations above are the reasons the result comes with advice to talk to a clinician rather than a prescription, and why the calculator labels itself a guide.

BMI and shift work: the staff angle

NHS working patterns make weight management genuinely harder, and it is worth saying so plainly rather than pretending willpower is the whole story. A twelve-and-a-half-hour shift, especially a night shift, disrupts every routine that healthy eating depends on. Meal times slide to whenever the ward goes quiet. The canteen closes or shrinks to a vending machine overnight. The gap between shifts is spent sleeping, commuting or catching up on life, not cooking or exercising. None of this is a personal failing; it is the shape of the job.

A few adjustments fit better around rotating rosters than generic advice does. Eating a proper meal before a night shift, rather than grazing through it, keeps energy steadier and reduces the 3am vending-machine run. Batch cooking on days off removes the decision fatigue that leads to takeaways after a long shift. Short bouts of activity count: a brisk twenty-minute walk on a break, the stairs instead of the lift across a shift, or a bodyweight routine at home on rest days all add up, and they fit rosters that a fixed gym schedule never will. Sleep matters too, and it is the one most shift workers sacrifice first: blackout blinds, earplugs and a cool, dark room protect the daytime sleep that makes everything else easier.

Your employment terms can help if you use them deliberately. Annual leave is recovery time, not just holiday time: a run of days off is the best window most staff get to reset eating and sleeping patterns. Flexible working arrangements, where the rota allows, can carve out regular slots for activity that a fixed long-day pattern never offers. And many trusts run staff health MOTs, wellbeing hubs and discounted gym or cycle schemes through their staff benefits offering, so the support is often already paid for: it just needs claiming. The tax side of NHS work expenses is covered in our NHS tax code guide, which is a different kind of staff saving but part of the same picture of looking after yourself on NHS pay.

What to do with your result

A BMI result is information, and information is only useful if it leads somewhere. What "somewhere" means depends on the category.

Healthy weight. The job is maintenance. Keep doing what keeps you there, and recheck every few months: the trend is the early-warning system. If your waist measurement is high despite a healthy BMI, mention it at your next check-up.

Overweight. Treat this as the cheapest point to act. Small, sustained changes beat dramatic ones: one fewer sugary drink a day, a daily walk, cooking one extra meal from scratch a week. The NHS Better Health programme offers free apps, meal ideas and structured plans, and your GP practice can advise on local weight-management services if you want structured help.

Obese or severely obese. Get support rather than going it alone. Your GP can refer you to tiered weight-management services, which combine dietary advice, activity support and behavioural help, and to specialist clinics at higher levels. Occupational health at your trust is another route, and it is confidential: using it is not recorded on your employment file in any way that affects your career. If cost is a barrier to healthier food or activity, the money guides on this site, from tax relief on work expenses to the benefits roundup, exist to put a little of your own pay back in your pocket.

Underweight. Do not ignore it. Unintentional weight loss or a persistently low BMI can have medical causes, from thyroid and gut conditions to stress and mental health, and it deserves the same clinical attention as the other end of the scale. See your GP, and mention any symptoms alongside the number.

Whatever your result, keep perspective. BMI is one number from one minute of one day. It does not measure your fitness, your strength, your energy or your worth, and it certainly does not measure the quality of the care you give. Use it the way clinicians do: as a prompt, in context, and in combination with everything else you know about your health.

Frequently asked questions

What is a healthy BMI for an adult?

For adults, a BMI between 18.5 and 24.9 is classed as a healthy weight. Below 18.5 is underweight, 25 to 29.9 is overweight, 30 to 39.9 is obese, and 40 or above is severely obese. These are the standard adult bands used across NHS services.

Is BMI accurate for muscular or very active people?

Not always. BMI cannot tell muscle from fat, so people with a lot of muscle mass can score as overweight or obese while being perfectly healthy. In that situation, waist circumference is a useful extra check: for most adults, keeping your waist below half your height is the commonly used guide.

Does the BMI calculator work for children?

No. The calculator on this page uses the adult formula and adult bands, and it should not be used for anyone under 18. Children's BMI is assessed against age- and sex-specific growth charts (centiles), which is a different calculation. The NHS provides a separate calculator for children and teenagers.

How do I convert stone and pounds for the BMI formula?

The imperial BMI formula is: BMI = (weight in pounds / height in inches squared) x 703. So convert your height to inches and your weight to pounds first, then apply the formula. The calculator above does this automatically if you choose imperial units.

What should I do if my BMI is in the overweight or obese range?

Treat it as a prompt, not a verdict. Book a check with your GP or use your trust's occupational health service; many trusts also run staff health MOTs and wellbeing hubs. The NHS Better Health programme offers free tools and support for weight management, and small sustained changes tend to work better than crash diets.

Can shift work affect my BMI?

Shift patterns can make healthy eating and exercise harder: night shifts disrupt normal meal times, canteen options shrink overnight, and short turnarounds between shifts leave little time for activity. None of this is inevitable, and the section on shift work below has practical suggestions that fit around rotating rosters.

Is my BMI result stored or shared anywhere?

No. The calculation runs entirely in your browser. Nothing you type is sent to our servers, stored, or shared, and no account or sign-up is needed. When you close the page, the numbers are gone.

Sources and independence

Source: NHS: Calculate your body mass index (BMI) (verified live 16 October 2026).

This is an independent guide and is not affiliated with the NHS. BMI is a screening measure for adults, not a medical diagnosis. If you have concerns about your weight, speak to your GP or your trust's occupational health service.