Calculate your Body Mass Index quickly with our BMI Calculator UK.
Body Mass Index has been the NHS's first screening step for healthy weight since it entered routine UK practice, and it's still the fastest way to see where your height and weight sit against national health guidance. The UK BMI calculator above goes further than a single number — it works in whichever units you actually think in (cm, feet and inches, kilograms, stones and pounds), applies the NHS's ethnicity-adjusted thresholds where they matter, and layers in your Basal Metabolic Rate, estimated daily calorie needs, waist-based ratios and an ideal weight range, so you get a rounder picture in one pass rather than four separate tools.
How to use this BMI calculator
The calculator is built so a first-time visitor can get a result in under ten seconds, while anyone who wants a deeper read on their metabolism or body shape can open the advanced panel for more. Here's the full walkthrough.
Pick your units
Use the toggle at the top to switch height between centimetres and feet/inches, and weight between kilograms, stones & pounds, or pounds alone. You can mix systems freely — height in feet and inches with weight in kilograms works exactly the same as any other combination, since everything is converted internally before the calculation runs.
Enter your height and weight
These are the only two fields the calculator strictly needs. Type them in and you're already able to press Calculate — everything below this step is optional extra detail.
Open "Advanced options" for the extra metrics
Expand this panel to add your age and sex (unlocks BMR and daily calories), an activity level (turns BMR into a full daily calorie estimate), waist and hip measurements (unlocks two separate risk ratios), and your ethnic background (applies the NHS's adjusted thresholds where research shows they're more appropriate). Every one of these fields is optional and the calculator will simply skip a metric if the fields it needs aren't filled in.
Press "Calculate my BMI"
You'll see a gauge with your exact BMI, its category, and a full set of supporting figures beneath it. Press Recalculate to go back to the form with your numbers still in place, or Reset to clear everything and start over.
Understanding every result it gives you
Rather than a single verdict, the calculator deliberately shows several angles on the same measurements, because no one number tells the whole story about a body. Here's what each one means and where it comes from.
🎯 BMI & category
Your weight in kilograms divided by your height in metres squared, matched against the NHS's six weight bands from underweight to severely obese.
📊 BMI Prime
Your BMI divided by the upper healthy limit (25, or 23 if ethnicity-adjusted thresholds apply). A Prime of 1.0 sits exactly on the boundary — below it is healthy, above it is not.
📏 Healthy BMI range
The NHS's 18.5–24.9 kg/m² band, shown alongside the actual weight-in-kg range that translates to for your specific height.
🧊 Ponderal Index
Weight divided by height cubed rather than squared. Because it scales with volume, it's considered a fairer measure than BMI for people at the very tall or very short end of the height range.
🔥 BMR (Basal Metabolic Rate)
How many calories your body burns in a day of complete rest, calculated with the Mifflin-St Jeor equation — the formula most nutrition professionals treat as the current gold standard.
🍽️ Daily calories (TDEE)
Your BMR multiplied by an activity factor you choose, giving a rough estimate of the calories you'd need each day to hold your current weight steady.
📐 Waist-to-height ratio
Your waist measurement divided by your height. NICE guidance for the NHS recommends keeping this under 0.5 — in plain terms, a waist less than half your height.
⚖️ Waist-to-hip ratio
Waist divided by hip measurement, an independent indicator of where you carry weight, based on WHO's 2008 expert consultation thresholds.
🧮 Ideal weight range
Four long-standing clinical formulas (Devine, Robinson, Miller and Hamwi) averaged together, giving a height-and-sex-based range that sits alongside the BMI-based one for comparison.
UK BMI categories (NHS thresholds)
The NHS uses the same core BMI formula as the rest of the world, but with a UK-specific twist: adults of South Asian, Chinese, Black African, Black Caribbean or Middle Eastern heritage are moved onto lower cut-offs, because research consistently shows that health risk climbs at a lower BMI in these groups. The calculator applies whichever set of bands matches what you select.
| Category | BMI range (kg/m²) |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese (Class I) | 30.0 – 34.9 |
| Obese (Class II) | 35.0 – 39.9 |
| Obese (Class III / severely obese) | 40.0 and above |
| Category | BMI range (kg/m²) |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 22.9 |
| Overweight | 23.0 – 27.4 |
| Obese | 27.5 and above |
The BMI scale, visualised
Because a single number can be hard to place in context, here's the same NHS scale as a continuous strip, running from the underweight threshold up to the point where BMI is classed as severely obese.
Standard adult NHS bands. The calculator redraws this same scale with the adjusted 18.5 / 23 / 27.5 cut-offs when an applicable ethnic background is selected.
BMI chart by height
If you'd rather glance at a weight range than do the arithmetic, here's roughly what the healthy 18.5–24.9 BMI band works out to in kilograms and stones, for a run of common UK adult heights.
| Height | Healthy weight (kg) | Healthy weight (stones) |
|---|---|---|
| 5'0" (152 cm) | 42.7 – 57.6 kg | 6 st 10 lb – 9 st 1 lb |
| 5'2" (157 cm) | 45.6 – 61.4 kg | 7 st 3 lb – 9 st 9 lb |
| 5'4" (163 cm) | 49.2 – 66.2 kg | 7 st 10 lb – 10 st 6 lb |
| 5'6" (168 cm) | 52.2 – 70.3 kg | 8 st 3 lb – 11 st 1 lb |
| 5'8" (173 cm) | 55.4 – 74.5 kg | 8 st 10 lb – 11 st 10 lb |
| 5'10" (178 cm) | 58.6 – 78.9 kg | 9 st 3 lb – 12 st 6 lb |
| 6'0" (183 cm) | 62.0 – 83.4 kg | 9 st 11 lb – 13 st 2 lb |
| 6'2" (188 cm) | 65.4 – 88.0 kg | 10 st 4 lb – 13 st 12 lb |
These are rounded to the nearest common height for quick reference — use the calculator above for your exact figure, since a couple of centimetres shifts the healthy range by a kilogram or more.
BMR, activity level and daily calories
Basal Metabolic Rate is the energy your body spends just staying alive — breathing, circulating blood, keeping your organs running — before you've taken a single step. The calculator estimates it with the Mifflin-St Jeor equation, published in 1990 and repeatedly shown in later studies to out-predict the older Harris-Benedict formula for most adults. From there, multiplying BMR by an activity factor gives your Total Daily Energy Expenditure, a rough calorie target for holding your weight steady.
| Activity level | Description | Multiplier |
|---|---|---|
| Sedentary | Little or no regular exercise | × 1.2 |
| Light | Light exercise 1–3 days a week | × 1.375 |
| Moderate | Moderate exercise 3–5 days a week | × 1.55 |
| Active | Hard exercise 6–7 days a week | × 1.725 |
| Very active | Physically demanding job or twice-daily training | × 1.9 |
Waist-to-height and waist-to-hip ratio
BMI can't see where on your body weight is carried, and that turns out to matter: fat stored around the abdomen carries more cardiometabolic risk than the same amount carried on the hips and thighs. Two simple ratios fill that gap, and the calculator works out both if you add your waist and hip measurements in the advanced panel.
- Waist-to-height ratio — NICE's guidance for the NHS is to keep your waist under half your height. A ratio under 0.5 is considered the healthier range; 0.5–0.59 signals increased risk, and 0.6 or above is classed as high risk.
- Waist-to-hip ratio — the World Health Organization's 2008 expert consultation sets the higher-risk threshold at 0.90 for men and 0.85 for women, regardless of overall BMI.
Ideal weight range formulas
Long before BMI became the standard screening tool, clinicians used height-and-sex-based formulas to estimate a target weight — originally for calculating safe drug doses, later adopted more broadly. None of them is definitively "correct", which is exactly why the calculator averages all four rather than picking one.
| Formula | Men | Women |
|---|---|---|
| Devine (1974) | 50 + 2.3/in | 45.5 + 2.3/in |
| Robinson (1983) | 52 + 1.9/in | 49 + 1.7/in |
| Miller (1983) | 56.2 + 1.41/in | 53.1 + 1.36/in |
| Hamwi (1964) | 48 + 2.7/in | 45.5 + 2.2/in |
Frequently asked questions
For most adults, the NHS classes 18.5 to 24.9 kg/m² as the healthy weight range. If you're of South Asian, Chinese, Black African, Black Caribbean or Middle Eastern heritage, the healthy range is 18.5 to 22.9, since research shows health risk rises at a lower BMI in these groups.
Divide your weight in kilograms by your height in metres, squared. For example, someone weighing 70 kg at 1.70 m tall works it out as 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 = 24.2. In imperial units, multiply your weight in pounds by 703, then divide by your height in inches squared.
No single measurement is perfect. BMI can't distinguish muscle from fat, so very muscular people are sometimes placed in the overweight or obese range despite low body fat. It's also not designed for pregnancy or for children and teenagers, who are assessed on age- and sex-specific growth centiles instead. That's why this calculator pairs BMI with the Ponderal Index, waist ratios and an ideal-weight range, rather than relying on one number alone.
The Ponderal Index divides weight by height cubed instead of squared. Because human body mass scales more closely with volume than with a flat area, this makes it a steadier measure for people at the extremes of height — BMI tends to run a little high for very tall people and a little low for very short people, and the Ponderal Index corrects for that. A healthy adult range is roughly 11 to 15 kg/m³.
Studies commissioned as part of NHS and NICE guidance found that people of South Asian, Chinese, Black African, Black Caribbean or Middle Eastern background tend to develop type 2 diabetes and cardiovascular disease at a lower BMI than the wider population, partly due to differences in where body fat is stored. To catch these risks earlier, the NHS lowers the overweight and obesity cut-offs for these groups rather than waiting until BMI 25 or 30.
BMR is the calories your body burns at complete rest — essentially your baseline. TDEE (Total Daily Energy Expenditure) takes that baseline and multiplies it by an activity factor to account for movement, exercise and daily life, giving a more realistic estimate of how many calories you actually need in a day to maintain your current weight.
NICE guidance for the NHS recommends keeping your waist circumference below half your height, which works out to a waist-to-height ratio under 0.5. This applies across sexes and ethnic groups, which is part of why it's increasingly used alongside BMI rather than instead of it.
Yes. The unit toggle at the top lets you switch weight between kilograms, stones and pounds combined, or pounds on their own, and height between centimetres or feet and inches — in any combination you like.
No. Adult BMI bands don't apply to anyone under 18, who need age- and sex-specific growth centile charts instead, or to pregnant women, whose healthy weight gain varies through each trimester in a way adult BMI categories don't capture. The calculator detects both cases from the age field and pregnancy checkbox and shows a note pointing you to the right resource rather than a misleading adult result.
No — this tool is a screening aid, not a diagnosis. It's built on the same NHS thresholds and published formulas that clinicians use as a starting point, but your GP or practice nurse can interpret your results alongside your full medical history, which no calculator can do.
This calculator and article are provided for general information only and are not a substitute for professional medical advice. BMI, BMR, TDEE and the other figures shown are population-level estimates based on published NHS, WHO and clinical formulas — they can't account for your full individual health picture. If you have concerns about your weight or health, please speak to a GP, practice nurse or registered dietitian.
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