BMI: what it shows and what it doesn't
BMI is quick, free and very simple. That's exactly why it's useful and why it's often wrong: it has no idea what your weight is made of.
Information checked on 2026-09-28. General information, not medical advice. How we calculate
How to work out your BMI
The formula is simple: body weight in kilograms divided by height in metres, squared.
Example: 78 kg, 165 cm. Height in metres is 1.65. 1.65 × 1.65 = 2.72. 78 / 2.72 ≈ 28.7.
Your BMI is also worked out automatically in the calorie calculator, alongside your daily calorie need.
The BMI ranges
| BMI | How it's described |
|---|---|
| below 18.5 | below the recommended range |
| 18.5–24.9 | within the recommended range |
| 25.0–29.9 | slightly above the recommended range |
| 30.0 and over | above the recommended range |
In medicine the upper bands also have clinical names, but we deliberately don't use them: these are statistical ranges, not descriptions of a person. The number says nothing about how hard you try, what you eat or who you are.
What BMI is genuinely good for
- As a first signpost. All you need is scales and your height – no equipment.
- For large groups. At population level BMI tracks trends well – that's what it was built for.
- As a safety limit. We use BMI 18.5 as a floor: the nutrition quiz won't offer a weight-loss plan if your BMI is already below 18.5.
Where BMI gets it wrong
BMI only sees weight and height. It doesn't know how much of that weight is muscle, water or fat, or where the fat sits. So it often misleads for:
- People who train. Plenty of muscle means a high BMI with no excess fat at all.
- Older adults. Muscle shrinks over the years while fat can rise at the same weight. BMI then looks “fine” although body composition has changed. That's why keeping muscle matters more after 60 – we cover it in the protein guide.
- Very tall and very short people. The squared formula distorts their result slightly.
- Pregnant and breastfeeding women. BMI ranges don't apply to them.
- Children and teenagers. They need growth charts and a doctor, not the adult table.
- People of different ancestry. For some populations health risk rises at a lower BMI, so several guidelines use lower thresholds for them.
What says more: your waist divided by your height
For an individual, the more practical measure is waist circumference divided by height. It shows how much fat sits around your middle – and fat stored there is the most closely linked to health risk.
| Waist / height | How it's read |
|---|---|
| below 0.5 | healthy range |
| 0.5–0.6 | increased risk |
| 0.6 and above | high risk |
The bands follow the UK NICE guideline (NG246, 2025). The simple version: your waist should be less than half your height. For someone 170 cm tall, that means a waist under 85 cm.
How to measure. In the morning, before eating, around your middle just above the belly button. Don't pull your stomach in and don't pull the tape tight – it should just sit against you. The same measurement every 2 weeks tells you more than daily weighing. More on belly fat specifically: how to lose belly fat.
How Dumpli uses BMI
We'd rather be clear about it – we use it for exactly two things:
- A safety limit. If your BMI is below 18.5 we don't offer a weight-loss plan and point you to a doctor.
- Protein maths. When BMI is over 30, we work out your protein target for the weight that matches a BMI of 25 – otherwise the number comes out unrealistically high.
Your calorie target is not calculated from BMI, but from your basal metabolic rate and your activity. All the formulas and limits are on our methodology page.
What to do with the number
BMI works best as a question, not an answer. If it's above the recommended range, it's more useful to look at the things you can change: your waist, the protein on your plate, your steps, your sleep and your blood test results. Losing 0.3–0.5 kg a week is a perfectly good pace.
Before you change how you eat to lose weight, talk to your doctor first if:
- you are pregnant or breastfeeding;
- you have diabetes or take medicines that affect your blood sugar;
- you have kidney disease, heart disease or another long-term condition;
- you are under 18;
- you have or have had an eating disorder;
- you are losing weight without trying.
If your relationship with food worries you, you can talk to a doctor or a psychologist. If you're struggling right now, call your local emergency number or find a free helpline at findahelpline.com.
Frequently asked questions
How do I calculate my body mass index?
BMI = your weight in kilograms divided by your height in metres squared. For example, 78 kg and 1.65 m: 78 / (1.65 × 1.65) ≈ 28.7.
What BMI counts as normal?
The recommended range is usually given as 18.5–24.9. Below 18.5 is lower than recommended; 25–29.9 is slightly above; 30 and over is above the recommended range. These are statistical bands, not a diagnosis and not a judgement.
Does BMI work for people who train?
Often not. Muscle weighs more than fat, so a trained person's BMI can look “too high” with no excess fat at all. Waist measurement is more useful in those cases.
Does BMI work for children?
Not the same way as for adults. For children and teenagers, BMI is read against age and sex growth charts, and that's a job for a paediatrician. Adult ranges must not be applied to children.
What tells you more than BMI?
Your waist divided by your height. Below 0.5 is considered a healthy range; 0.5–0.6 increased risk; 0.6 and above high risk (NICE NG246, 2025). Measure at the level of your belly button, without pulling your stomach in.
Read next
- How much protein do you need a day? – How much protein you need for your weight and goal, where to find it cheaply, and when to ask your doctor.
- How to lose belly fat – Why losing fat “just from your belly” doesn't work, what really shrinks your waist and when waist size matters for health.
- How Dumpli calculates calories – Formulas, data sources, safe limits and what our numbers don't know. All out in the open.