Height and weight, nothing else. You get your BMI and the WHO category straight away — and then the honest part, which is what BMI cannot tell you.
Your Height and Weight WHO categories
Please enter a height between 120 and 230 cm and a weight between 30 and 250 kg.
That is a screening number, not a diagnosis, and it cannot tell you what to eat. The two numbers that can: your body fat percentage, which knows muscle from fat, and your TDEE, which tells you how much food is actually maintenance. Both free, no signup.
BMI Categories (WHO, adults 18+)
What BMI Cannot Tell You
BMI is your weight divided by your height squared. That is the whole calculation. It has no way of knowing how much of that weight is muscle and how much is fat, where the fat sits, how strong you are, how well you sleep or what you ate this week. A rugby second row and a man of the same height and weight who has never trained get the same BMI and land in the same category, and only one of them has anything to think about.
It also runs the other way. Someone can sit comfortably inside the healthy band while carrying very little muscle and a lot of fat around the middle — the pattern people call skinny fat. BMI will tell that person everything is fine. A tape measure will not.
None of that makes BMI useless. Across a whole population it tracks risk well enough that the WHO and the HSE still use it, and it costs nothing to work out. It is a screening number: a cheap first look that tells you whether a question is worth asking. It is not a diagnosis and it is not medical advice. If your number worries you, or you have symptoms you are unsure about, talk to your GP — that is the right next step and it is not one a website can do for you.
The honest summary: BMI answers one narrow question, and it answers it in about two seconds. Then you need better numbers.
Better Numbers, All Free
What BMI got wrong — the body fat calculator uses a tape measure to separate muscle from fat, which is the single thing BMI cannot do.
What to actually eat — the TDEE calculator gives you maintenance calories, and the macro calculator splits them. Protein targets are what stop you losing muscle on the way down.
Where to start — the free Day One pack is the whole method on one page. No call, no signup, nothing to book.
How to track it — the free calorie tracker app (or use it in the browser) logs Irish supermarket food without an account. If the number is not moving, this is usually why.
The World Health Organization puts the healthy range at 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese. The same cut-offs apply to adult men and women. They mark where risk across a population starts to bend, which is not the same thing as a verdict on you. Plenty of healthy people sit outside the band and plenty of unhealthy people sit inside it.
No. Muscle is denser than fat, so anyone carrying a decent amount of it can land in the overweight band while being visibly lean. BMI sees a heavier number on the scale and has no way to ask why. If you train, use the body fat calculator and the mirror, and treat BMI as background noise. The same applies in reverse to anyone who has never trained: a flattering BMI is not a clean bill of health.
Treat it as a prompt, not an answer. If it sits in the healthy band and you feel well, carry on. If it is high and you want to change it, the lever is energy intake, not the chart: work out your maintenance calories, eat under them, keep protein high enough to hold onto muscle, and check again in eight weeks. Counting calories properly moves the number. Re-reading the chart does not.
No. Children and teenagers are read against age and sex percentile charts rather than the adult cut-offs, and BMI is not meaningful during pregnancy. In both cases ask a GP or public health nurse rather than a calculator. This tool is for adults over 18 and is for general information only.
About the author
Keith O’Beirne went from 90kg to 72kg around a full-time job and two kids, after paying a personal trainer to learn the basics. He is not a doctor, dietitian or coach and does not sell coaching — he writes No Nonsense Fitness to give away what he paid to learn, free, with no ads, no subscriptions and no commission on any link. Based in Dublin.
For general information only, not medical advice, and not intended to diagnose any condition. How every calculator on this site works is set out on our methodology page, and the rest of the free tools are on the free tools page.