Calculators

BMI calculator

We thought hard about putting a BMI calculator on No Spoons. BMI is the number that gets quoted whenever weight comes up in a medical appointment, and it’s far easier to question a number you understand than one you’ve been handed.

So here’s the calculator, plus what BMI can and can’t tell you. Use it as a conversation starter with your care team, not a verdict on your body.

Your numbers

What your results mean

BMI

Your weight in kilograms divided by your height in metres squared.

Resting energy (BMR)

Roughly how much energy your body uses to keep going while you do nothing.

Estimated daily energy needs

Your BMR adjusted for how active you are.

The calculator asks for sex because the resting energy equation uses it. BMI itself doesn’t. All three results are estimates from standard equations, and none of them know about your medications, your health conditions or how your energy changes from day to day.

BMI ranges for adults

BMILabel clinicians use
Under 18.5Underweight
18.5 to 24.9Healthy weight range
25 to 29.9Overweight
30 and overObesity

Some guidelines use different cut-offs for some population groups, for example lower cut-offs for people of Asian descent. Your GP can tell you which they use.

What BMI is

BMI compares your weight with your height. It was designed to compare groups of people, which is part of why it’s blunt for one person. Most of us met it at school or in a GP’s office. Think of it as a screening tool: it can prompt questions, but it can’t answer them.

The limits

What BMI can’t tell you

Muscle, fat and bone

It doesn’t tell them apart. Someone with a lot of muscle can land in a higher range, and someone with little muscle can land in the healthy range. The scale has the same blind spot, which is why taking measurements can show change the number misses.

Who it was built on

BMI was largely developed from studies of people of European descent, so it can misclassify other groups. It also shouldn’t be used for children and teenagers, whose bodies are still growing. They’re assessed on growth charts instead, so ask a GP or child health nurse.

Where fat sits

Two people with the same BMI can carry weight in different places, and that matters to clinicians. BMI can’t see it.

The rest of your health

Age, activity, medications, medical conditions and what you eat aren’t part of the sum.

Where the lines fall

The cut-offs are lines drawn on a scale. Someone just either side of a line isn’t meaningfully different from the person next to them.

Calories and resting energy

I think looking at calories alone is a bit daft. A calorie figure tells you how much energy, not what’s on the plate. If you’d rather look at food than numbers, the macro calculator splits energy into protein, carbs and fat, and Macros made simple explains what that means.

At the doctor

Taking your numbers to an appointment

If BMI comes up, you can ask:

What else are you looking at besides BMI?
Does my BMI change what you’d suggest, or is it just for the record?
Is there anything else I should be tracking?

For advice that fits you, a GP or an Accredited Practising Dietitian can help. Our Editorial, Health and Sourcing Policy explains how we handle health information.

If weight and food are a hard topic

If thinking about weight or calories tends to send you somewhere unhelpful, you’re allowed to close this page. The Butterfly Foundation’s helpline (1800 33 4673) supports people with body image and eating concerns, and your GP is a good first call too.

Looking at food instead of numbers?

Try the macro calculator, or read How macros fuel our body and Keto vs low carb: what’s the difference?.

Try the macro calculator