BMI calculator
Body mass index is a quick way to relate your weight to your height. This free calculator gives you your BMI and its World Health Organization category the moment you enter your height and weight — in metric or imperial, with no account or email. BMI is a useful screening number, but it is deliberately simple: it treats muscle and fat the same and knows nothing about your fitness, age, or body shape. So we show your result plainly and without judgement. Think of it as one data point among many, not a scorecard. If you want a number you can act on day to day, your calorie needs and food choices tell you far more than a single index ever will.
Your BMI
24.2
Healthy weight
- Underweight
- below 18.5
- Healthy weight
- 18.5 – 24.9
- Overweight
- 25 – 29.9
- Obesity
- 30 and above
How the BMI calculation works
BMI is a single ratio: your weight divided by the square of your height. In metric units the formula is straightforward:
BMI = weight(kg) ÷ height(m)²
If you enter imperial units we first convert them — pounds to kilograms (÷ 2.205) and feet-and-inches to centimetres (× 2.54), then centimetres to metres — so the calculation is the same underneath. A person who is 170 cm and 70 kg, for instance, has a BMI of 70 ÷ 1.70² ≈ 24.2, which lands in the healthy-weight range.
Your BMI is then matched to the WHO adult categories: under 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, and 30 or above is obesity. We show these as neutral information. BMI is a blunt instrument — it can not tell muscle from fat, ignores where weight is carried, and was never meant to judge an individual. Athletes with high muscle mass often read as overweight despite being very lean. Use BMI as a rough orientation, pair it with measures like waist size and activity, and speak to a healthcare professional for anything that matters to your health.
Frequently asked questions
What is BMI?
BMI (body mass index) is your weight in kilograms divided by your height in metres squared. It is a quick screening number that sorts adults into broad weight categories. It was designed to describe populations, so it is a rough guide for an individual, not a diagnosis.
What are the BMI categories?
The World Health Organization uses: below 18.5 underweight, 18.5 to 24.9 healthy weight, 25 to 29.9 overweight, and 30 or above obesity. These are the same thresholds worldwide for adults, and they are informational — a starting point for a conversation, not a verdict on your health.
Is BMI accurate for everyone?
No. BMI does not distinguish muscle from fat or account for where fat is stored, so it can misclassify athletes, older adults, and some ethnic groups. It is one data point among many. For a fuller picture, look at waist measurement, activity, bloodwork, and how you feel, and talk to a healthcare professional.
Estimate a real plate instead
Numbers are a start — logging is where progress happens. Snap a photo of your meal and NibbleCal estimates the calories and macros in about 3 seconds.
Try it with a photo — free, no signup