BMI Calculator
Check your Body Mass Index and which category it falls into.
What BMI means
Body Mass Index is a simple ratio of your weight to your height. It was designed in the 1830s to describe populations, not individuals, and it remains a rough screening tool rather than a measure of health or body fat. It's useful as one cheap data point, and genuinely misleading if you treat it as a verdict — it can't tell muscle from fat.
The BMI formula
BMI = weight in kg ÷ (height in metres)²
Imperial: BMI = (weight in lb ÷ (height in inches)²) × 703
Standard adult categories: under 18.5 underweight, 18.5-24.9 normal, 25-29.9 overweight, 30 and above obese. These cut-offs come from population data and don't adjust for muscle mass, age, sex, or ethnicity.
Worked example
Someone 175 cm tall weighing 80 kg:
- Height in metres: 1.75
- 1.75² = 3.0625
- 80 ÷ 3.0625 = BMI 26.1
That lands in the 'overweight' band — but a muscular 80 kg athlete at that height would get the same number as someone carrying significant fat. This is exactly the limitation to keep in mind.
Where BMI falls short
- It can't distinguish muscle from fat, so athletes and lifters are routinely misclassified as overweight.
- It ignores fat distribution. Waist circumference and waist-to-height ratio predict metabolic risk better.
- Cut-offs vary by population — some health bodies use lower thresholds for people of South Asian descent.
- It's not validated the same way for children, pregnant people, or the elderly, who need different references.
- Better companions: waist-to-height ratio (aim under 0.5), progress photos, strength trends, and bloodwork.
Frequently asked questions
How accurate is this bmi calculator?
It uses the Mifflin-St Jeor equation, the most accurate general formula for estimating metabolic rate, combined with standard activity multipliers. Results are a solid starting estimate — individual needs vary, so adjust based on your real-world results over a few weeks.
Is it free to use?
Yes, completely free with no sign-up. Everything is calculated in your browser and nothing you enter is stored.
Should I use metric or imperial?
Either — switch units at the top of the calculator and enter your numbers in whichever system you prefer. The math is identical.
This calculator provides a general estimate for informational purposes only and is not medical or nutritional advice. Consult a qualified professional before making significant changes to your diet or exercise routine.