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BMI Calculator: Check Your Body Mass Index Instantly

Tooly · 2026-08-02 · 5 min read

Body Mass Index is the quickest health screening number there is — a simple ratio of weight to height used by doctors, gyms, and insurers. A BMI calculator gives you the number and its category in seconds, in metric or imperial. Here's how it works, what it means, and its limits.

How BMI is calculated

• Metric — weight (kg) ÷ height (m)²
• Imperial — (weight (lb) ÷ height (in)²) × 703

Calculate your BMI free with Tooly

The Tooly BMI Calculator makes it instant:

• Enter your weight and height — metric or imperial
• Get your BMI and category immediately
• Runs in your browser — nothing is uploaded
• Free, no signup, no limits

BMI categories

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Normal weight
25 – 29.9	Overweight
30 – 34.9	Obesity (Class I)
35 – 39.9	Obesity (Class II)
40+	Obesity (Class III)

What BMI does and doesn't tell you

• Good screening tool — cheap, instant, and correlates with population health trends
• Doesn't measure body fat directly — athletes with high muscle mass often show "overweight"
• Doesn't consider age, sex, or body composition — use it as a starting point, not a verdict
• Pair with other signals — waist circumference, bloodwork, and how you feel matter more

Frequently Asked Questions

Is this BMI calculator really free? Yes — unlimited use, no account, no limits.

Is my data private? Everything runs in your browser — your measurements never leave your device.

Is BMI accurate for athletes? No — it can misclassify muscular people. Use it as a general screen, not a diagnosis.

What else should I check? The Age Calculator and Unit Converter pair well for health tracking.

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