How to calculate your BMI

Body mass index from your height and weight — in metric or imperial — plus the healthy range for your height.

BMI (body mass index) is a quick screening number that relates your weight to your height. It's used by health services worldwide to flag potential under- or overweight — though it's a starting point, not a diagnosis, since it can't tell muscle from fat. The Toolsfully BMI calculator gives you the number, your category, and the healthy weight range for your height. Health data is about as personal as data gets, which is why this calculator runs entirely on your device.

Step-by-step

1. Open the BMI calculator. Go to Calculators and select the BMI tab.
2. Pick your units. Choose Metric (cm, kg) or Imperial (ft/in, lb) with the toggle.
3. Enter height and weight. The BMI value, category, and healthy range update instantly as you type.

Tips

• The healthy range is height-specific. The third tile shows the weight span that corresponds to a BMI of 18.5–24.9 for your height — a more actionable number than the index itself.
• Muscle skews it. Athletes often read "overweight" on BMI while being perfectly lean. Waist measurement and body-fat percentage tell a fuller story.
• Track the trend. A single BMI reading matters less than its direction over months.

What the number is, and what it was built for

BMI is weight divided by height squared, and it is nearly two centuries old — devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was studying the characteristics of populations and explicitly not assessing individuals. The name "body mass index" came much later, in the 1970s, when it was adopted as a convenient screening measure precisely because it needs only two numbers that are easy to collect at scale.

That history explains both its usefulness and its limits. As a way of describing how a large group compares with another large group, it works and it is cheap. As a statement about one person standing in front of you, it is a rough indicator that a conversation might be worth having, which is roughly how clinicians treat it.

What it cannot see

The formula has two inputs, so everything else is invisible to it. It cannot distinguish muscle from fat, which is why athletes with considerable muscle mass often land in categories that plainly do not describe them. It says nothing about where weight is carried, although distribution is known to matter for health. It does not account for bone density, and it behaves differently for the very tall and the very short because dividing by height squared is an approximation that fits the middle of the range better than the ends.

The category boundaries are also softer than they look. They are conventional cut-points on a continuous scale rather than thresholds where anything changes, so a value a fraction either side of a line is not meaningfully different from one just inside it — and the World Health Organization has noted that different cut-points may be appropriate for some populations, since the relationship between BMI and health risk is not identical everywhere. This page calculates the number and shows you where it falls. What it means for you is a question for a clinician who can see the rest of the picture.

Frequently asked questions

What do the categories mean?

Under 18.5 is classed as underweight, 18.5–24.9 as a healthy weight, 25–29.9 as overweight, and 30+ as obese — per the World Health Organization's adult ranges.

Is BMI accurate for everyone?

No — it doesn't account for muscle mass, age, sex, or body composition, and adult ranges don't apply to children. Talk to a healthcare provider about what's right for you.

Is my data private?

Yes. The calculation runs in your browser and your measurements never leave your device.