BMI Calculator: Calculate Your Body Mass Index

BMI is one division and one multiplication. It scales body mass to the square of height so that people of different heights can be compared on the same axis. There are two ways to write it, and they describe the same quantity.

The 703 is not a fudge factor. BMI is defined in metric units, so the US version has to carry the conversion inside it. One pound is 0.45359237 kilograms and one inch is 0.0254 meters, so converting pounds and inches into kilograms and meters squared works out to 0.45359237 ÷ 0.0254², which is 703.06957 and change. The published formula rounds that to 703. That rounding is why a US-unit BMI and a metric BMI can differ in the second decimal place, which is also why this calculator computes from one set of units and converts for display rather than running the arithmetic twice.

This is the original definition, published in the 19th century by the Belgian mathematician Adolphe Quetelet as the Quetelet index. He was describing populations, not patients, and the measure was not built to assess an individual. The name body mass index came more than a century later.

That is the whole calculation. Notice how little went into it: two measurements, no labs, no waist, no training history, no age past the cutoff for adult categories. What comes out is a screening estimate, not a diagnosis.

For adults 20 and older, 18.5 to 24.9 is the range international health bodies label normal weight. The World Health Organization fact sheet on obesity and overweight sets out those adult cutoffs and the reasoning behind them.

Read that range for what it is. It describes statistical associations across large populations between a height-scaled weight and health outcomes. It is not a verdict on an individual, and a BMI inside it is not a clean bill of health any more than a BMI outside it is a diagnosis. Two people at 23.0 can have completely different waist measurements, lipid panels, fasting insulin, and training histories, and those differences matter more than the shared number.

The useful question is never "is my BMI good." It is what the number, taken together with everything BMI leaves out, suggests a clinician should look at next.

None of the following replaces BMI so much as it makes BMI usable, by supplying the information a height-and-weight ratio cannot carry.

• Waist circumference. A tape measure at the level of the navel captures abdominal fat, which BMI cannot see at all.
• Waist-to-height ratio. Dividing waist by height normalizes the waist measurement across body sizes and is simple enough to repeat at home.
• Resting labs. A1c, a lipid panel, and fasting insulin describe how the body is actually handling fuel, which is the thing BMI is being used as a stand-in for.
• Strength and lean-mass trend over time. What direction your lean mass and your working strength are moving says more about the year ahead than a single ratio does.

Taken together, those shift the question from weight to body composition, which is the more honest frame. If you want the energy side of that picture, the TDEE calculator (your total daily burn) estimates how much your body uses in a day, which is a different question from what your mass divided by your height squared happens to be.

Protocol MD is a physician-guided peptide therapy practice. Licensed clinicians review labs, history, and goals across performance, longevity, and body composition, alongside recovery, sleep, energy, and cellular health.

If you want the background before anything else, here is what peptide therapy is and how it is used.

A calculator cannot tell you whether any treatment is appropriate for you. Only a licensed physician can, after a full assessment. If that is the conversation you want to have, you can start the physician-guided assessment.