Calculate Your Body Mass Index & Understand Your Health

Enter your height and weight and this calculates your body mass index, plus the numbers most calculators leave out: basal metabolic rate, an ideal weight range, and an estimated body fat percentage. Works for adults and kids. Everything runs on your device; nothing you type ever reaches a server.

Understanding Body Mass Index

What is BMI?

Body Mass Index is your weight in kilograms divided by your height in meters, squared. That's the whole formula. No lab work, no equipment.

It's also older than most people assume. A Belgian mathematician named Adolphe Quetelet worked out the ratio in the 1830s, and he wasn't trying to diagnose anyone. He was trying to describe the "average man" for population statistics. The name "Body Mass Index" didn't exist yet; a physiologist named Ancel Keys coined that in 1972, well over a century later. The WHO and most public health bodies still use Quetelet's original math today, just repurposed from population studies to individual screening.

The formula: BMI = weight (kg) ÷ height (m)². Working in pounds and inches instead? Multiply weight by 703, then divide by height in inches, squared. This calculator handles either unit system. You don't need to convert anything by hand.

BMI Categories

Here's how the WHO breaks down adult BMI, for anyone 20 and older:

ClassificationBMI (kg/m²)Health Risk
Underweight< 18.5Increased
Normal weight18.5 - 24.9Least
Overweight25.0 - 29.9Increased
Obese Class I30.0 - 34.9High
Obese Class II35.0 - 39.9Very high
Obese Class III>= 40.0Extremely high
Limitations of BMI

Body mass index has a well-known blind spot: it can't tell muscle from fat. A bodybuilder and someone with a sedentary lifestyle can land on the exact same number and be in completely different places health-wise. The formula is counting mass, not composition.

Age changes the picture too. The CDC has found that older adults tend to carry more body fat than younger adults do at the same reading, and women typically carry more body fat than men at an equivalent number. None of this makes the metric useless. It just means it's a starting point, not a verdict. Pair it with a waist measurement or a body fat estimate if you want the fuller picture, and bring actual blood work into the conversation if you're making decisions with a doctor.

BMI vs. Body Fat Percentage

Body mass index and body fat percentage are answering two different questions. The index checks whether your weight makes sense for your height. Body fat percentage tells you what that weight is actually made of: muscle, bone, water, or fat.

This calculator estimates body fat with the Deurenberg formula, which factors in your reading, age, and sex to approximate a number without any special equipment. It's an estimate, not a measurement. If you need a precise figure, a DEXA scan or a bioelectrical impedance test done by a professional will get you much closer.

About This Tool

Built by Junaid Farooq. I put this together because most body mass index calculators give you one number and leave you guessing. This one shows the full picture: your category, basal metabolic rate, ideal weight range, and a body fat estimate, with context about what the numbers actually mean.

Every calculation runs in your browser. No health data is sent to any server.

Last updated: July 2026

Sources

This calculator uses established formulas and classifications from the following sources:

  • World Health Organization. Obesity: Preventing and Managing the Global Epidemic. WHO Technical Report Series 894. Geneva: World Health Organization; 2000. WHO publication
  • World Health Organization: Obesity and Overweight
  • CDC: Adult BMI Categories
  • NIH: BMI Calculator
  • Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness: age- and sex-specific prediction formulas. British Journal of Nutrition. 1991;65(2):105-114. PubMed
  • Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. The American Journal of Clinical Nutrition. 1990;51(2):241-247. PubMed
  • Robinson JD, Lupkiewicz SM, Palenik L, Lopez LM, Ariet M. Determination of ideal body weight. Drug Intelligence & Clinical Pharmacy. 1983;17(7-8):561-565.
  • Miller DR, Carlson JD, Lloyd BJ, Day BJ. Determination of ideal body weight. Journal of the American Pharmaceutical Association. 1983;23(5):374-376.
  • Hamwi GJ. Therapy: changing dietary concepts. In: Diseases of Metabolism and Nutritional Therapy. 1964:197-209.

Medical disclaimer: This calculator is for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of a qualified healthcare provider with any questions you may have regarding your health or a medical condition. Never disregard professional medical advice because of something you read on this website.

Frequently Asked Questions

Common questions about body mass index, accuracy, and how to use your results.

Is BMI accurate for athletes and bodybuilders?
Not reliably, no. BMI can't distinguish muscle from fat, so someone with a lot of muscle mass and low body fat can score in the overweight or even obese range on BMI alone. If you train seriously, a body fat percentage estimate, or a proper composition scan, will tell you more than the BMI number will.
What is a healthy BMI for women over 50?
The WHO's standard categories (18.5 to 24.9 as normal weight) apply broadly across adults, including women over 50, but body composition does shift with age and with menopause specifically. Body fat tends to increase somewhat even at a stable weight. Many clinicians treat BMI as one data point for this age group rather than the whole picture, alongside waist circumference and bloodwork. Worth discussing your specific number with a doctor rather than treating any single threshold as a hard rule.
How do I calculate BMI manually?
Metric: divide your weight in kilograms by your height in meters, squared. Imperial: multiply your weight in pounds by 703, then divide by your height in inches, squared. Either way, you land in the same categories on the WHO table above. This calculator just does the arithmetic and the unit conversion for you.
What is the difference between BMI and body fat percentage?
BMI is a ratio of weight to height. It tells you nothing about what that weight is made of. Body fat percentage measures how much of your total weight is fat tissue specifically, which is the number more directly tied to health risk. Two people can share a BMI and have very different body fat percentages depending on muscle mass, and this calculator gives you both numbers so you're not relying on BMI alone.
Can children use a standard BMI calculator?
No. This is one of the most common mistakes with BMI tools. Kids and teens aren't measured against fixed adult categories; their BMI needs to be plotted against CDC growth charts as a percentile for their specific age and sex, because a normal number at age 8 is different from a normal number at age 15. Use this calculator's pediatric mode for anyone under 20, not the standard adult categories.
What is BMI Prime?
BMI Prime is your BMI divided by the upper threshold of the normal range, 25. A BMI Prime of exactly 1.0 puts you right at that boundary; anything under 1.0 falls within the normal range, anything over it doesn't. It's mostly used in academic and clinical contexts as a quick way to express distance from the normal-weight cutoff, rather than something most people need day to day.
Why does BMI matter for health insurance and medical screening?
Insurers and healthcare providers use BMI as a fast, cheap, standardized screening number. It costs nothing to calculate and correlates, at a population level, with certain health risks. That population-level correlation is exactly why it's used so widely, and exactly why it's a poor tool for judging any one individual: it wasn't built to be precise for a single person, only informative across large groups.
Is BMI different for Asian populations?
Yes, and it's a genuinely important adjustment. Research has shown people of Asian descent tend to face increased health risks at lower BMI thresholds than the WHO's standard categories reflect, so the WHO has published separate, lower cutoff points for Asian populations in some clinical contexts. If you're of Asian descent, it's worth asking a healthcare provider which threshold set applies to you rather than defaulting to the standard categories.
How often should I check my BMI?
There's no fixed schedule, and checking daily won't tell you anything useful. Normal day-to-day weight fluctuation from water and food will move the number without meaning anything. Monthly is plenty for tracking a real trend, or alongside a regular checkup if a healthcare provider is tracking it as part of your care.
Does this calculator store my personal data?
No. Your height, weight, age, and results are calculated entirely in your browser and never sent to a server. The only thing that persists is your last entry, saved locally on your own device so you don't have to retype it. That's optional, cleared the moment you clear your browser data.