Tinker Tools

BMI Calculator Instantly

Calculate your Body Mass Index using your height and weight. Metric and Imperial units supported with visual BMI range indicator.

ResultMetric mode

Enter your height and weight, then click Calculate BMI.

How it works

1. Enter Measurements

Input your height and weight in either Metric (cm/kg) or Imperial (ft-in/lbs) units. Switch between unit systems with one click.

Metric / Imperial

2. Get Your BMI

Your BMI is calculated using the standard formula: weight(kg) divided by height(m) squared. The result is displayed with its WHO category.

Instant Result

3. Review Your Range

See where you fall on the visual BMI scale. Review the category descriptions and healthy weight range for your height.

Visual Guide

What is BMI?

Body Mass Index — commonly known as BMI — is a numerical value derived from your weight and height. The formula is simple: divide your weight in kilograms by the square of your height in meters (kg/m^2). A person who weighs 70 kg and stands 1.75 m tall, for example, has a BMI of roughly 22.9. That single number places them within the WHO Normal weight category. Health professionals around the world use this metric as a quick screening tool to flag potential weight-related health risks, though it was never designed to serve as a standalone diagnosis.

The concept dates back to the 1830s, when Belgian mathematician Adolphe Quetelet developed what he called the Quetelet Index. He was studying population-level statistics, not individual health. It was not until the 1970s that researcher Ancel Keys renamed it the Body Mass Index and promoted it as a practical proxy for body fat. The simplicity of the calculation — no lab work, no specialized equipment — made it attractive for large-scale public health studies. That same simplicity, however, is also the source of its well-known blind spots.

The World Health Organization defines four primary BMI categories for adults: Underweight (below 18.5), Normal weight (18.5 to 24.9), Overweight (25.0 to 29.9), and Obese (30.0 and above). These thresholds apply to adults aged 20 and older regardless of sex. For children and adolescents, BMI is interpreted differently using age- and sex-specific percentile charts. Keep in mind that BMI is a population-level screening tool. It tells you something about statistical risk, but it cannot measure body composition directly. Two people with an identical BMI may carry very different amounts of muscle and fat.

Key Features and Benefits

  • Instant Calculation — Enter your height and weight and get your BMI in under a second. No waiting, no appointments, no equipment beyond a scale and a tape measure.
  • WHO Category Classification — Your result is automatically mapped to the standard WHO categories — Underweight, Normal, Overweight, or Obese — so you can see where you fall on the recognized international scale.
  • Metric and Imperial Support — Whether you think in kilograms and centimeters or pounds and inches, the calculator handles unit conversion behind the scenes. You pick the system that feels natural.
  • Historical Tracking — Record your BMI over weeks or months to spot trends. A single snapshot is useful, but a trend line over time is far more informative for tracking the impact of lifestyle changes.
  • Educational Context — Raw numbers mean little without context. The tool explains what each category implies for general health risk and reminds you of the limitations that come with any single-number metric.
  • Privacy-First Design — All calculations happen locally in your browser. Your weight and height data are not sent to any server, stored in any database, or shared with third parties.

How to Calculate Your BMI

  1. 1

    Measure Your Height Accurately

    Stand barefoot on a flat surface with your back against a wall. Look straight ahead — not up or down — and have someone mark the wall at the top of your head. Measure from the floor to that mark. Record the value in meters or feet and inches. Small errors here get squared in the formula, so they matter more than you might expect. If you measured in feet and inches, convert to meters by multiplying your total inches by 0.0254.

  2. 2

    Weigh Yourself Consistently

    Use a reliable digital scale on a hard, level floor. Weigh yourself at the same time of day — ideally in the morning before eating. Wear minimal clothing and remove shoes. Body weight fluctuates by 1 to 2 kg throughout a normal day due to food, water, and activity, so consistency in timing reduces noise. Record the value in kilograms. If your scale reads in pounds, multiply by 0.4536 to convert.

  3. 3

    Enter Your Values

    Select your preferred unit system — metric or imperial — and type in your height and weight. Double-check the numbers before proceeding. A misplaced decimal point can dramatically skew the result. If you are using imperial units, the calculator converts everything to metric internally before applying the formula.

  4. 4

    Review Your BMI Result

    The calculator displays your BMI rounded to one decimal place alongside the corresponding WHO category. A BMI of 22.4 falls in the Normal range, while 27.8 places you in the Overweight category. Take a moment to read the brief explanation of what your category means in terms of statistical health risk. Remember — this is a screening tool, not a medical verdict.

  5. 5

    Interpret With Context

    BMI does not distinguish between muscle mass and fat mass. A competitive weightlifter and a sedentary office worker can share the same BMI yet have completely different body compositions. Age, sex, ethnicity, and bone density all influence what a given BMI actually means for your health. If your result surprises you — high or low — consider following up with a healthcare provider who can assess body fat percentage, waist circumference, blood markers, and other indicators that paint a fuller picture.

Expert Tips for Understanding BMI

BMI works best as a starting point, not a finish line. The metric was designed for population-level analysis, and applying it to individuals requires caution. Athletes with significant muscle mass often register as Overweight or even Obese by BMI standards despite having low body fat. Conversely, older adults may have a Normal BMI while carrying excess visceral fat because they have lost muscle mass with age — a condition sometimes called sarcopenic obesity. Neither case is well served by the number alone. If you train regularly or are over 65, treat your BMI as one data point among many rather than a definitive assessment.

Waist circumference is one of the most useful complementary measures. The WHO suggests that a waist measurement above 94 cm for men or 80 cm for women indicates increased metabolic risk, even when BMI falls within the Normal range. This is because fat stored around the abdomen — visceral fat — is more metabolically active and more strongly linked to cardiovascular disease, type 2 diabetes, and certain cancers than fat stored in the hips or thighs. Grabbing a flexible tape measure and wrapping it around your midsection at navel height takes about ten seconds and adds real information that BMI misses.

Ethnic background also matters. Research has shown that certain populations — particularly those of South Asian, Southeast Asian, and East Asian descent — tend to develop metabolic complications at lower BMI thresholds than the standard WHO cutoffs suggest. Some countries have adopted adjusted categories to reflect this. Japan, for instance, defines obesity as a BMI of 25 or above rather than 30. If the standard categories do not align with risk factors that run in your family, discuss ethnicity-specific thresholds with your doctor.

This calculator is an educational tool. It is not medical advice, and it is not a substitute for professional evaluation. If you have concerns about your weight, body composition, or overall health, consult a qualified healthcare provider. They can order lab tests, evaluate your medical history, and create a plan tailored to your specific situation. No online calculator — no matter how accurate its math — can replace that kind of individualized care.

Related Tools

Calorie Calculator

Estimate daily calorie needs based on your activity level

Health Insurance Calculator

Check health insurance premiums that may factor in BMI-related conditions

Currency Converter

Compare healthcare costs across countries

BMI gives you a quick, standardized snapshot of where your weight sits relative to your height. Pair it with the Calorie Calculator to understand your energy balance, or use the Health Insurance Calculator to see how weight-related health factors might affect your coverage costs. If you are researching medical procedures or treatments abroad, the Currency Converter helps you compare expenses across different currencies. Each tool adds a different angle, and together they give you a more grounded view of the financial and health dimensions of weight management.

Frequently Asked Questions

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