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BMI Calculator

A detailed BMI calculator that works out your Body Mass Index from weight, height, gender, and age in Standard (lb, ft/in) or Metric (kg, cm) units, with a category gauge, a full weight-range table for your exact height, your healthy weight range, and BMI Prime.

How the BMI Calculator works

Choose Standard (lb, ft/in) or Metric (kg, cm) units, enter your gender, optional age, height, and weight, then select "Calculate your BMI." The calculator instantly shows your exact BMI value, a color-coded gauge showing where you fall between 18.5 and 30+, and a full weight-range table — built specifically for your height — showing exactly which weight ranges correspond to underweight, healthy, overweight, and obesity.

How BMI is calculated

In metric units, BMI = weight (kg) ÷ height (m)². In imperial units, BMI = weight (lb) ÷ height (in)² × 703. Both formulas produce the same BMI value for the same body — the 703 constant simply converts pounds and inches into the metric-equivalent ratio. The calculator also reports BMI Prime (your BMI divided by the upper healthy limit of 25), a simple ratio doctors sometimes use to see at a glance how far above or below the healthy ceiling someone falls.

Your personal weight-range table

Rather than showing generic BMI cutoffs, the calculator converts each category boundary (18.5, 25.0, 30.0) into an actual weight in your chosen unit, calculated for your exact height — the same style of table used by the NIH's National Heart, Lung, and Blood Institute and Harvard Health BMI calculators. It also tells you exactly how many pounds or kilograms you'd need to lose or gain to move into the healthy range.

What BMI does and doesn't tell you

BMI is a fast, free screening tool used by health organizations worldwide, but it doesn't directly measure body fat and doesn't account for muscle mass, bone density, age, or sex. A muscular athlete can have a "high" BMI without excess body fat. Gender and age are collected here for context, but standard adult BMI thresholds are applied the same way regardless of the values entered. Use BMI as a starting point, not a diagnosis — a healthcare provider can give a complete picture.

Common Mistakes to Avoid

Mixing metric and imperial values

Solution:

Entering weight in pounds with height in centimeters (or any mixed combination) produces a meaningless result — always keep weight and height in the same unit system, or switch the calculator's unit toggle.

Entering height in the wrong unit

Solution:

Height must be in meters (not centimeters) for the raw metric formula — this calculator handles that conversion for you automatically, but doing it by hand is the single most common BMI mistake.

Treating BMI as a direct measure of body fat

Solution:

BMI is a ratio of weight to height, not a direct body-fat measurement — it doesn't distinguish muscle from fat, so athletic or muscular people can show a misleadingly high BMI.

Applying adult BMI categories to children

Solution:

BMI categories for children and teens are age- and sex-specific percentiles, not the same fixed thresholds used for adults — a pediatric BMI chart should be used instead.

Ignoring that BMI ranges are population averages

Solution:

The standard underweight/healthy/overweight/obesity cutoffs are general population guidelines — individual factors like frame size, ethnicity, and muscle mass can shift what's healthy for a specific person.

Reading only the single BMI number and ignoring the range table

Solution:

The weight-range table shows exactly how many pounds or kilograms separate you from the next category — a much more actionable detail than the raw BMI number alone.

Frequently Asked Questions

For most adults, a BMI between 18.5 and 24.9 is considered a healthy weight, according to the WHO/CDC classification used by this calculator.
BMI is a useful general screening tool but has known limitations — it doesn't account for muscle mass, bone density, age, sex, or body fat distribution, so it can be less accurate for athletes, older adults, and some ethnic groups.
Yes — switch between Standard (pounds, feet, and inches) and Metric (kilograms and centimeters) using the unit selector, and the calculator converts automatically.
They're collected for context and to make the result feel personal, but the standard adult BMI formula and category thresholds are the same for every adult regardless of gender or age — the calculation itself doesn't change based on those fields.
It converts each BMI category boundary (18.5, 25.0, 30.0) into an actual weight for your exact height, so instead of an abstract BMI number you see the real weight ranges for underweight, healthy, overweight, and obesity — the same approach used by the NIH and Harvard Health BMI calculators.
BMI Prime is your BMI divided by 25 (the upper boundary of the healthy range). A BMI Prime under 1.0 means you're below the healthy ceiling; above 1.0 means you're above it — it's a quick way to see how far you are from that threshold.
No — it uses the standard adult BMI formula and categories. Children and teens require age- and sex-specific growth-chart percentiles instead of fixed adult thresholds.
No — every calculation runs locally in your browser. Nothing you enter is uploaded, logged, or stored on any server.
No — BMI is a helpful starting point, but it's best combined with other measures (waist circumference, body composition, overall fitness) and a conversation with a healthcare provider for a complete assessment.

Free Online BMI Calculator

Calculate your Body Mass Index (BMI) instantly using Standard (lb, ft/in) or Metric (kg, cm) units. Enter your gender, age, height, and weight to see your exact BMI, a visual category gauge, and a full weight-range table built for your exact height — just like the calculators used by the NIH and Harvard Health. Optionally add your waist measurement and age for a waist-to-height ratio and target heart rate zones too.

What You Get

  • Instant BMI calculation in Standard or Metric units
  • A color-coded gauge showing exactly where your BMI falls
  • A personal weight-range table converting each category cutoff into your actual weight, in your unit, for your height
  • Your healthy weight range and how much you'd need to lose or gain to reach it
  • BMI Prime — a quick ratio showing how far you are from the healthy ceiling
  • Waist-to-height ratio — an independent cardiometabolic risk indicator based on your waist measurement
  • Target heart rate zones for moderate and vigorous activity, based on your age
  • No signup, no data stored — everything runs in your browser

Who Uses a BMI Calculator

People tracking general health and fitness goals, healthcare intake forms, insurance and wellness programs, and anyone curious how their weight, height, and waist size relate to standard health screening ranges use this calculator for a quick, detailed, and free estimate.