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BMI Calculator

Calculate adult body mass index from height and weight in US or metric units, with the adult category and the healthy weight range for your height.

Level: Grades 6–7

  • Common Core standard 6.EE.A.2.cEvaluate expressions at specific values of their variables. Include expressions that arise from formulas used in real-world problems. Perform arithmetic operations, including those involving whole-number exponents, in the conventional order when there are no parentheses to specify a particular order (Order of Operations).
  • Common Core standard 6.RP.A.3.dUse ratio reasoning to convert measurement units; manipulate and transform units appropriately when multiplying or dividing quantities.
Printable worksheet

BMI Calculator

Know the answer you need? Pick an input to work backward.

Inputs
Units

Result

Body mass index

23.6 kg/m²

A BMI of 23.6 falls in the "Healthy weight" range for adults. At this height, a BMI of 18.5–24.9 corresponds to 125.3 – 168.6 lb.

Adult BMI category
Healthy weight
BMI 18.5–24.9 at this height
125.3 – 168.6 lb
Your weight compared with that range
Within the range
BMI Prime (BMI ÷ 25)
0.95

See the step-by-step solution ↓

Adult BMI categories and the matching weights at 5 ft 9 in
Adult BMI categories and the matching weights at 5 ft 9 in
CategoryBMIWeight at your heightYou
UnderweightBelow 18.5Under 125.3 lb
Healthy weight18.5 – 24.9125.3 lb – 168.6 lbYour result
Overweight25.0 – 29.9169.3 lb – 202.5 lb
Obesity, class 130.0 – 34.9203.2 lb – 236.3 lb
Obesity, class 235.0 – 39.9237 lb – 270.2 lb
Obesity, class 340.0 and above270.9 lb or more

Step-by-step solution

  1. Change your height to inches, then to meters, because the BMI formula uses meters (1 inch is exactly 0.0254 m)

    (5×12+9)×0.0254=1.7526 m

  2. Change your weight to kilograms (1 pound is exactly 0.45359237 kg)

    160 lb×0.453592=72.575 kg

  3. Divide your weight by your height times itself. That is your BMI

    72.5751.75262=23.6

  4. Round to one decimal place and see which adult category it falls in (they start at 18.5, 25, 30, 35 and 40)

    23.6 → Healthy weight

  5. Find BMI Prime by dividing by 25, the top of the healthy-weight range. A result of 1.00 sits right at that top

    23.6325=0.95

  6. Find the weights that give a BMI of 18.5 to 24.9 at your height: multiply each limit by your height times itself

    18.5×1.75262=56.8 kg; 24.9×1.75262=76.5 kg

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

Please note: This calculator gives a general screening estimate, not a diagnosis. Talk to a qualified healthcare professional about your individual health.

What this calculator does

Find your BMI (body mass index), a quick number that compares your weight with your height. Enter your height and weight, and you'll see your BMI and the adult category it falls in. You'll also see the weight range that gives a BMI of 18.5–24.9 at your height.

BMI is your weight in kilograms divided by your height in meters, squared (multiplied by itself). It only needs two measurements, so it's widely used as a quick screening number for adults.

BMI is not a diagnosis. It can't tell muscle from fat, or show where on the body fat is carried. So the same number can mean different things for different people. Use your result as a starting point for a conversation with a healthcare professional, not a verdict.

Where it's used

Why this math matters outside the classroom:

  • Checkup screening. Clinicians use BMI as a quick weight-for-height screening number at routine checkups, alongside other measures.
  • Population health. Health agencies compare BMI across large national surveys to see how weight trends change over the years.
  • Unit conversion. Converting 160 lb and 5 ft 9 in to about 72.6 kg and 1.75 m before dividing shows where the US formula's factor of 703 comes from.
  • Evaluating a formula. Students substitute real values into kg ÷ m², such as 72 ÷ 1.75², which comes to about 23.5.
  • Fitness tracking. Fitness apps recalculate BMI each time someone logs a new weight, so the trend over several months is easy to see.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.EE.A.2.c, 6.RP.A.3.d. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose US units (feet, inches, pounds) or metric units (centimeters, kilograms).
  2. Enter your height and weight. In US units, enter whole feet first, then the extra inches.
  3. Read your BMI, the adult category it falls in, and the weight range that gives a BMI of 18.5–24.9 at your height.
  4. Check the table to see where each category starts and ends at your height. It also shows how far your weight is from the 18.5–24.9 range.

What the inputs mean

Units
This only changes the units you type in. US measurements are converted to metric exactly before the math is done.
Height (feet)
Whole feet only. Put the rest in the inches box, so 5 ft 9 in is 5 and 9.
Height (inches)
The inches on top of the whole feet, from 0 up to just under 12. Decimals are fine.
Weight
Your body weight in pounds.
Height
Your height in centimeters, at least 50 cm.

Formula and method

BMI = weight (kg) ÷ height (m)²
height (m) = (feet × 12 + inches) × 2.54 ÷ 100
weight (kg) = pounds × 0.45359237
BMI Prime = BMI ÷ 25
weight at a given BMI = BMI × height (m)²

In the formulas, kg is kilograms, m is meters, and a small ² means "times itself". So height (m)² is your height in meters multiplied by itself.

The math is always done in metric. US numbers are converted using the exact official sizes of the inch (2.54 cm) and the pound (0.45359237 kg). So nothing is rounded before your BMI is worked out.

Your category comes from your BMI rounded to one decimal place. It uses the WHO (World Health Organization) adult cut-offs. Under 18.5 is underweight, and 18.5–24.9 is healthy weight. 25.0–29.9 is overweight, and 30.0–34.9 is obesity class 1. 35.0–39.9 is obesity class 2, and 40.0 and above is obesity class 3.

The weight range shown is every weight that would give a BMI from 18.5 to 24.9 at your height. That's 18.5 × height² up to 24.9 × height². The table does the same for every category. The comparison line shows how far your weight is from that range, in the units you chose.

BMI Prime was suggested by Gadzik in 2006. It divides BMI by 25, the top of the healthy-weight range. A BMI Prime of 0.74 to 1.00 roughly matches the healthy-weight range. A value of 1.20 means a BMI 20% above that top limit.

Worked example

An adult is 5 ft 9 in tall and weighs 160 lb.

  1. Height: (5 × 12 + 9) × 2.54 = 175.26 cm = 1.7526 m. Squared: 3.0716 m².
  2. Weight: 160 × 0.45359237 = 72.575 kg.
  3. BMI: 72.575 ÷ 3.0716 ≈ 23.63, shown as 23.6.
  4. Weight range for BMI 18.5–24.9 at 1.7526 m: about 125.3 – 168.6 lb. So 160 lb is inside the range.
  5. BMI Prime: 23.63 ÷ 25 ≈ 0.95.
  6. From the table, overweight (BMI 25.0–29.9) would start at about 169.3 lb at this height.

Result: The BMI is 23.6 (BMI Prime 0.95), which is in the healthy weight category for adults.

Understanding the result

A BMI in the healthy weight range doesn't promise good health. A BMI outside it doesn't, on its own, mean something is wrong. Waist size, fitness, blood pressure, diet and family history all add context that BMI leaves out.

If your result is outside the 18.5–24.9 range, or you have any worry about your weight, talk with a doctor or other healthcare professional. They can look at it alongside other measures.

Some health organizations use different cut-offs for certain groups of people. That's because the link between BMI and health risk isn't the same for every group.

Assumptions and limitations

  • The person is an adult, and the measurements are accurate. CDC guidance uses these categories from age 20. Younger people are measured with growth percentiles instead.
  • Categories use the WHO adult cut-offs on the BMI rounded to one decimal place. So 24.94 counts as healthy weight, and 24.95 as overweight.
  • BMI can't tell muscle from fat. Athletes and people with a muscular build can be classed as overweight while carrying little body fat.
  • It isn't designed for children and teens. Their BMI is compared with growth percentiles, charts that rank them against other young people of the same age and sex. This calculator doesn't use them.
  • It can be misleading during pregnancy and for older adults, because their bodies are made up differently from most adults'.
  • It says nothing about where fat is carried on the body. That matters on its own, apart from total weight.

Frequently asked questions

Is BMI accurate?

The math is exact, but BMI only estimates body fat indirectly. It works fairly well for screening large groups of people. It can be misleading for one person with an unusual build. A healthcare professional can use other measurements to fill in the picture.

What is a healthy BMI for adults?

The WHO adult scale calls 18.5 to 24.9 the healthy-weight range. 25.0 to 29.9 is overweight, and 30.0 or more is obesity. These cut-offs are the same for men and women. Some health agencies use lower cut-offs for people of Asian descent. So your clinician may read the same number differently.

How much would I need to weigh to be in the 18.5–24.9 range?

Multiply 18.5 and 24.9 by your height in meters squared. At 5 ft 4 in (1.6256 m), for example, that's about 48.9 to 65.8 kg, or roughly 107.8 to 145.1 lb. The calculator does this for your height. It also shows how far your current weight is from either end.

What is BMI Prime?

BMI Prime is your BMI divided by 25. It compares your BMI with the top of the healthy-weight range. 0.90 means 10% below that top, and 1.10 means 10% above it. It holds exactly the same information as BMI, on a different scale.

Can I use this calculator for my child?

No. For children and teens, the same BMI means different things at different ages, and it differs between boys and girls. So it's compared with growth percentiles instead. Ask a pediatrician, or use a tool built for children.

Why does my category change at 24.95 instead of 25?

Your category comes from your BMI rounded to one decimal place, to match how the cut-offs are written. A BMI of 24.95 rounds to 25.0, which is where the overweight range starts.

What does the weight range at my height mean?

It's the range of body weights that would put someone of your height in the 18.5–24.9 BMI range. It's a reference point from the formula, not a personal goal. A healthcare professional can tell you what's right for you.

Is my height and weight saved or shared?

The math runs in your browser, and this calculator doesn't offer a shareable link with your measurements in it.

How this calculator is checked

Method: Standard BMI definition (kg/m²); WHO adult BMI classification cut-offs; BMI Prime as BMI ÷ 25 (Gadzik, 2006); 1959 international inch and pound definitions for unit conversion. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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