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Simple Calculators

BMI Calculator

Calculate your Body Mass Index and understand your weight category with personalized health insights

kg
cm

Age and gender are optional; they tailor the health notes below, not the BMI itself.

Understanding BMI

World Health Organization categories for adults (age 20+), and what BMI can and can't tell you.

WHO BMI categories

Severe thinness
< 16
Moderate thinness
16 – 17
Mild thinness
17 – 18.5
Normal
18.5 – 25
Overweight
25 – 30
Obese class I
30 – 35
Obese class II
35 – 40
Obese class III
> 40

BMI limitations

  • Cannot distinguish between muscle and fat mass
  • May not be accurate for athletes with high muscle mass
  • Older adults tend to have more body fat than younger adults with the same BMI
  • Women tend to have more body fat than men for an equivalent BMI
  • May not be suitable for very tall or very short individuals

BMI is fairly indicative of body fat for 90–95% of people, but use it alongside other measures, such as waist size, to judge a healthy weight.

Risks of being overweight

  • High blood pressure
  • Type II diabetes
  • Coronary heart disease
  • Stroke
  • Sleep apnea
  • Certain cancers

Risks of being underweight

  • Malnutrition & vitamin deficiencies
  • Osteoporosis
  • Decreased immune function
  • Growth & development issues
  • Reproductive issues
  • Surgery complications

Related calculators

All health & fitness

Quick summary

BMI (body mass index) is your weight in kilograms divided by your height in meters squared. For adults, a BMI under 18.5 is underweight, 18.5–24.9 is healthy, 25–29.9 is overweight, and 30 or higher is obese. BMI is a quick screening tool, not a diagnosis: it cannot tell muscle from fat, so talk through your result with a healthcare provider.

Reviewed by Simple Calculators TeamLast reviewed

For information only — not medical advice. Results are estimates from published formulas and can't account for your health history. Talk to a doctor or other qualified clinician before making health decisions.

How to use this calculator

  1. Under Measurement Unit, choose Metric (kg, cm) or Imperial (lbs, inches).
  2. Enter your Weight in kilograms or pounds, using your current weight without heavy clothing or shoes.
  3. Enter your Height in centimeters or total inches (for example, 5 ft 9 in is 69 inches).
  4. Optionally fill in Age (Optional) and Gender (Optional); they tailor the health insights but do not change the BMI number itself.
  5. Click Calculate BMI, then read your BMI (rounded to one decimal), your weight category, and the healthy weight range for your height, which corresponds to a BMI of 18.5–24.9.

BMI Formulas & Examples

BMI Formula (Metric)

BMI=weight (kg)height (m)2BMI = \frac{\text{weight (kg)}}{\text{height (m)}^2}
weight
Body weight in kilograms
height
Height in meters

Worked example

Calculate BMI for a person weighing 70 kg and 1.75 m tall

Given
weight = 70 kg
height = 1.75 m
Calculation
BMI=701.752=703.0625=22.857BMI = \frac{70}{1.75^2} = \frac{70}{3.0625} = 22.857
Result
BMI ≈ 22.9 (Normal weight; the calculator rounds to one decimal)

BMI Formula (Imperial)

BMI=weight (lbs)×703height (in)2BMI = \frac{\text{weight (lbs)} \times 703}{\text{height (in)}^2}
weight
Body weight in pounds
height
Height in inches

Worked example

Calculate BMI for a person weighing 154 lbs and 69 inches tall

Given
weight = 154 lbs
height = 69 in
Calculation
BMI=154×703692=108,2624,761=22.74BMI = \frac{154 \times 703}{69^2} = \frac{108{,}262}{4{,}761} = 22.74
Result
BMI ≈ 22.7 (Normal weight; the calculator rounds to one decimal)

BMI Categories

Category={Underweightif BMI<18.5Normalif 18.5≤BMI<25Overweightif 25≤BMI<30Obeseif BMI≥30\text{Category} = \begin{cases} \text{Underweight} & \text{if BMI} < 18.5 \\ \text{Normal} & \text{if } 18.5 \leq \text{BMI} < 25 \\ \text{Overweight} & \text{if } 25 \leq \text{BMI} < 30 \\ \text{Obese} & \text{if BMI} \geq 30 \end{cases}
BMI
Calculated Body Mass Index value

Worked example

BMI categories and their health implications

Given
BMI = 22.9
Calculation
18.5≤22.9<2518.5 \leq 22.9 < 25
Result
Normal weight category

Frequently asked questions

What is BMI and why is it important?

BMI (body mass index) is a number calculated from your height and weight that screens for weight categories linked to health risk. Doctors and public health agencies such as the CDC use it because it is quick, free, and consistent. It does not measure body fat directly or diagnose any condition, so it works best as a starting point for a conversation with your healthcare provider.

How is BMI calculated?

BMI is weight in kilograms divided by height in meters squared. In imperial units, the same result comes from pounds × 703 ÷ inches squared. For example, 70 kg at 1.75 m gives 70 ÷ 3.0625 ≈ 22.9. This calculator converts imperial entries to kilograms and meters, applies the metric formula, and rounds the result to one decimal place.

What are the BMI categories?

The standard adult BMI categories are underweight (below 18.5), healthy weight (18.5 to 24.9), overweight (25 to 29.9), and obese (30 or higher). These are the cut-offs used by the CDC and the World Health Organization and the ones this calculator applies. Obesity is sometimes split further into class 1 (30–34.9), class 2 (35–39.9), and class 3 (40 or higher).

What is a healthy weight for my height?

A healthy weight for adults is any weight that puts your BMI between 18.5 and 24.9. The calculator works this range out for your height and shows it in your results; for someone 175 cm tall it is roughly 56.7–76.3 kg. It is a broad range on purpose: where you fall within it depends on build, muscle, and other factors best discussed with a clinician.

Is BMI accurate for everyone?

No, BMI is a useful screen but it is not accurate for everyone. It cannot tell muscle from fat, so muscular athletes may score as overweight, while older adults who have lost muscle may score as healthy despite high body fat. It also does not show where fat is stored. For children and teens, BMI must be read against age- and sex-specific percentiles, which this adult calculator does not do.

What is the difference between BMI and body fat percentage?

BMI compares weight to height, while body fat percentage estimates how much of your weight is fat. Two people with the same BMI can have very different body fat levels. BMI is easier to measure and good for screening; body fat estimates such as the US Navy tape method or a DEXA scan say more about body composition. Waist size adds information about fat around the abdomen.

What does the Weight to Lose or Weight to Gain figure mean?

It is the smallest change that would bring your BMI to the nearest edge of the healthy range: down to a BMI of 24.9 if you are above it, or up to 18.5 if you are below it. If your BMI is already between 18.5 and 24.9, no figure is shown. For example, 80 kg at 170 cm (BMI 27.7) shows about 8.0 kg to lose. It is a screening figure, not a personal goal; talk to a doctor before starting any weight change plan.

How often should I check my BMI?

For most adults, checking BMI every few months, or at routine checkups, is plenty. Day-to-day weight swings from water and food make frequent checks misleading. Focus on overall habits such as diet, activity, and sleep rather than the number alone, and talk to a healthcare provider about any large or unexplained change in weight or BMI.

Key terms

BMI
Body Mass Index - weight in kilograms divided by height in meters squared, used to screen adult weight categories
Body Composition
The proportion of fat, muscle, bone, and water in the body
Metabolic Health
How well your body processes energy and maintains healthy blood sugar levels
Waist-to-Hip Ratio
Another measure of body fat distribution that may be more predictive of health risks

Methodology, sources & limitations

This page shows the formula, assumptions, and limitations used for this estimate. Rates, thresholds, and other time-sensitive inputs can change, so verify them against the relevant official source before relying on a result.

For our review approach and the primary sources we consult, see the methodology and source policy. If you spot an outdated figure or formula, please contact us with a correction.