Using the BMI calculator
This tool estimates body mass index from height and weight, then maps your value to WHO adult categories. The semicircle gauge on the results panel shows where your BMI sits across underweight, normal, overweight, and obesity ranges — built with ShoutingNow theme styling rather than a generic stock graphic. New to BMI? Read BMI Calculator Explained: What Your Number Really Means for category definitions, health risks, and when the formula falls short.
- Select US units or Metric units.
- Enter age (2–120), gender, height, and weight.
- Click Calculate — the needle animates to your BMI on the meter.
- Use Save in the result header to store the calculation in your browser.
What is BMI?
Body mass index relates weight to height and is widely used as a population-level screening tool for body weight status. The number alone does not measure body fat directly, but it helps flag when further evaluation may be useful. Our calculator also reports BMI Prime and the Ponderal Index so you can compare your reading against common clinical reference points.
BMI table for adults (WHO)
WHO categories for men and women age 20+:
| Classification | BMI (kg/m²) |
|---|---|
| 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 chart for adults
The chart below plots height against weight with colored WHO bands and dashed contour lines at common BMI values (18.5, 20, 22, 25, and so on). Points above a contour line correspond to higher BMI at that height:
- Underweight < 18.5
- Normal 18.5 – 25
- Overweight 25 – 30
- Obese 30+
Dashed lines mark BMI values: 18.5, 20, 22, 25, 27, 30, 33, 35, and 40 kg/m².
Children and teens (ages 2–20)
CDC uses BMI-for-age percentiles rather than fixed adult cutoffs:
| Category | Percentile range |
|---|---|
| Underweight | < 5th |
| Healthy weight | 5th – 85th |
| At risk of overweight | 85th – 95th |
| Overweight | > 95th |
Pediatric interpretation depends on age and sex. For children, consult growth charts published by the CDC or your national health authority and discuss results with a pediatric clinician.
Risks linked to high BMI
Elevated BMI is associated with increased likelihood of several conditions, including:
- High blood pressure and unfavorable cholesterol profiles
- Type 2 diabetes and insulin resistance
- Coronary heart disease and stroke
- Gallbladder disease and fatty liver
- Osteoarthritis and joint pain
- Sleep apnea and breathing difficulties
- Certain cancers (including breast, colon, kidney, and endometrial)
- Reduced quality of life and higher mortality risk at population level
Many people benefit from keeping BMI below 25 kg/m², but individual targets should come from a qualified clinician who reviews your full health picture.
Risks linked to low BMI
Very low BMI may signal inadequate nutrition or underlying illness. Possible concerns include:
- Malnutrition, vitamin deficiencies, and anemia
- Osteoporosis and bone fragility
- Weakened immune function
- Growth or developmental delays in youth
- Reproductive and hormonal disruption in some women
- Greater surgical complication risk when underweight
If you or someone you know is underweight without an obvious cause, seek medical evaluation — especially when eating disorders or chronic illness may be involved.
Limitations of BMI
In adults: BMI estimates excess weight, not excess fat. Age, sex, ethnicity, muscle mass, and activity level all influence whether a given BMI reflects healthy composition. A sedentary older adult and a muscular athlete can share the same BMI with very different body-fat levels.
- Older adults often carry more fat than younger adults at the same BMI.
- Women typically have higher body fat than men at the same BMI.
- Trained athletes may read as overweight because muscle weighs more than fat.
In children: Height velocity and puberty affect BMI interpretation. BMI is more reliable for identifying obesity in youth than for spotting overweight, where lean mass gains can also raise the index.
For most people BMI remains a useful screening metric when paired with waist circumference, activity history, and clinical labs — not as a standalone diagnosis.
BMI formulas
Worked example using a 5′10″ (70 in), 160 lb person — the same defaults as the US tab above:
US customary units
Metric (SI) units
BMI Prime
BMI Prime divides your BMI by the upper normal limit (25 kg/m²). It is dimensionless and makes it easy to see how far you sit from the normal ceiling:
| Classification | BMI | BMI Prime |
|---|---|---|
| Severe thinness | < 16 | < 0.64 |
| Moderate thinness | 16 – 17 | 0.64 – 0.68 |
| Mild thinness | 17 – 18.5 | 0.68 – 0.74 |
| Normal | 18.5 – 25 | 0.74 – 1.0 |
| Overweight | 25 – 30 | 1.0 – 1.2 |
| Obese Class I | 30 – 35 | 1.2 – 1.4 |
| Obese Class II | 35 – 40 | 1.4 – 1.6 |
| Obese Class III | > 40 | > 1.6 |
Examples and use cases
Worked example
A 5′10″ (70 in), 160 lb adult — the US tab defaults:
- BMI ≈ 23.0 kg/m² — within the normal range (18.5–24.9)
- Healthy weight band for this height ≈ 128–174 lb
Real-world use cases
- Wellness screening: Someone tracks BMI quarterly alongside waist measurement during a structured fitness program.
- Clinical intake: A patient records height and weight before a telehealth visit when the clinic asks for BMI category.
- Athlete context: A rugby player with high muscle mass notes BMI may read “overweight” despite low body fat — uses the tool as one data point, not a diagnosis.
Ponderal Index
The Ponderal Index cubes height in the denominator instead of squaring it. That can stabilize readings for very tall or short individuals where BMI over- or under-estimates leanness:
US customary units
Metric (SI) units