On This Page
- How to use this calculator
- What is BMI?
- How to calculate BMI
- BMI classifications
- Healthy weight by height
- BMI by age and for children
- Reverse BMI
- When BMI misleads
- Worked examples
- Frequently asked questions
How to Use This Calculator
Choose your unit system (Metric or US), select a mode, enter your measurements, and the result appears immediately.
Calculate BMI mode
Enter height and weight. The result shows your BMI, its WHO classification, the healthy weight range for your exact height, and how much to gain or lose to reach the Normal range if you are outside it. The age field is optional, it adds a note for anyone under 18 or over 65, where standard adult thresholds don't apply directly.
Find Target Weight mode (Reverse BMI)
Enter your height and a target BMI. The result shows the weight required to reach that BMI, in both kilograms and pounds. Useful for finding the weight at the midpoint of the Normal range or at any other goal BMI.
What Is BMI?
BMI, body mass index, is a ratio of weight to height squared. Belgian statistician Adolphe Quetelet devised it in the 1830s to describe body size across populations, not to assess individual health. The WHO and CDC adopted it as a population screening measure in the late 20th century because it requires only two measurements and produces a consistent, comparable number across large groups.
It does not measure body fat. It is a rough proxy, reliable enough for population-level screening, but limited for individual assessment. The gaps matter most for athletes, older adults, and people of Asian descent. See When BMI Misleads below for specifics, and the Asian BMI Calculator for WHO-adjusted thresholds if that applies to you.
How to Calculate BMI
Metric (kg and cm)
Convert height to metres, square it, then divide weight by that value.
- Convert height: 170 cm ÷ 100 = 1.70 m
- Square it: 1.70 × 1.70 = 2.89 m²
- Divide weight: 70 kg ÷ 2.89 = 24.2: Normal weight
US units (lbs and inches)
Same calculation, with a conversion factor of 703 that aligns the pounds-and-inches ratio with the metric scale.
- Total inches: 5 ft 7 in = (5 × 12) + 7 = 67 in
- Square it: 67 × 67 = 4,489 in²
- Divide weight: 154 lbs ÷ 4,489 = 0.03431
- Multiply by 703: 0.03431 × 703 = 24.1
The factor 703 is sometimes listed as 704 in older sources. The precise value is 703.07; the CDC and NIH use 703. Both produce the same result when rounded to one decimal place. The calculator handles all conversions automatically when you select a unit system.
BMI Classifications
The WHO uses four primary categories. Most clinical settings, and this calculator, use the six-tier NIH breakdown that splits obesity into three classes, which becomes relevant for treatment decisions above BMI 35.
| BMI range | Category | Health risk |
|---|---|---|
| Below 18.5 | Underweight | Elevated risk of malnutrition, bone loss, and immune issues |
| 18.5 – 24.9 | Normal weight | Lowest risk in the general population |
| 25.0 – 29.9 | Overweight | Moderately elevated risk |
| 30.0 – 34.9 | Obese, Class I | High risk |
| 35.0 – 39.9 | Obese, Class II | Very high risk |
| 40 and above | Obese, Class III | Severely elevated risk |
These thresholds apply to adults 18 and over, regardless of sex. The calculator result shows your position on this scale and your personal healthy weight range for the Normal category.
At 1.70 m: Normal range = 18.5 × 2.89 to 24.9 × 2.89 = 53.5 – 71.9 kg
Healthy Weight by Height
The Normal range (BMI 18.5–24.9) corresponds to a different weight band at every height. The tables below show those bands in kilograms and pounds. The boundaries are the same for men and women, BMI does not adjust by sex.
Women
| Height | Healthy weight range (BMI 18.5–24.9) |
|---|---|
| 5 ft 0 in (152 cm) | 107 – 143 lbs (48.5 – 65.0 kg) |
| 5 ft 3 in (160 cm) | 119 – 160 lbs (53.9 – 72.5 kg) |
| 5 ft 5 in (165 cm) | 126 – 169 lbs (57.2 – 76.7 kg) |
| 5 ft 7 in (170 cm) | 132 – 178 lbs (60.1 – 80.7 kg) |
| 5 ft 9 in (175 cm) | 140 – 188 lbs (63.3 – 85.4 kg) |
Men
| Height | Healthy weight range (BMI 18.5–24.9) |
|---|---|
| 5 ft 7 in (170 cm) | 132 – 178 lbs (60.1 – 80.7 kg) |
| 5 ft 9 in (175 cm) | 140 – 188 lbs (63.3 – 85.4 kg) |
| 5 ft 11 in (180 cm) | 148 – 199 lbs (67.3 – 90.4 kg) |
| 6 ft 1 in (185 cm) | 157 – 211 lbs (71.3 – 95.7 kg) |
| 6 ft 3 in (190 cm) | 166 – 224 lbs (75.4 – 101.4 kg) |
Men and women at the same height share the same BMI thresholds but not the same body composition. Women carry roughly 8–10% more body fat than men at any given BMI, owing to hormonal differences and reproductive physiology. The cutoffs don't shift, but what those cutoffs mean physiologically differs by sex.
BMI by Age and for Children
Adults 18–64
BMI thresholds don't change with age for working-age adults. A BMI of 27 is Overweight at 30 and at 60.
Adults 65 and over
Some clinical guidelines treat a BMI between 25 and 27 as acceptable, or even preferable, in older adults. The risks of being underweight at this age (fractures, muscle loss, poor recovery from illness) tend to outweigh the modest risks of mild overweight. The calculator adds a note if you enter an age of 65 or over.
US population averages by age group (NHANES data)
| Age group | Average BMI (women) | Average BMI (men) | Classification |
|---|---|---|---|
| 20–39 | 27.4 | 27.5 | Overweight |
| 40–59 | 29.3 | 29.3 | Overweight |
| 60–79 | 29.1 | 29.3 | Overweight |
These are population averages, what is common, not what is healthy. A BMI in the Normal range is consistently linked to lower rates of cardiovascular disease, type 2 diabetes, and related conditions across every age group in the research base.
Children and teenagers (ages 2–17)
The formula is the same, but the fixed adult cutoffs don't apply. For anyone under 18, BMI is compared to age- and sex-specific percentile charts, the same numeric result means something very different at 10 years old versus 35.
| Percentile | Weight status (CDC, ages 2–17) |
|---|---|
| Below 5th | Underweight |
| 5th – 84th | Healthy weight |
| 85th – 94th | Overweight |
| 95th and above | Obese |
Use the CDC's BMI-for-age calculator for anyone under 18. The adult scale on this page does not give valid results for children.
Reverse BMI, Find Your Target Weight
Reverse BMI rearranges the standard formula to solve for weight instead of BMI. Given a target BMI and a height, it returns the exact weight required to reach it.
Switch to "Find Target Weight" mode, enter your height, and type a target BMI. Common reference points:
- 18.5: lower boundary of Normal weight
- 22: midpoint of the Normal range, commonly used as a clinical planning target
- 24.9: upper boundary of Normal weight
- 23: the Overweight threshold on the Asian BMI scale
Example
Height: 175 cm (1.75 m). Target BMI: 22.
- Height squared: 1.75 × 1.75 = 3.0625 m²
- Target weight = 22 × 3.0625 = 67.4 kg (148.5 lbs)
When BMI Misleads
BMI captures weight relative to height, nothing else. Because it can't distinguish fat from muscle, or account for where fat is distributed, it produces misleading results in several common situations.
Athletes and muscular individuals
Muscle is denser than fat. A competitive athlete can be lean and still register Overweight on the BMI scale simply because muscle mass is heavy. Many professional rugby players and Olympic-level strength athletes sit at BMI 27–30 with low body fat. For anyone who trains seriously, body fat percentage, measured by DEXA or hydrostatic weighing, is a more informative number. The Lean Body Mass Calculator can estimate fat and lean mass from height and weight if you need a reference point.
Fat distribution
Where fat is stored matters more than how much exists in total. Fat concentrated around the abdomen and internal organs carries significantly higher cardiovascular and metabolic risk than fat stored elsewhere. Two people with the same BMI can have very different risk profiles based on fat distribution alone. Waist circumference is a more direct proxy for this risk than BMI.
Asian populations
The standard WHO cutoffs were established primarily from European population data. People of South Asian, East Asian, and Southeast Asian descent tend to develop metabolic complications, elevated blood glucose, insulin resistance, high blood pressure, at BMI values below 25. The WHO's adjusted guidance recommends screening for metabolic risk from BMI 23 in these groups. The Asian BMI Calculator applies those thresholds.
Older adults
With age, lean mass declines and fat often redistributes toward the abdomen, even with little change in total weight. BMI can stay flat while body composition worsens. Waist-to-height ratio tends to be a better screening measure for metabolic risk in people over 65.
Worked Examples
Example 1, Metric, Normal weight
Woman, 165 cm, 62 kg.
- 1.65 × 1.65 = 2.7225 m²
- 62 ÷ 2.7225 = 22.8 → Normal weight
- Healthy range at 165 cm: 18.5 × 2.7225 to 24.9 × 2.7225 = 50.4 – 67.8 kg
Example 2, US units, Overweight
Man, 5 ft 11 in, 205 lbs.
- 71 inches × 71 = 5,041 in²
- (205 ÷ 5,041) × 703 = 28.6 → Overweight
- Weight to reach Normal (BMI 24.9): (24.9 × 5,041) ÷ 703 = 178.5 lbs, about 26.5 lbs to lose
Example 3, Reverse BMI
Person, 180 cm, target BMI 22.
- 1.80 × 1.80 = 3.24 m²
- 22 × 3.24 = 71.3 kg (157.1 lbs)
- Full Normal range at 180 cm: 59.9 – 80.7 kg
Example 4, Teenager (percentile context)
A 14-year-old girl, 162 cm, 55 kg. BMI = 55 ÷ (1.62)² = 20.9. By adult standards that reads as Normal weight, but at age 14 it sits around the 75th–80th percentile on CDC growth charts, still Healthy weight for her age, but toward the upper end. Adult cutoffs cannot be applied to children.
Frequently Asked Questions
What is a normal BMI?
A BMI of 18.5 to 24.9 is classified as Normal weight by the WHO and NIH. Below 18.5 is Underweight; 25–29.9 is Overweight; 30 and above is Obese. These cutoffs apply to adults 18 and over and don't change by sex, though guidelines generally accept mild overweight (up to BMI 27) as reasonable in adults over 65.
How do you calculate BMI?
Divide weight in kilograms by height in metres squared: BMI = kg ÷ m². For US units: BMI = (lbs ÷ in²) × 703. Example in metric: 70 kg at 1.70 m = 70 ÷ 2.89 = 24.2.
Is BMI the same for men and women?
The formula and classification thresholds are identical for both sexes. Men and women at the same BMI typically have different body fat percentages, women carry roughly 8–10% more due to physiological differences, but the classification cutoffs themselves don't adjust by sex.
What is a healthy BMI for women?
18.5 to 24.9, the same range as for men. The calculator result shows the exact weight band for the Normal category at your specific height.
How do I calculate BMI for kids?
Use the same formula, but compare the result to CDC or WHO age- and sex-specific growth charts rather than the adult cutoffs. The fixed thresholds on this page are not valid for anyone under 18.
What BMI is considered obese?
30 or above. The NIH divides this into Class I (30–34.9), Class II (35–39.9), and Class III (40+). Each class carries progressively higher risk for type 2 diabetes, cardiovascular disease, and related conditions.
What does the reverse BMI calculator do?
It solves the BMI formula for weight: target weight (kg) = target BMI × height (m)². Enter your height and any target BMI to find the corresponding weight. Switch to "Find Target Weight" mode in the calculator above to use it.
How accurate is BMI?
It tracks body size reasonably well at the group level, but has real blind spots for individuals. It can classify muscular athletes as overweight and miss high-risk fat patterning in people with a normal-looking BMI. Treat it as a first screen, not a definitive measure. Waist circumference, body fat percentage, and metabolic blood markers together give a much more complete picture.
References
- CDC Adult BMI Calculator: CDC classification guidelines and BMI interpretation for adults.
- NIH BMI reference: National Heart, Lung, and Blood Institute classification tables.
- WHO, Obesity and overweight: Global BMI data and WHO classification thresholds.
- CDC BMI Percentile Calculator for Children and Teens: Age- and sex-specific percentile charts for ages 2–17.