What is BMI Calculator?
BMI Calculator is a free tool that calculates your Body Mass Index from your height and weight. BMI is a widely used screening number defined as weight in kilograms divided by height in meters squared. The calculator supports both metric (cm, kg) and imperial (feet/inches, pounds) inputs and shows your result alongside the standard weight category ranges.
The standard BMI categories are: underweight (below 18.5), healthy weight (18.5–24.9), overweight (25–29.9), and obese (30 and above). These thresholds were established by the World Health Organization and are used globally as a general health screening tool. BMI does not distinguish between muscle and fat mass, so athletes and very muscular individuals often measure as overweight despite being healthy.
BMI is most useful as a quick initial screening for potential weight-related health risks, not as a definitive health assessment. A doctor or dietitian uses BMI alongside other measures like waist circumference, blood pressure and blood glucose to get a full picture. The calculator is free, private and works entirely in your browser with no data stored.
⚡
Instant results
No server processing or wait
🔒
100% private
Files stay on your device
💰
Always free
No subscription or fees
📱
Mobile ready
Works on any device
How to use BMI Calculator
- Enter your height — choose centimeters or feet and inches
- Enter your current weight in kilograms or pounds
- Click Calculate to get your BMI value and category instantly
- Review the healthy BMI range and what your result means
Why use OnlineToolsPlus?
OnlineToolsPlus processes everything locally in your browser. Your files, text and data never leave your device, which eliminates server-side privacy risks. With 200+ free tools covering PDF, image, AI writing, developer utilities, calculators, SEO, color tools and more, there is no need to juggle multiple subscriptions or install software.
Frequently asked questions
Is BMI an accurate measure of health?
BMI is a useful population-level screening tool but has significant limitations for individuals. It does not measure body fat directly — it estimates it from height and weight. Muscular individuals are often misclassified as overweight. It also does not capture fat distribution, which is closely linked to cardiovascular and metabolic risk. For a comprehensive health assessment, consult a healthcare provider who can consider BMI alongside body fat percentage, waist circumference, blood markers, and family history.
What is a healthy BMI for my age?
The standard adult BMI categories (18.5–24.9 for normal weight) apply to adults 20 and over regardless of age. However, research suggests that the health risk associated with a given BMI may change slightly with age — older adults (65+) may have better outcomes at a slightly higher BMI (around 25–27). For children and teenagers, BMI is measured as a percentile relative to age and sex, not against adult categories. Always discuss your specific results with a healthcare provider.
Does BMI apply to children?
Standard adult BMI categories do not apply to people under 20. For children and adolescents, BMI-for-age percentiles are used: below the 5th percentile is underweight, 5th–84th is healthy weight, 85th–94th is overweight, and 95th and above is obese. These percentile charts are maintained by the CDC (US) and WHO. A pediatrician should interpret BMI results for children in the context of growth charts and development.
How is BMI calculated?
BMI = weight (kg) ÷ height (m)². In imperial units: BMI = [weight (lbs) ÷ height (inches)²] × 703. For example, a person weighing 70 kg at 1.75 m has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9, which falls in the normal weight range.
What is the difference between BMI and body fat percentage?
BMI is calculated from height and weight only. Body fat percentage measures the actual proportion of fat mass to total body mass. A professional athlete and a sedentary person of the same height and weight have the same BMI but very different body fat percentages. Body fat percentage can be measured by DEXA scan, hydrostatic weighing, bioelectrical impedance, or skinfold calipers. BMI is a free, instant screening tool; body fat percentage requires measurement equipment.
Is a BMI of 25 considered overweight?
Yes, according to WHO and most national health guidelines, a BMI of 25.0–29.9 is classified as overweight. However, this classification is based on population averages and statistical associations with disease risk. Some individuals at BMI 25 are metabolically healthy, while others at BMI 23 may have risk factors. BMI is one indicator among many.
Does BMI differ for men and women?
The BMI calculation and categories are the same for men and women. However, women naturally carry a higher percentage of body fat than men at the same BMI, due to hormonal differences and body composition. A man and a woman with BMI 22 have the same calculated BMI but different body compositions. Some researchers have proposed sex-specific BMI thresholds, but the standard categories remain widely used.
Should I use metric or imperial units?
Both give the same result — the tool converts automatically. Use whatever units you are most comfortable with. In the US, imperial (pounds and inches) is most familiar. In most other countries, metric (kilograms and centimeters) is standard. If you know your height and weight from a recent medical visit, use those exact measurements for accuracy.
Can BMI predict life expectancy?
At the population level, BMI correlates with mortality risk — very low and very high BMI both associate with higher mortality. But BMI is not a reliable predictor of individual life expectancy. Other factors — physical activity, diet, smoking, sleep, stress, genetics, and access to healthcare — have greater influence on longevity than BMI alone. A person with a "normal" BMI who smokes and is sedentary has worse health prospects than an active, non-smoking person with a BMI of 27.
What should I do if my BMI is outside the normal range?
A BMI outside the 18.5–24.9 range is a starting point for a conversation with a healthcare provider, not a diagnosis. Your provider will consider your complete health picture — blood pressure, cholesterol, blood sugar, physical activity level, diet, and family history — before making any recommendations. BMI alone is not sufficient to determine whether intervention is needed or appropriate.
- Does not distinguish muscle from fat. Athletes and bodybuilders with high muscle mass often have a "overweight" or "obese" BMI despite very low body fat. A 90 kg rugby player and a 90 kg sedentary person have the same BMI but very different health profiles.
- Different accuracy across ethnic groups. Research shows that health risks associated with excess body fat begin at lower BMI thresholds for South Asian, East Asian, and some other populations. The WHO has separate BMI guidelines for Asian populations.
- Not reliable for children and adolescents. BMI-for-age percentiles (not adult categories) are used for individuals under 20.
- Does not account for fat distribution. Central obesity (fat around the abdomen) carries higher health risk than fat distributed elsewhere, regardless of total BMI. Waist circumference is a complementary and often more informative measure.
BMI is a screening tool, not a diagnostic measure. It has well-documented limitations:
BMI Limitations You Should Know
- Below 18.5 — Underweight: May indicate nutritional deficiency or underlying health conditions. Consultation with a healthcare provider is recommended.
- 18.5 to 24.9 — Normal weight: Associated with the lowest risk of weight-related health conditions for most adults. This range is the target for general health maintenance.
- 25.0 to 29.9 — Overweight: Elevated risk for cardiovascular disease, type 2 diabetes, and hypertension. Weight management through diet and exercise is generally recommended.
- 30.0 and above — Obese: Significantly elevated risk for a range of chronic conditions. Medical guidance is recommended for a structured weight management plan.
- 40.0 and above — Severely obese: High risk for serious health complications. Medical intervention may be appropriate.
BMI is calculated by dividing weight (in kilograms) by height squared (in meters). The World Health Organization defines the following categories for adults:
Understanding Your BMI Result
More Calculator Tools
Age CalculatorCalculate age and date differencesPercentage CalcPercent of, change, discountTip CalculatorSplit bills, calculate tipsLoan CalculatorMonthly payments and total interestAge CalculatorCalculate age and date differences
Last updated: April 11, 2026
View all 200+ free tools →