# BMI Calculator > Free BMI calculator with your category, health risk and healthy weight range. WHO / CDC / NIH standards, imperial or metric, for men, women, children and teens. **URL:** https://calculators.im/bmi-calculator **Category:** health **Last updated:** 2026-07-03 Calculate your Body Mass Index (BMI), check where you fall on the BMI chart, find your ideal weight range, and understand what your BMI means for your health. Our free BMI calculator helps you calculate your body mass index using height and weight, view your BMI classification, find your healthy weight range, and set weight goals. Supports both metric (kg/cm) and imperial (lb/in) units for men, women, and all ages. ## Formula ``` BMI = weight (kg) / height² (m) ``` **Variables:** - **BMI** — Body Mass Index (kg/m²) - **weight** — Body weight in kilograms - **height** — Height in meters BMI is calculated by dividing your weight in kilograms by your height in meters squared. For imperial units, multiply by 703: BMI = (weight in lbs / height² in inches) × 703. ## Worked Example ### Example: 70 kg, 175 cm A person weighing 70 kg with height 175 cm has a BMI of 22.9, which falls in the 'Normal weight' category. Their healthy weight range is 56.7-76.6 kg. ## How to Calculate Your BMI: 4 Quick Steps 1. **Select Unit System** — Choose metric (kg/cm) or imperial (lb/in) measurements. 2. **Enter Height & Weight** — Input your current height and weight accurately. 3. **View Your BMI** — See your BMI score, category, and what it means. 4. **Check Healthy Range** — Find your ideal weight range based on BMI guidelines. ## Pro Tips - BMI between 18.5-24.9 is considered normal/healthy weight - BMI doesn't distinguish between muscle and fat mass - Athletes may have high BMI due to muscle mass, not fat - BMI is a screening tool, not a diagnostic measure - Consult a healthcare provider for a complete health assessment - Waist circumference is another important health indicator ## Frequently Asked Questions ### How do I calculate my BMI? Enter your height and weight into the calculator. BMI is calculated as weight (kg) divided by height squared (m²). For imperial units, the formula is (weight in lbs ÷ height² in inches) × 703. Our calculator handles the math automatically. ### What is a healthy BMI range? A healthy BMI is between 18.5 and 24.9. Below 18.5 is underweight, 25-29.9 is overweight, and 30+ is obese. However, BMI is just one indicator — muscle mass, age, and other factors also matter. ### Is BMI different for men and women? The BMI formula is the same for men and women. However, women typically have more body fat than men at the same BMI. The calculator shows healthy ranges that apply to both genders, though individual assessments may vary. ### Does age affect BMI interpretation? For adults, standard BMI categories apply regardless of age. For children and teens (2-19 years), BMI is interpreted using age- and sex-specific percentiles. Older adults may have slightly different healthy ranges. ### What BMI should I have for my height? For a healthy BMI (18.5-24.9), your ideal weight depends on your height. The calculator shows your healthy weight range based on your height. For example, someone 170 cm tall should weigh between 53-72 kg. ### How accurate is BMI as a health indicator? BMI is a useful screening tool but has limitations. It doesn't account for muscle mass, bone density, or fat distribution. Athletes may have high BMI due to muscle. For a complete assessment, consider waist circumference, body fat percentage, and other metrics. ### Can I use BMI for weight loss goals? Yes! The calculator shows your healthy weight range based on BMI. If your current weight is above this range, you can set a target weight within the healthy BMI zone. Aim for gradual weight loss of 0.5-1 kg per week. ### What's the difference between BMI and body fat percentage? BMI estimates body fat based on height and weight only. Body fat percentage measures actual fat mass. BMI can misclassify muscular people as overweight. For precise body composition, consider DEXA scans or skinfold measurements. ### How do I convert between metric and imperial for BMI? Our calculator supports both systems. For metric: use kg and cm. For imperial: use lbs and inches. The BMI result is the same regardless of which system you use — just select your preferred unit system. ### What are the BMI categories? WHO BMI categories: Underweight (<18.5), Normal weight (18.5-24.9), Overweight (25-29.9), Obese Class I (30-34.9), Obese Class II (35-39.9), Obese Class III (40+). The calculator shows your category and health implications. ### How is BMI calculated for children and teens? For ages 2–19, BMI is interpreted using age- and sex-specific CDC percentiles, not fixed numbers. Under 5th percentile is underweight; 5–84th is healthy weight; 85–94th is overweight; 95th+ is obese. A child's BMI number doesn't classify them directly — it's compared to peers of the same age and sex. Use our calculator to get the BMI number, then interpret it via the CDC BMI Percentile Calculator for children. Always consult a pediatrician for medical interpretation. ### Which groups should not rely on BMI alone? Competitive athletes and bodybuilders (muscle mass inflates BMI), adults over 65 (muscle loss masks elevated body fat), pregnant and breastfeeding women (use pre-pregnancy BMI), people with amputations, and those at extreme height (below 5'0 or above 6'4). If any apply to you, pair BMI with waist circumference, body-fat percentage, and clinical guidance. ### Does BMI change during menopause? BMI calculation doesn't change — same formula — but many women gain 5–10 lbs (2–5 kg) during and after menopause due to hormonal shifts and muscle loss, pushing their BMI up without lifestyle changes. Waist circumference often captures menopausal metabolic risk more accurately. Work with your clinician to set a personalized target, especially if you have a history of cardiovascular disease or diabetes. ### What is a healthy BMI after pregnancy? Do not use BMI as a benchmark immediately postpartum. Allow 6–12 months for body composition to normalize. Pre-pregnancy BMI is the correct reference for tracking weight gain during pregnancy (via Institute of Medicine / NHS guidance) and for setting postpartum goals. Consult your healthcare provider before starting weight-loss efforts, especially if breastfeeding. ### What is waist-to-height ratio (WHtR) and should I use it? WHtR = waist circumference ÷ height (same units). Under 0.5 is healthy — the 'keep your waist less than half your height' rule. WHtR measures abdominal fat, which drives cardiovascular and metabolic disease better than BMI can. NHS (UK) updated its guidelines in 2023 to recommend WHtR alongside BMI. Use both: a normal BMI with WHtR above 0.5 still signals elevated risk (skinny fat); a high BMI with WHtR below 0.5 usually means muscle. ### What is BMI Prime? BMI Prime is your BMI divided by 25 (the overweight threshold). A value of 1.0 means you're at the overweight borderline; 0.8 is bottom of healthy range; 1.2 means you're 20% above the overweight cutoff. It's a quick way to see how far above or below the healthy-weight ceiling you are. ### What is the Ponderal Index? Ponderal Index (weight ÷ height³) is a BMI alternative that accounts better for height. Especially useful for very tall or very short adults whom BMI can misclassify. A healthy Ponderal Index is roughly 11–15 kg/m³. It remains informational only — BMI is still the primary screening tool used by CDC, NHS, Health Canada and Heart Foundation Australia. ## Deep Dive Calculate your Body Mass Index (BMI) instantly with our free BMI calculator. Find out if you're in a healthy weight range, view the BMI chart categories, and discover your ideal weight based on your height. Works with both metric (kg/cm) and imperial (lb/in) units. ### How to Calculate BMI **BMI** is calculated by dividing your weight in kilograms by your height in meters squared (kg/m²). For **imperial units**, the formula is (weight in pounds ÷ height in inches²) × 703. Simply enter your height and weight, and our calculator does the math instantly. ### BMI Chart & Categories The **WHO BMI classification**: - Underweight (below 18.5) - Normal weight (18.5-24.9) - Overweight (25-29.9) - Obese Class I (30-34.9) - Obese Class II (35-39.9) - Obese Class III (40 and above) Your BMI category helps assess potential health risks. ### Ideal Weight Calculator Based on your height, our calculator shows your **healthy weight range** where BMI falls between 18.5 and 24.9. This gives you a target range for weight management, whether you need to lose or gain weight. ### BMI for Men and Women While the **BMI formula is identical for both genders**, body composition differs. Women naturally have higher body fat percentages. Our calculator provides the same healthy BMI range (18.5-24.9) for both, with optional gender input for context. ### BMI Limitations & Accuracy **BMI** is a useful screening tool but doesn't measure body fat directly. It may overestimate fat in athletes and underestimate it in older adults. For a complete health picture, consider: - waist circumference - body fat percentage - and cardiovascular markers such as the blood-lipid numbers you can review with a cholesterol calculator ### BMI for Children and Teens (Percentile-Based) Unlike adults, children and teens ages 2–19 are assessed using **age- and sex-specific BMI percentiles**, not fixed number categories. A child's BMI is plotted against CDC growth charts: - below the 5th percentile is underweight - 5th–84th is healthy weight - 85th–94th is overweight - and 95th or above is obese This is because growing bodies have different body-fat profiles at each age. A BMI of 19 might be healthy for a 15-year-old but overweight for a 6-year-old. The CDC BMI Percentile Calculator for Children and Teens uses the same height and weight inputs but interprets them against percentile tables. If you're calculating BMI for a child, use the number from this calculator as input to percentile tools — and always consult a pediatrician for interpretation. ### BMI vs Body Fat Percentage: When Each Metric Is Right **BMI and body-fat percentage answer different questions.** BMI is a quick screening tool based only on height and weight — it takes 5 seconds and requires no equipment. Knowing how to measure body fat percentage means using calipers, a DEXA scan or a bio-impedance scale to directly gauge fat mass relative to total body mass. Use **BMI** for population-level screening, tracking trends over time, or when you don't have access to body-composition tools. Use **body-fat percentage** if you are a competitive athlete, heavily muscled, over 65, pregnant, or specifically working on body recomposition. BMI classifies many muscular athletes as 'overweight' when their body-fat percentage is actually low. For most adults without extreme body composition, BMI tracks body-fat percentage closely enough to be useful as a screening tool. ### Who Should Not Rely on BMI Alone BMI is a screening tool designed for the general adult population and is **less accurate for specific groups**: - Competitive athletes and bodybuilders often have BMI above 25 from muscle mass, not fat — use body-fat measurement instead. - Adults over 65 lose muscle mass while retaining fat, so their BMI may read normal even when body-fat is elevated; waist circumference is a better marker. - Pregnant and breastfeeding women should use pre-pregnancy BMI as reference, not current weight. - People with amputations and those at very tall or very short stature (below 5'0 or above 6'4) may need adjusted interpretation. If any of these apply to you, treat BMI as one data point alongside waist circumference, body-fat percentage, and clinical guidance from your healthcare provider. ### BMI for Women: Menopause, Pregnancy and Postpartum The **BMI formula is identical for women and men**, but women's bodies go through phases where interpretation needs nuance. - During pregnancy, do not use current BMI — your healthcare provider tracks weight gain against pre-pregnancy BMI using Institute of Medicine (US) or NHS (UK) guidelines. - After childbirth, allow 6–12 months for weight normalization before using BMI as a benchmark. - At menopause and beyond, many women gain 5–10 lbs (2–5 kg) due to hormonal shifts and muscle loss, pushing BMI up even without lifestyle changes; waist circumference often captures this risk more accurately than BMI alone. - Women with polycystic ovary syndrome (PCOS) frequently have BMI in the overweight range with insulin resistance — a healthy BMI for a woman with PCOS is best set with her doctor. In short: BMI 18.5–24.9 is the same target for women, but context (life stage, waist measurement, hormonal status) should shape how you act on the number. ### Waist-to-Height Ratio: The Modern BMI Supplement **Waist-to-Height Ratio (WHtR)** is increasingly recommended alongside BMI by the UK NHS (2023 guideline update), Lancet Diabetes & Endocrinology, and several US cardiology bodies. The formula is simple: waist circumference ÷ height (same units). A **WHtR under 0.5** is considered healthy for most adults regardless of sex — the 'keep your waist to less than half your height' rule. WHtR captures abdominal fat — the visceral fat that drives cardiovascular and metabolic disease — far better than BMI, which can't distinguish muscle from fat or tell fat distribution. Use WHtR with BMI: a 'normal' BMI with WHtR above 0.5 still carries elevated metabolic risk (sometimes called 'skinny fat'), and a high BMI with WHtR below 0.5 is usually muscle, not health risk. To measure waist correctly: - stand - exhale normally - measure at the midpoint between the lowest rib and the top of the hip bone - keeping the tape horizontal and snug but not compressing skin ### BMI Prime and Ponderal Index Explained Our calculator outputs two supplementary metrics alongside your BMI. **BMI Prime** is your BMI divided by 25 (the upper threshold of the healthy range): - A BMI Prime of 1.0 means you're exactly at the overweight borderline - 0.8 means you're at the bottom of the healthy range - 1.2 means you're 20% above the overweight cutoff It makes it easy to see how far above or below the healthy-weight ceiling you are at a glance. **Ponderal Index** (weight ÷ height³) is an alternative to BMI that accounts better for height — particularly useful for very tall or very short adults whom BMI can misclassify. A typical healthy Ponderal Index is 11–15 kg/m³. Both metrics are informational only; BMI remains the primary screening tool used by the CDC, NHS, Health Canada, and Heart Foundation Australia. ## Related Calculators - [calorie-calculator](https://calculators.im/calorie-calculator) - [ideal-weight-calculator](https://calculators.im/ideal-weight-calculator) - [body-fat-percentage-calculator](https://calculators.im/body-fat-percentage-calculator) - [blood-alcohol-content-calculator](https://calculators.im/blood-alcohol-content-calculator) - [body-surface-area-calculator](https://calculators.im/body-surface-area-calculator) - [waist-to-height-ratio-calculator](https://calculators.im/waist-to-height-ratio-calculator) ## Authoritative Sources - [World Health Organization](https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight) — WHO BMI classification standards - [CDC - About Adult BMI](https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html) — CDC guidelines on BMI interpretation - [NIH - Calculate Your BMI](https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm) — National Institutes of Health BMI information - [NHS - BMI healthy weight calculator](https://www.nhs.uk/health-assessment-tools/calculate-your-body-mass-index/calculate-bmi-for-adults) — UK NHS BMI assessment for adults - [Health Canada - Canadian guidelines for body weight classification](https://www.canada.ca/en/health-canada/services/food-nutrition/healthy-eating/healthy-weights/canadian-guidelines-body-weight-classification-adults.html) — Health Canada official BMI guidelines for adults - [Heart Foundation Australia - BMI calculator](https://www.heartfoundation.org.au/your-heart/bmi-calculator) — Australian Heart Foundation BMI and health risk tool --- *Source: https://calculators.im/bmi-calculator — calculators.im, 299+ free online calculators.*