BMI Calculator Guide
The complete guide to understanding and using BMI calculators, including step-by-step calculation examples, chart interpretation, and practical tips for healthy weight management.
BMI Calculators & Body Composition Tools
Choose from our suite of free health calculators. Each tool is designed for a specific purpose — from standard BMI screening to specialized calculations for women, men, children, and body composition analysis.
Quick Guide
- 8 free calculators — each built for a different purpose
- WHO-endorsed formulas — the same math used in clinical practice
- Instant results — no sign-up, no data stored
- Works on any device — desktop, tablet, and mobile
- Imperial & metric — switch between lbs/ft and kg/cm
Calculator Overview
8
Free Calculators
18.5-24.9
Healthy BMI Range
2-99
Age Range Covered
4
Ideal Weight Formulas
BMI Categories at a Glance
Visual representation of WHO BMI classification ranges. The healthy range (18.5-24.9) is where most people should aim to maintain their weight.
According to the CDC, approximately 42% of U.S. adults have obesity (BMI ≥30). Learn more in our comprehensive BMI guide.
All Calculators
Browse all eight calculators below. Each card includes a quick description, who it is best for, a worked example, and a link to the related in-depth guide.
Standard BMI Calculator
The classic Body Mass Index calculator for all adults. Enter your height and weight to get an instant BMI result with category classification, healthy weight range, and personalized guidance. This is the most widely used BMI tool worldwide.
Best for: Quick BMI screening for any adult
Example: A person who is 5'7" and weighs 160 lbs has a BMI of 25.1 — Overweight. Their healthy weight range at this height is 118–159 lbs.
BMI Calculator for Women
A BMI calculator with women-specific healthy weight interpretation and recommendations. Accounts for the fact that women naturally carry a higher body fat percentage than men, and provides tailored guidance including waist circumference thresholds and life-stage considerations.
Best for: Women seeking gender-specific BMI guidance
Example: A woman who is 5'4" and weighs 140 lbs has a BMI of 24.0 — Normal. She is within the healthy range (108–145 lbs for her height).
BMI Calculator for Men
A BMI calculator tailored for men with male-specific interpretation and health guidance. Addresses the tendency to store fat abdominally and provides waist circumference guidelines, muscle mass context, and cardiovascular risk factors specific to men.
Best for: Men, especially those with higher muscle mass
Example: A man who is 5'10" and weighs 180 lbs has a BMI of 25.8 — Overweight. However, if he exercises regularly and has a waist under 40", his actual risk may be lower.
BMI Calculator by Age
An age-adjusted BMI calculator that provides different healthy range recommendations based on your age group. Recognizes that body composition changes as we age — older adults may benefit from a slightly higher BMI, while younger adults can reference standard ranges.
Best for: Adults over 45 and seniors seeking age-specific guidance
Example: A 65-year-old who is 5'6" and weighs 155 lbs has a standard BMI of 25.0. With age adjustment, this falls within the recommended range for seniors (23–28), where a slightly higher BMI is protective.
Pediatric BMI Calculator
A CDC percentile-based BMI calculator designed specifically for children and teens ages 2–19. Unlike adult BMI, pediatric BMI compares your child to age- and sex-matched peers using growth chart percentiles, providing a developmentally appropriate assessment.
Best for: Parents of children ages 2–19
Example: A 10-year-old boy who is 4'6" and weighs 70 lbs has a BMI of 16.8, placing him at approximately the 50th percentile — Healthy Weight for his age and sex.
New BMI Calculator (Trefethen)
Calculate your BMI using the updated 2013 Trefethen formula that corrects the traditional formula's well-known height bias. The standard formula uses height squared, which tends to underestimate BMI for shorter people and overestimate for taller people. Trefethen's formula uses height to the power of 2.5 to correct this.
Best for: Very short or tall individuals curious about height bias correction
Example: A 6'2" person weighing 195 lbs has a traditional BMI of 25.0 but a New BMI of 24.3 — the traditional formula slightly overestimates BMI for taller individuals. The difference narrows for average-height people.
Ideal Weight Calculator
Calculate your ideal body weight using multiple clinical formulas including Devine, Robinson, Miller, and Hamwi. Instead of giving you a single number, this tool compares results across all four formulas and accounts for frame size to provide a realistic, well-rounded target weight range.
Best for: Anyone seeking a clinically-backed target weight range
Example: A 5'6" woman with a medium frame has an ideal weight range of approximately 120–144 lbs across the four clinical formulas (Devine: 130 lbs, Robinson: 136 lbs, Miller: 126 lbs, Hamwi: 130 lbs).
Lean Body Mass Calculator
Estimate your lean body mass (muscle, bone, organs, water) and body fat percentage using the Boer, James, and Hume formulas. Unlike BMI, this tool separates your weight into lean mass and fat mass, giving athletes and fitness enthusiasts a much clearer picture of body composition.
Best for: Athletes, fitness enthusiasts, and anyone tracking body composition
Example: A 5'8" male weighing 170 lbs has an estimated lean body mass of approximately 141 lbs (Boer formula), meaning about 29 lbs (~17%) of his weight is fat — within the healthy range for men.
Calculator Comparison at a Glance
This quick-reference table summarizes the key differences between all eight calculators so you can compare them side by side.
| Calculator | Formula Used | Age Range | Special Feature |
|---|---|---|---|
| Standard BMI | WHO standard (kg/m²) | Adults 20+ | Quickest calculation, universally recognized |
| BMI for Women | WHO standard (kg/m²) | Women 20+ | Female-specific interpretation, life-stage guidance |
| BMI for Men | WHO standard (kg/m²) | Men 20+ | Male-specific interpretation, waist guidelines |
| BMI by Age | WHO standard + age adjustment | Adults 20+, focus 45+ | Age-adjusted healthy ranges for seniors |
| Pediatric BMI | CDC growth chart percentiles | Ages 2–19 | Percentile-based, age- and sex-specific |
| New BMI | Trefethen 2013 (1.3 × kg/m^2.5) | Adults 20+ | Corrects height bias in traditional formula |
| Ideal Weight | Devine, Robinson, Miller, Hamwi | Adults 18+ | Multi-formula comparison, frame size adjustment |
| Lean Body Mass | Boer, James, Hume | Adults 18+ | Separates lean mass from fat mass |
How to Choose the Right Calculator
Not sure which tool to use? This decision guide will help you pick the most relevant calculator for your situation. You can always use more than one for a fuller picture.
| If you want to... | Use this calculator | Why |
|---|---|---|
| Get a quick BMI check | Standard BMI | The fastest way to calculate your BMI using the WHO formula |
| Get women-specific guidance | BMI for Women | Accounts for higher natural body fat and provides life-stage guidance |
| Get men-specific guidance | BMI for Men | Addresses muscle mass, abdominal fat patterns, and male health thresholds |
| Factor in your age (especially 45+) | BMI by Age | Provides age-adjusted healthy ranges — a slightly higher BMI may be protective for seniors |
| Calculate BMI for a child (ages 2–19) | Pediatric BMI | Uses CDC growth chart percentiles instead of fixed adult cutoffs |
| Correct for height bias (very short/tall) | New BMI (Trefethen) | Uses height^2.5 instead of height^2 to produce fairer results across different heights |
| Find a target weight goal | Ideal Weight | Compares four clinical formulas (Devine, Robinson, Miller, Hamwi) for a realistic range |
| Analyze body composition (muscle vs. fat) | Lean Body Mass | Estimates lean mass and body fat percentage — ideal for athletes and fitness tracking |
For the most complete health picture, consider using 2–3 calculators together. For example, pair the Standard BMI with the Lean Body Mass calculator and your Ideal Weight range.
Understanding BMI Categories
The World Health Organization defines the following BMI categories for adults. These ranges are the same for women and men, though health risks at each level may differ by sex, age, and ethnicity. For a detailed breakdown, see our BMI calculator guide.
| Category | BMI Range (kg/m²) | Risk Level |
|---|---|---|
| Severe Thinness | < 16.0 | High |
| Moderate Thinness | 16.0 – 16.9 | Moderate |
| Mild Thinness | 17.0 – 18.4 | Low |
| Normal Weight | 18.5 – 24.9 | Minimal |
| Overweight | 25.0 – 29.9 | Increased |
| Obese Class I | 30.0 – 34.9 | High |
| Obese Class II | 35.0 – 39.9 | Very High |
| Obese Class III | ≥ 40.0 | Extremely High |
Source: World Health Organization — Obesity and Overweight Fact Sheet. For Asian populations, overweight begins at BMI 23 and obesity at 27.5.
Related Guides
Our in-depth articles complement the calculators above. Each guide provides the background knowledge to help you interpret your results and take informed action.
BMI Chart for Women
Women-specific BMI chart with healthy weight ranges by height, age-related adjustments, and guidance on how body composition differs for women across life stages.
BMI Chart for Men
Men-specific BMI chart with healthy weight ranges by height, waist circumference guidelines, and how muscle mass affects BMI interpretation for men.
BMI Formula Explained
Detailed walkthrough of the BMI formula in both metric and imperial units, worked examples, the 703 conversion factor, and the proposed new BMI formula by Trefethen.
Healthy BMI Range
Everything you need to know about what constitutes a healthy BMI range, how it varies by age, sex, and ethnicity, and what the science says about optimal BMI for longevity.
BMI Calculator by Age
How BMI interpretation changes across age groups, from pediatric percentile-based assessment through adult ranges to age-adjusted recommendations for older adults.
Trusted Resources
Our calculators are built on formulas and guidelines from the world's leading health organizations. These authoritative sources provide additional context for understanding your BMI results.
World Health Organization (WHO)
The global authority on BMI classification and obesity guidelines. Defines the standard BMI cutoffs used worldwide.
WHO Obesity Fact Sheet →Centers for Disease Control (CDC)
Provides the growth charts used in our pediatric calculator and maintains the primary U.S. BMI guidelines for adults.
CDC BMI Overview →Harvard Health Publishing
Evidence-based analysis of BMI's usefulness and limitations from one of the world's top medical schools.
Harvard on BMI →Mayo Clinic
Comprehensive patient-facing information on obesity, BMI interpretation, and evidence-based weight management strategies.
Mayo Clinic on Obesity →NHS (UK National Health Service)
The UK's public health authority with practical guidance on BMI, healthy weight, and when to see a doctor about your results.
NHS BMI Calculator →Medical Disclaimer: These calculators provide estimates for informational purposes only. BMI is a screening tool, not a diagnostic measure. It does not account for muscle mass, bone density, age, sex, or ethnicity. Always consult a healthcare professional for medical advice. The information on this page should not be used as a substitute for professional medical advice, diagnosis, or treatment.
Frequently Asked Questions
For a quick health screening, start with the Standard BMI Calculator. If you want gender-specific interpretation, use the Women or Men version. Adults over 45 should try the BMI by Age calculator for age-adjusted healthy ranges, as research suggests a slightly higher BMI may be protective for older adults. Parents should use the Pediatric BMI Calculator for children ages 2–19 — it uses CDC growth chart percentiles rather than fixed adult cutoffs. If you are very short or very tall, the New BMI Calculator corrects the traditional formula's height bias. For body composition analysis, combine the Ideal Weight and Lean Body Mass calculators.
Our calculators use the same WHO-endorsed formulas that healthcare providers use worldwide. The mathematical calculations are precise. However, BMI itself has well-known limitations as a health metric: it cannot distinguish between muscle and fat, does not account for fat distribution, and may be less accurate for certain ethnic groups. BMI is most useful as a population-level screening tool rather than an individual diagnostic. For the most complete assessment, we recommend combining your BMI with other measurements such as waist circumference, body fat percentage (see our Lean Body Mass Calculator), and standard blood work from your healthcare provider.
The BMI formula itself is identical for men and women — BMI = weight (kg) / height (m)² regardless of sex. The WHO classification ranges (18.5–24.9 = Normal, etc.) are also the same. However, the health implications differ because men and women have fundamentally different body compositions. Women naturally carry 6–11% more body fat than men at the same BMI. Men tend to store fat abdominally, which is associated with higher cardiovascular risk. This is why our Women's BMI Calculator and Men's BMI Calculator provide tailored interpretation and guidance, even though the underlying BMI number is calculated the same way.
The Standard, Women, Men, and New BMI calculators are designed for adults aged 20 and older. The BMI by Age Calculator covers all adults but is especially valuable for those over 45, where age-adjusted ranges may apply. The Pediatric BMI Calculator is specifically for children and teens ages 2–19 and uses CDC growth chart percentiles. For infants under 2, BMI is not recommended — weight-for-length charts should be used instead under pediatrician guidance. The Ideal Weight and Lean Body Mass calculators are intended for adults of all ages.
You can, but be aware that BMI frequently misclassifies athletes and muscular individuals. Because BMI cannot distinguish between muscle and fat, a football player with a BMI of 30 and 12% body fat is categorized the same as a sedentary person with 35% body fat. For athletes, the Lean Body Mass Calculator is the most appropriate tool, as it estimates your fat-free mass and body fat percentage. You can also use the New BMI Calculator, which may produce slightly different (and sometimes more accurate) results for taller athletes. For a target weight, the Ideal Weight Calculator offers a useful reference, though athletes in strength sports will typically exceed these ranges due to muscle mass.
For adults with stable weight, checking your BMI a few times per year is sufficient. If you are actively trying to lose or gain weight, monthly checks can help track progress without causing unnecessary stress over day-to-day fluctuations. Daily weight can vary by 2–4 lbs due to water retention and food intake, so always weigh yourself at the same time of day (ideally morning) for consistency. For children, BMI should be tracked at regular pediatric checkups, typically annually. Focus on long-term trends rather than individual readings — a gradual change over months is more meaningful than a single measurement. If you notice a consistent upward or downward trend, consult your healthcare provider.