BMI Calculator is a free online tool that helps you calculate your Body Mass Index based on your height and weight. It quickly shows whether you are underweight, normal, overweight, or obese.
BMI Calculator
Calculate your Body Mass Index, check the full BMI chart, and find your healthy weight range in seconds
About This BMI Calculator
Body Mass Index (BMI) is a simple measure that uses your height and weight to estimate whether your weight is in a healthy range. This calculator defaults to U.S. standard units (feet, inches, and pounds) and gives you a complete breakdown including your healthy weight range, BMI category, and a visual scale showing exactly where you stand — consistent with guidelines from the CDC and WHO.
- Defaults to U.S. Imperial units (ft / in / lb) — switch to metric anytime.
- Get your healthy weight range based on your exact height, plus how much you'd need to gain or lose to reach it.
- Visual BMI scale shows exactly where your result falls — underweight, normal, overweight, or obese.
- Full CDC/WHO BMI category table for quick reference and comparison.
- Personalized health tips based on your result.
What Is BMI? A Simple, Honest Answer
BMI (Body Mass Index) is a single number worked out from your height and weight. Doctors and public health agencies in the U.S. use it as a fast, free first-pass check to see whether an adult's weight falls in the underweight, normal, overweight, or obese range. It's a screening number, not a diagnosis — think of it as a starting point for a bigger health conversation, not the final word.
The idea behind BMI goes back almost 200 years, to a Belgian statistician named Adolphe Quetelet who was trying to describe the "average" build of a population — not judge any one person. His formula stuck around because it's cheap, quick, and needs nothing more than a scale and a tape measure. By the 1970s it had been rebranded "Body Mass Index" and adopted by researchers studying weight-related disease risk across large groups.
In U.S. healthcare today, BMI shows up everywhere from your annual physical to insurance forms, largely because agencies like the CDC and the World Health Organization (WHO) treat it as a practical screening shortcut. It won't tell you your body-fat percentage or whether you're metabolically healthy — but a BMI that's unusually high or low is often a nudge to look closer, whether that's a follow-up with your doctor or a second measurement like waist circumference.
BMI Formula: How to Calculate BMI by Hand
Not near a calculator? You don't need one — the BMI calculation formula only takes two numbers: your height and your weight. Here's the exact formula U.S. clinics use, in both unit systems.
How to Calculate BMI Manually, Step by Step
- Convert your height to inches (or meters). For example, 5'9" = (5 × 12) + 9 = 69 inches.
- Square the height — multiply it by itself. 69 × 69 = 4,761.
- Divide your weight by that squared height. 154 ÷ 4,761 = 0.03234.
- Multiply by 703 (the conversion constant for pounds/inches). 0.03234 × 703 = 22.7.
Worked Example (Imperial — U.S. Standard)
A person who weighs 154 lb and stands 5 ft 9 in (69 inches) tall works out as: (154 ÷ 69²) × 703 = (154 ÷ 4,761) × 703 = 22.7. That lands squarely in the normal weight range of 18.5–24.9.
Worked Example (Metric)
A person who weighs 70 kg and is 1.75 m tall calculates as: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 — also within the normal range.
If manual math isn't your thing, the calculator at the top of this page runs the exact same BMI calculation formula instantly and also throws in your BMI Prime, Ponderal Index, and estimated ideal weight — figures a plain formula alone won't give you.
BMI Chart: Weight Ranges by Height
If you'd rather scan a chart than punch in numbers, this BMI chart lays out the underweight, normal, overweight, and obese weight ranges for common adult heights — each figure calculated directly from the same BMI formula above, in pounds.
| Height | Underweight | Normal | Overweight | Obese |
|---|---|---|---|---|
| 4'10" (147 cm) | Under 88 lb | 88 – 119 lb | 120 – 143 lb | 144 lb+ |
| 5'0" (152 cm) | Under 95 lb | 95 – 128 lb | 129 – 153 lb | 154 lb+ |
| 5'2" (157 cm) | Under 101 lb | 101 – 136 lb | 137 – 164 lb | 165 lb+ |
| 5'4" (163 cm) | Under 108 lb | 108 – 145 lb | 146 – 174 lb | 175 lb+ |
| 5'6" (168 cm) | Under 115 lb | 115 – 154 lb | 155 – 185 lb | 186 lb+ |
| 5'8" (173 cm) | Under 122 lb | 122 – 164 lb | 165 – 196 lb | 197 lb+ |
| 5'10" (178 cm) | Under 129 lb | 129 – 174 lb | 175 – 208 lb | 209 lb+ |
| 6'0" (183 cm) | Under 137 lb | 137 – 184 lb | 185 – 220 lb | 221 lb+ |
| 6'2" (188 cm) | Under 144 lb | 144 – 194 lb | 195 – 233 lb | 234 lb+ |
| 6'4" (193 cm) | Under 152 lb | 152 – 205 lb | 206 – 246 lb | 247 lb+ |
This chart applies the standard adult BMI cutoffs (18.5, 25, and 30) to each height, so the numbers match exactly what you'd get by running the formula yourself — no rounding shortcuts. If your height falls between two rows, plug your exact numbers into the calculator above for a precise result instead of estimating.
BMI Categories Explained (CDC / WHO Classification)
The CDC and WHO define the following standard BMI categories for adults aged 18 and over. These ranges apply to both men and women, though individual health risk can vary based on factors such as age, muscle mass, and ethnicity.
| BMI Range | Category | General Health Risk |
|---|---|---|
| Below 16.0 | Severe Thinness | High risk of nutritional deficiency, weakened immunity |
| 16.0 – 16.9 | Moderate Thinness | Elevated risk of health problems |
| 17.0 – 18.4 | Mild Thinness | Slightly elevated risk |
| 18.5 – 24.9 | Normal Weight | Lowest risk for most people |
| 25.0 – 29.9 | Overweight | Moderately increased risk |
| 30.0 – 34.9 | Obese Class I | High risk |
| 35.0 – 39.9 | Obese Class II | Very high risk |
| 40.0 and above | Obese Class III (Severe Obesity) | Extremely high risk |
How to Use This BMI Calculator
- Choose your unit system — the calculator defaults to U.S. Imperial (feet, inches, and pounds). Switch to metric if preferred.
- Select your biological sex — used for the ideal weight estimate, which differs slightly between males and females.
- Enter your age — used for context in the breakdown, as BMI interpretation can vary slightly by life stage.
- Enter your height and weight — type the values into the input fields.
- Tap "Calculate BMI" — instantly see your BMI score, category, healthy weight range, and a full detailed breakdown including BMI Prime, Ponderal Index, Body Surface Area, and an estimated ideal weight.
What Is a Healthy BMI? Understanding the Normal BMI Range
For most adults, a healthy — or "normal" — BMI falls between 18.5 and 24.9. This single range is used for both men and women aged 18 and up, since the standard BMI formula doesn't adjust for sex. A BMI below 18.5 is classed as underweight, and 25 or above is classed as overweight or obese.
A common question is whether normal BMI differs by gender. The formula itself treats men and women the same, but because men typically carry more muscle mass at a given weight, two people with an identical BMI can have quite different body compositions. That's one reason many U.S. clinicians pair a BMI reading with waist circumference or body-fat measurements before drawing conclusions, especially for anyone sitting near the edge of the normal range.
Age matters too. A "normal" BMI for a 25-year-old and a 70-year-old carry different implications, since muscle naturally declines with age even when weight stays the same. Some geriatric guidelines even suggest a slightly higher BMI (up to around 27) may be protective for older adults during illness or hospital stays — a nuance the standard 18.5–24.9 range doesn't capture.
Limitations of BMI: Why It Isn't the Whole Picture
While BMI is widely used in U.S. healthcare because it's quick, free, and requires no special equipment, it has several well-documented limitations:
- Doesn't distinguish muscle from fat: A muscular athlete and a sedentary person of the same height and weight will have identical BMI scores, even though their body composition and health risks may be very different.
- Doesn't account for fat distribution: Abdominal (visceral) fat is associated with higher health risks than fat stored elsewhere, but BMI cannot detect this. Waist circumference measurements are often used alongside BMI in U.S. clinical assessments.
- Less reliable across age groups: Older adults naturally carry more body fat for the same BMI due to age-related muscle loss (sarcopenia).
- Ethnic variation: Research — including studies referenced by the CDC — suggests certain populations may face elevated health risks at lower BMI thresholds than standard WHO cutoffs, prompting some U.S. clinicians to use adjusted ranges.
- Not designed for children, pregnant women, or very elderly individuals: These groups require different assessment methods. For children and teens, the CDC recommends age- and sex-specific BMI percentile charts.
Beyond BMI: Other Useful Metrics in This Calculator
BMI Prime
BMI Prime is your BMI divided by 25 (the upper limit of the normal range). A BMI Prime of 1.0 means you are right at the top of the normal weight boundary. Values below 1.0 indicate normal or underweight; values above 1.0 indicate overweight or obesity — making it easy to see proportionally how far you are from the healthy range.
Ponderal Index
The Ponderal Index uses the cube of height instead of the square, which some researchers consider more accurate for very tall or short individuals, since it scales more proportionally with body volume.
Body Surface Area (BSA)
BSA, calculated using the Mosteller formula, estimates the total surface area of your body in square meters. It is commonly used in U.S. medicine to calculate medication dosages, chemotherapy dosing, and fluid requirements.
Ideal Weight Estimate
This calculator uses the Devine formula (1974), still widely referenced in U.S. clinical settings — particularly for medication dosing — to estimate ideal body weight based on height and biological sex. It should be treated as a general reference, not a personal target.
How to Achieve or Maintain a Healthy BMI
Whether your BMI is below, within, or above the normal range, sustainable lifestyle habits tend to produce the best long-term results. The following align with recommendations from the CDC, the American Heart Association (AHA), and the Dietary Guidelines for Americans:
- Follow a balanced diet: Prioritize vegetables, fruits, whole grains, lean proteins, and healthy fats. The USDA's MyPlate guidelines (myplate.gov) offer a practical, U.S.-specific framework.
- Stay consistently active: The CDC recommends at least 150 minutes of moderate-intensity aerobic activity per week for adults, plus muscle-strengthening activities at least 2 days per week.
- Prioritize sleep: The CDC recommends adults get 7 or more hours of sleep per night. Poor sleep is linked to weight gain and hormonal imbalance.
- Track weekly trends, not daily fluctuations: Body weight can vary by 1–5 lb daily due to water retention, sodium intake, and hormonal cycles. Weekly or monthly trends are more meaningful.
- Set gradual, realistic goals: A rate of 1–2 lb per week is generally considered safe and sustainable by U.S. health authorities.
- Consult a professional: A U.S.-licensed physician, registered dietitian nutritionist (RDN), or certified personal trainer can create a plan tailored to your health history and goals.
Frequently Asked Questions
About This Tool & Editorial Standards
This BMI calculator applies the standard formula and classification ranges published by the CDC and WHO. All content is written and reviewed to reflect current, publicly available guidance from U.S. and international health authorities. This tool is provided for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment from a licensed U.S. healthcare provider.