BMI Calculator — Check Your Body Mass Index & Target Weight

What Is BMI?

Body mass index (BMI) is a calculated measure of weight relative to height. For adults, it is commonly used as a screening measure to classify weight status into underweight, healthy weight, overweight, or obesity. BMI is useful for screening, but it is not a direct measurement of body fat or a diagnosis of health.

The adult BMI categories used by this calculator are intended for adults age 20 and older. Children and teenagers require age- and sex-specific BMI-for-age interpretation.

The Mathematical Formula Behind BMI

In metric units, BMI is calculated by dividing body weight in kilograms by height in meters squared.

BMI = weight (kg) ÷ height² (m²)

When using pounds and inches, the conversion factor is included in the formula:

BMI = 703 × weight (lb) ÷ height² (in²)

For example, someone who is 5 ft 10 in tall and weighs 175 lb has a BMI of about 25.1. The calculator above performs the same calculation automatically and handles the conversion between US customary and metric units.

What Is a Healthy BMI?

The commonly used adult BMI categories are:

These categories are reference ranges for adults. BMI should be considered with other information when assessing an individual's health rather than treated as a diagnosis on its own.

What Is a Normal BMI for My Age and Height?

For adults age 20 and older, the commonly used healthy-weight BMI range is 18.5 to 24.9. Height matters because the same BMI corresponds to different body weights at different heights.

Age does not change the standard adult BMI categories. However, children and teenagers are still growing, so their BMI is interpreted using age- and sex-specific BMI-for-age percentiles rather than the adult categories shown here.

BMI and Target Weight

If you want to know what body weight corresponds to a particular BMI, the calculation can be reversed. For a given height, the calculator finds the weights corresponding to BMI 18.5 and BMI 24.9 and displays that as a reference range.

This can help answer searches such as “ideal weight for height,” but BMI alone cannot determine an individual's ideal or healthiest body weight. Muscle mass, fat distribution, medical history, waist size, and other factors can provide additional context.

If your goal is weight loss, you can use the Calorie Deficit Calculator to estimate a calorie target and projected timeline, then use real-world changes in body weight and performance to adjust your plan.

BMI Limitations for Athletes and Lifters

BMI is especially important to interpret carefully if you lift weights or carry substantial muscle mass. The calculation only uses height and weight, so it cannot tell whether that weight comes from muscle, fat, bone, or other lean tissue.

A muscular athlete can therefore have a BMI in the overweight or obesity range without having the same body-fat level as someone with less lean mass. That does not make the BMI calculation mathematically wrong; it means BMI may provide less information about body composition for that individual.

If body composition is your main question, use our Body Fat Calculator as another reference rather than relying on BMI alone.

Does Muscle Mass Make BMI Inaccurate?

Muscle mass can make BMI less informative because muscle contributes to body weight. BMI cannot distinguish fat mass from lean mass, so a highly muscular person may receive an overweight or obesity classification from BMI even when their body-fat level is relatively low.

For gym-goers, it is therefore useful to view BMI as one measurement in a broader picture. Waist circumference, body-fat estimates, training performance, health history, and other relevant measures can provide additional context.

Are There Different BMI Standards for Men and Women?

The standard adult BMI calculation is the same for men and women, and the commonly used adult category cutoffs are the same. BMI can still relate differently to body composition between individuals, so sex alone should not be used to turn a BMI number into a complete assessment of health.

How to Use BMI With Other Fitness Metrics

BMI answers a narrow question about weight relative to height. RepMaths has other calculators that can answer the next question in a fitness or nutrition workflow:

• Body Fat Calculator — estimate body-fat percentage using the US Navy tape-measure method.
• BMR Calculator — estimate basal metabolic rate from age, sex, height, and weight.
• TDEE Calculator — estimate daily maintenance calories from BMR and activity.
• Calorie Deficit Calculator — estimate a calorie target and weight-loss timeline.
• Macro Calculator — convert a calorie target into protein, carbohydrate, and fat targets.
• Protein Calculator — estimate a daily protein range based on body weight, activity, and goal.

Sources & Methodology

RepMaths uses the standard adult BMI calculation and the commonly used adult BMI categories of below 18.5, 18.5–24.9, 25.0–29.9, and 30.0 or higher. These categories and the BMI formula are consistent with guidance from the U.S. Centers for Disease Control and Prevention (CDC) and National Heart, Lung, and Blood Institute (NHLBI) .

The calculator is intended for adults age 20 and older. CDC describes BMI as a screening measure that should be considered alongside other health information rather than used as a diagnosis or complete measure of individual health. CDC's BMI methodology and limitations provide additional background.

BMI-based weight ranges shown by this calculator are mathematical conversions of the 18.5–24.9 reference range for the entered height. They are not personalized medical recommendations.

Frequently Asked Questions