A BMI calculator can provide a quick screening estimate using your height and weight. This guide shows how to calculate BMI in metric or imperial units, explains the adult BMI chart and its limitations, adds waist measurements for context, and includes a simple progress tracker so you can review trends without treating one number as a diagnosis.
Overview
Body mass index, or BMI, is an estimate calculated from body weight and height. It is useful because it is simple, inexpensive, and repeatable. However, BMI does not directly measure body fat, muscle, fitness, eating habits, or overall health. A result should be considered one piece of information alongside waist circumference, medical history, activity, sleep, laboratory results when appropriate, and how you feel.
For adults, commonly used BMI categories are:
- Underweight: below 18.5
- Healthy-weight range: 18.5 to 24.9
- Overweight: 25.0 to 29.9
- Obesity: 30.0 or higher
These ranges are screening categories, not individualized medical targets. They are not intended for children and teenagers, for whom age- and sex-specific growth charts are used. Pregnancy also requires a different approach; BMI before pregnancy may help inform care, but a standard adult BMI interpretation should not be used to assess pregnancy health or weight gain.
BMI calculator
Choose a unit system, enter your measurements, and select Calculate. Use current, reasonably accurate measurements rather than estimates when possible.
Your result will appear here.
This calculator is for adults and provides an estimate only. It does not diagnose a health condition.
How to estimate
The metric formula is:
BMI = weight in kilograms ÷ (height in metres × height in metres)
For imperial measurements, the formula is:
BMI = 703 × weight in pounds ÷ (height in inches × height in inches)
For example, a person who weighs 70 kilograms and is 1.75 metres tall has an estimated BMI of about 22.9. The same calculation can be completed with pounds and inches by converting the measurements or using the imperial formula. Round the final result to one decimal place; extra decimal points suggest more precision than the measurement supports.
BMI is not a recommended calorie target. If you are trying to change weight, avoid automatically setting an aggressive restriction based on the result. A more useful next step may be to review eating patterns, protein and fibre intake, movement, sleep, and the pace of change. A TDEE calculator guide can help explain maintenance-calorie estimates, while a calorie-deficit guide can help you think about sustainable adjustments.
Inputs and assumptions
Measure consistently
Measure height without shoes, standing upright on a firm floor. Weigh under similar conditions each time, such as at a consistent time of day and with similar clothing. Daily weight naturally changes with hydration, food volume, sodium intake, bowel movements, and hormonal fluctuations. Those changes do not necessarily represent changes in body fat.
Understand what BMI misses
Muscle is denser than fat, so a muscular person may fall into a higher BMI category without having excess body fat. Older adults may lose muscle while maintaining a similar body weight. BMI may also be less informative for some ethnic groups, who can experience health risks at different body-fat levels. These limitations are reasons to interpret the result with context, not reasons to ignore health indicators.
Add waist circumference
Waist circumference can provide additional information about abdominal fat distribution. Measure with a flexible tape around the waist, usually midway between the lowest rib and the top of the hip bone, after a normal exhalation. Keep the tape level and snug rather than compressing the skin. Use the same location and technique each time. A waist measurement is also not a diagnosis, and thresholds can vary by population and clinical guideline. Ask a clinician how to interpret it alongside your other risk factors.
People who are pregnant, under 18, recovering from an eating disorder, or managing an illness that affects weight should seek individualized advice rather than using BMI as a stand-alone goal. For pregnancy-related questions, see the guidance on pregnancy weight gain ranges.
Worked examples
Example one: metric measurements
Suppose an adult weighs 82 kilograms and is 1.80 metres tall. The calculation is 82 ÷ (1.80 × 1.80), producing an estimated BMI of about 25.3. That falls just into the adult overweight screening range. It does not show whether the person has excess body fat, where fat is stored, or whether a specific weight change would improve health.
Example two: imperial measurements
Suppose an adult weighs 165 pounds and is 68 inches tall. The calculation is 703 × 165 ÷ (68 × 68), producing an estimated BMI of about 25.1. Because the value is close to a category boundary, measurement technique and normal weight variation matter. It is more helpful to discuss the broader pattern than to react to a single decimal point.
Use a progress tracker
Copy or print this table for reassessment. Record measurements under similar conditions and review trends every few weeks rather than checking for meaning after every daily fluctuation.
| Date | Weight | Height | BMI | Waist | Notes |
|---|---|---|---|---|---|
When to recalculate
Recalculate BMI when your weight changes meaningfully, when you confirm that an earlier height measurement was inaccurate, or when you begin a new health or fitness plan and want a consistent baseline. Adults generally do not need to calculate it every day. A monthly or occasional review, paired with waist measurement and health goals, may provide a clearer trend.
Consider speaking with a clinician if your weight changes unexpectedly, you have symptoms such as persistent fatigue or shortness of breath, you are concerned about blood pressure or blood sugar, or BMI and your lived experience do not seem to match. Also seek support before pursuing weight loss if you have a history of disordered eating. A clinician or registered dietitian can help set goals that account for medications, medical conditions, nutrition needs, and activity level.
Your practical next step is to calculate your BMI once, record the date and measurements, and decide what additional information would make the result more useful. That may be waist circumference, strength and fitness progress, meal quality, sleep, or a professional assessment. Use BMI as a starting point for informed questions—not as a verdict on your health.
