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Calculate My BMI
Stop guessing whether your weight is healthy. When you want to calculate my BMI, you are asking for the same screening metric used by the NHS, CDC, and WHO to flag potential weight-related health risks. The index boils your height and weight down to 1 number that falls into a defined category, from underweight to obesity. Doctors use it worldwide because it is fast, free, and needs no special equipment beyond a bathroom scale and a tape measure.
What Is BMI?
Body Mass Index (BMI) is a screening tool, not a diagnostic test. It estimates whether your weight is appropriate for your height by comparing your total mass to the square of your stature. Developed in the 1830s by Belgian statistician Adolphe Quetelet, the formula remains the global standard for initial obesity screening. It does not measure body fat directly, but epidemiologists have shown that, across large populations, BMI trends track closely with body-fat percentage and related illnesses.
BMI Categories Reference
| Category | BMI Range |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity Class I | 30.0 – 34.9 |
| Obesity Class II | 35.0 – 39.9 |
| Obesity Class III | 40.0 and above |
These thresholds apply to adults aged 20 and older. Some Asian health authorities flag increased risk at BMI 23.0 and high risk at 27.5.
Disclaimer: This calculator provides general information and is not a substitute for professional medical advice, diagnosis, or treatment. BMI does not distinguish between muscle and fat and may misclassify athletes, pregnant women, children, and older adults.
How Do I Calculate My BMI?
You can measure BMI using metric or imperial units. Both formulas return the same number, so use whichever you find easiest.
Metric formula
BMI = weight (kg) ÷ height (m)²
For example, a person weighing 70 kg and standing 1.75 m tall would calculate:
70 ÷ (1.75 × 1.75) = 22.9
Imperial formula
BMI = weight (lb) ÷ height (in)² × 703
For example, someone who weighs 154 lb and is 69 inches tall (5 ft 9 in) would calculate:
154 ÷ (69 × 69) × 703 = 22.7
Always measure height without shoes and weight in light clothing. The calculator above applies these equations automatically and rounds the result to 1 decimal place, which is the clinical standard.
Adult BMI Categories and Ranges
Once you have your number, match it to the table below. These thresholds apply to men and women aged 20 and older.
| Category | BMI Range |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity Class I | 30.0 – 34.9 |
| Obesity Class II | 35.0 – 39.9 |
| Obesity Class III | 40.0 and above |
A healthy-weight BMI is generally linked to lower rates of hypertension, type 2 diabetes, and coronary heart disease. Some Asian health authorities additionally flag increased risk at BMI 23.0 and high risk at 27.5 because metabolic disease tends to appear at lower body fat percentages in certain populations.
When Is BMI Misleading?
BMI does not see inside your body. It cannot tell whether mass comes from fat, muscle, or bone. Because of that, the index can misclassify several groups:
- Athletes and bodybuilders. Dense muscle tissue weighs more than fat. A fit rugby player at 95 kg and 185 cm may record a BMI of 27.8–classed as overweight–while carrying very little body fat.
- Pregnant or breastfeeding women. Expected weight gain and fluid shifts render the standard adult ranges meaningless during gestation.
- Children and teens. For anyone aged 2 to 19, doctors plot BMI on sex-specific growth charts and use percentiles rather than fixed thresholds.
- Older adults. Adults over 65 may face greater health risks at BMI values below 23, and modestly higher BMIs can be protective against frailty and bone loss.
If you fall into one of these groups, pair BMI with waist circumference–men should generally aim below 102 cm (40 in) and women below 88 cm (35 in)–or ask a clinician for a DEXA scan or bio-impedance reading.
What Is the Next Step After I Know My BMI?
Your number is a starting point, not a verdict.
- Below 18.5: Schedule a check-up to rule out thyroid issues, malabsorption, or eating disorders. Focus on nutrient-dense foods and resistance training to build lean mass.
- 18.5 – 24.9: Maintain your current habits. Aim for roughly 150 minutes of moderate aerobic activity each week and a diet rich in fibre, lean protein, and vegetables.
- 25.0 – 29.9: A sustained 5% to 10% reduction in body weight can lower blood pressure and improve insulin sensitivity. Start with portion awareness and daily walking.
- 30.0 and above: Speak with a healthcare provider. They may screen for sleep apnoea, fatty liver, and metabolic syndrome, then build a supervised weight-management plan.
Regardless of your category, avoid crash diets. Sustainable changes of 0.5 kg to 1 kg per week are safer and more likely to last.
This calculator provides general information and is not a substitute for professional medical advice, diagnosis, or treatment.
Frequently Asked Questions
Does BMI change with age?
For adults over 20, the standard BMI ranges stay the same regardless of age. However, for children and teenagers, healthcare providers use BMI-for-age percentiles that account for sex and developmental stage.
Can I calculate BMI without a calculator?
Yes. Divide your weight in kilograms by your height in meters squared, or multiply your weight in pounds by 703 and divide by your height in inches squared. A pocket calculator or phone app can handle the arithmetic in seconds.
Why does my BMI say I am overweight when I am fit?
BMI cannot distinguish between muscle and fat. Athletes with high muscle mass may register as overweight or obese despite having low body fat. In these cases, waist circumference or body composition testing gives a more accurate picture.
Is a BMI of 22 good?
A BMI of 22 falls comfortably inside the healthy weight range of 18.5 to 24.9. Research generally associates this range with the lowest risk of heart disease and type 2 diabetes for most adults.
What BMI is considered underweight?
Any adult BMI below 18.5 is classified as underweight. Persistently low BMI can be linked to nutrient deficiencies, weakened immunity, and osteoporosis, so medical evaluation is advisable.
How accurate is BMI for predicting health?
BMI is a useful population-level screening tool, but it misses individual variations such as fat distribution and muscle mass. It is most accurate when combined with waist measurement and a review of metabolic markers by a clinician.