Calculate your BMI with age-specific guidance for children, adults, and older adults
This calculator provides estimates only. Consult your GP or a healthcare professional for personalised advice.
Body Mass Index (BMI) is a widely used screening tool, but its interpretation changes significantly depending on your age. For adults aged 18 to 64, the standard WHO thresholds apply. For children and teenagers aged 2 to 17, BMI must be compared against age- and sex-specific percentile charts because normal body fat levels change as children grow. For older adults aged 65 and over, research suggests that a slightly higher BMI range of 22 to 27 may actually be associated with better health outcomes, as modest weight can be protective in later life. This age-adjusted BMI calculator takes your weight, height, age, and sex into account to provide a more nuanced assessment than a standard BMI calculator. For children, it estimates the percentile range based on CDC growth chart data. For older adults, it applies adjusted thresholds based on geriatric research. For all ages, it provides context-specific guidance to help you understand what your BMI means for your particular age group. It is important to remember that BMI is a screening tool, not a diagnostic measure. It does not distinguish between muscle and fat, nor does it account for fat distribution. If you are concerned about your weight or your child's growth, speak to your GP for a thorough assessment that may include waist circumference, body composition analysis, or blood tests.
To use this age-adjusted BMI calculator: 1. Enter your weight in kilograms. If you know your weight in stones, multiply by 6.35 to convert. For example, 11 stone = 69.85 kg. For children, use their current weight. 2. Enter your height in metres using the decimal format. For example, 5 foot 6 inches = 1.68 metres. For young children, measure their height (or length if under 2) carefully, as even small errors significantly affect the result. 3. Enter your age in years. The calculator supports ages 2 to 100. For children aged 2 to 17, the calculator switches to percentile-based assessment. For adults 65 and over, it uses adjusted healthy weight ranges. 4. Select your biological sex. This is particularly important for children and adolescents, as growth patterns differ between boys and girls. 5. Review your results. The calculator displays your BMI, your weight category, an age-specific note explaining how to interpret the result, and (for children) an estimated percentile range.
The BMI formula is the same regardless of age: BMI = weight (kg) / height (m) squared What changes with age is how the result is interpreted: For adults aged 18 to 64, the standard WHO categories apply: below 18.5 is underweight, 18.5 to 24.9 is healthy weight, 25 to 29.9 is overweight, and 30 or above is obese. For adults aged 65 and over, some geriatric guidelines suggest adjusted thresholds. The healthy range shifts to approximately 22 to 27, reflecting evidence that moderate weight in older adults is associated with lower mortality risk. BMI may also underestimate body fat in this age group because of age-related muscle loss (sarcopenia). For children and adolescents aged 2 to 17, BMI is compared against age- and sex-specific reference data. A child at the 85th percentile has a higher BMI than 85% of children of the same age and sex. The categories are: below the 5th percentile (underweight), 5th to 84th percentile (healthy weight), 85th to 94th percentile (overweight), and 95th percentile or above (obese). This calculator uses a simplified estimation based on median BMI values from CDC growth charts; for an exact percentile, your GP or health visitor can plot your child on official NHS growth charts.
Inputs: Weight: 70 kg, Height: 1.65 m, Age: 25, Sex: Female
Inputs: Weight: 35 kg, Height: 1.40 m, Age: 10, Sex: Female
Inputs: Weight: 70 kg, Height: 1.70 m, Age: 72, Sex: Male
This calculator provides an estimate only and should not replace professional medical advice. For children, the percentile estimate is approximate; exact percentiles require plotting on official growth charts such as the UK-WHO growth charts used by the NHS. For older adults, the adjusted BMI ranges are based on observational studies and may not apply to everyone. Individual factors such as muscle mass, ethnicity, and existing health conditions all influence what constitutes a healthy weight. The NHS recommends that people of South Asian, Chinese, and other Asian ethnic backgrounds may face increased health risks at a lower BMI. If your BMI falls outside the healthy range for your age group, or if you have any concerns about your weight or your child's growth, please speak to your GP.