Free BMI Calculator - Find Out Your Body Mass Index Instantly
Your body mass index (BMI) is one of the most widely used health screening tools in the world. At Prescribe MyFitness, we’ve made it completely free to calculate yours in seconds, no sign-up, no app download, no subscription needed.
Simply enter your height, weight, age, and gender below and our BMI calculator will instantly show you your score, your weight category, and your healthy weight range.
BMI Calculator
BMI Scale Reference
Standard adult BMI categories (WHO classification)
What Is BMI?
BMI stands for Body Mass Index. It is a number calculated from your height and weight that gives a quick snapshot of whether your weight is in a healthy range for your height.
It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has been used by the NHS, WHO, and health professionals worldwide ever since as a simple, low-cost screening tool.
BMI is not a direct measure of body fat, but it is strongly correlated with more direct measures of body fat and is useful for identifying potential weight-related health risks.
BMI Categories Explained
The NHS and World Health Organisation use the following BMI categories for adults:
• Under 18.5: Underweight. You may need to gain weight. Speak to your GP.
• 18.5 to 24.9: Healthy weight. This is the ideal range for most adults.
• 25 to 29.9: Overweight. You may benefit from losing a small amount of weight.
• 30 to 39.9: Obese. Your health risk is increased. A GP referral is advisable.
• 40 and above: Severely obese. Medical support is strongly recommended.
Note: BMI categories differ slightly for people of South Asian, Chinese, and Black African-Caribbean backgrounds, where health risks can occur at lower BMI values.
How Is BMI Calculated?
The BMI formula is straightforward:
BMI = weight (kg) ÷ height (m)²
For example, if you weigh 75 kg and are 1.75 m tall:
BMI = 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5
Our calculator does this automatically — just enter your measurements and get your result instantly.
Is BMI Accurate?
BMI is a useful screening tool but it does have limitations. It does not account for muscle mass, bone density, or where body fat is distributed. This means:
• Athletes and bodybuilders may show as overweight despite having low body fat
• Older adults may have a healthy BMI but high body fat due to muscle loss
• Pregnant women should not use standard BMI calculations
For a more complete picture, use our Free Progress Tracker to log body fat percentage and measurements alongside your BMI.
What to Do After Checking Your BMI
Once you know your BMI, here are your next steps:
• In the healthy range? Use our Free Home Workout Tracker to maintain it.
• Overweight? Try our Free Weight Loss Tracker to set a target and track your progress.
• Underweight? Use our Free Meal Plan for safe advice on gaining healthy weight.
All tools at Prescribe MyFitness are completely free, no subscription required.
Frequently Asked Questions
Is BMI the same for men and women?
The same BMI formula and categories apply to both men and women, although body fat distribution differs between sexes. Men typically carry more muscle, which can make their BMI slightly higher than a woman at the same health level.
What is a healthy BMI for my age?
For adults aged 18 and over, a BMI of 18.5 to 24.9 is considered healthy. For children and teenagers, BMI is assessed differently using age and sex-specific percentile charts.
How often should I check my BMI?
Once every 3 to 6 months is generally sufficient unless you are actively trying to lose or gain weight, in which case monthly tracking is useful.
Ready to take the next step? Try our Free Weight Loss Tracker — completely free at Prescribe MyFitness.