What is BMI?
Body Mass Index (BMI) is a number calculated from your height and weight that is used as a screening tool to identify weight categories associated with health risks. It was developed by Belgian mathematician Adolphe Quetelet in the 19th century and adopted by the World Health Organization as a standard population-level health metric. The formula is simple: weight in kilograms divided by the square of height in metres.
Despite its simplicity, BMI correlates well with more precise body fat measurements in large population studies, which is why it remains widely used in clinical settings, public health research, and insurance assessments. A BMI between 18.5 and 24.9 is considered healthy for most adults. Below 18.5 indicates underweight; 25–29.9 indicates overweight; 30 or above indicates obesity.
The main limitation of BMI is that it cannot distinguish between fat mass and muscle mass. A muscular athlete may have a high BMI while carrying very little body fat, while a sedentary person with a normal BMI may have an unhealthy body composition. For this reason, BMI is best used alongside other measures such as waist circumference or body fat percentage rather than in isolation.
How to interpret your BMI
A BMI in the normal range (18.5–24.9) suggests your weight is appropriate for your height and that weight-related health risks are lower. A BMI in the overweight range (25–29.9) does not mean you are unhealthy — many people in this range are metabolically healthy and can reduce any excess risk through modest lifestyle changes. However, if overweight is combined with other risk factors such as high blood pressure or elevated blood sugar, addressing weight becomes more important.
A BMI of 30 or above is linked to meaningfully higher risks of type 2 diabetes, cardiovascular disease, sleep apnoea, joint problems, and certain cancers. Even a modest weight loss of 5–10% of body weight can significantly improve blood pressure, cholesterol, and blood glucose levels. The goal does not need to be reaching the "normal" range — any reduction in BMI from an obese range produces real health benefits.
An underweight BMI (below 18.5) may indicate inadequate nutrition, an underlying medical condition, or both. It is associated with increased risk of bone fractures, immune problems, and nutritional deficiencies. If you are underweight, consulting a healthcare provider or registered dietitian is strongly recommended.
BMI and different body types
BMI was originally developed from data on European men and does not account for natural variation in body composition across different ethnicities, ages, and sexes. Research has shown that people of Asian descent tend to have higher health risks at lower BMI values compared to people of European descent — some health organisations recommend a lower "overweight" threshold of 23 for Asian adults. Conversely, people of African descent may have lower health risks at equivalent BMI values due to differences in bone density and muscle mass distribution.
Age also affects the BMI-health relationship. Older adults (65+) may be at lower risk at a slightly higher BMI, as modest excess weight has been associated with better survival outcomes in this age group — a phenomenon researchers call the "obesity paradox." Children and teenagers use age- and sex-specific BMI percentile charts rather than the adult cutoffs used here.
For women who are pregnant or breastfeeding, standard BMI categories do not apply. Healthcare providers use separate guidelines for healthy weight gain during pregnancy based on pre-pregnancy BMI.
Frequently asked questions about BMI
Is BMI accurate for women?
BMI uses the same formula for men and women, but women naturally carry more body fat than men at the same BMI. This means a woman at BMI 22 typically has a higher body fat percentage than a man at BMI 22. Despite this difference, the standard BMI categories are still used for both sexes as population-level screening tools, as the health risk associations remain broadly similar.
Why is my BMI high but I look thin?
If your BMI appears high but you have a lean appearance, you may have a large, dense bone structure or higher-than-average muscle mass — both of which add weight without adding fat. Athletes and people who do regular resistance training commonly find that their BMI overstates their true level of fatness. In this case, body fat percentage is a more informative measure than BMI.
How quickly can I lower my BMI?
BMI drops as you lose weight. At a sustainable rate of 0.5–1 lb (0.25–0.5 kg) per week, BMI typically decreases by about 0.5–1 unit per month, depending on your height. A person who is 5'9" (175 cm) and loses 10 pounds will see their BMI drop by approximately 1.5 points. Use our weight loss timeline calculator to see when you could reach your target BMI.
Is BMI or body fat percentage more accurate?
Body fat percentage is more informative than BMI because it directly measures the proportion of your weight that is fat tissue. However, accurate body fat measurement requires specialist equipment or careful tape measurements. BMI is easier to calculate and still useful as an initial screening tool. Ideally, use both together — BMI for a quick check and body fat percentage for more detail. Try our body fat percentage calculator for a more complete picture.
What BMI is considered healthy for my age?
For adults aged 18–65, the standard WHO healthy BMI range of 18.5–24.9 applies regardless of age. For adults over 65, some evidence suggests that a slightly higher BMI (up to 27) may be acceptable and even protective against frailty. For children and teenagers, BMI is assessed using age- and sex-specific percentile charts — the adult cutoffs shown in this calculator do not apply to people under 18.
Steps to reach a healthy BMI
If your BMI is above the healthy range, the most evidence-based approach to lowering it is creating a moderate and sustainable calorie deficit through a combination of reduced food intake and increased physical activity. A deficit of 500 calories per day produces approximately one pound of fat loss per week without being so restrictive that it becomes unsustainable. Use our calorie deficit calculator to find your personalised daily target.
Prioritising protein intake (0.7–1g per pound of bodyweight) helps preserve muscle mass during weight loss, which is important both for health and for maintaining your metabolic rate. Foods such as chicken breast, eggs, Greek yogurt, fish, and legumes are excellent high-protein choices that also tend to be lower in calories.
Regular physical activity — particularly a combination of resistance training and cardiovascular exercise — accelerates fat loss, preserves muscle, and improves cardiovascular health regardless of weight change. Even 30 minutes of moderate-intensity exercise five days a week produces significant health benefits and supports long-term weight maintenance.