Calculate your body mass index instantly. See your BMI category, healthy range, and personalised guidance — free, no sign-up.
| Category | BMI Range | Risk Level |
|---|---|---|
| Underweight | Below 18.5 | Increased risk |
| Normal weight | 18.5 – 24.9 | Low risk |
| Overweight | 25.0 – 29.9 | Moderate risk |
| Obese | 30.0 and above | High risk |
Body Mass Index (BMI) is a simple calculation using your height and weight to estimate body fat and screen for weight-related health risks. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has been the standard population-level screening tool for over a century.
The formula is: BMI = weight (kg) ÷ height² (m²). In imperial units, BMI = [weight (lbs) ÷ height² (in²)] × 703.
While BMI doesn't directly measure body fat, large-scale studies consistently show it correlates with metabolic health risks including type 2 diabetes, heart disease, and hypertension.
The World Health Organisation defines four primary BMI categories for adults:
A BMI below 18.5 is Underweight and may indicate nutritional deficiency or other health concerns. The Normal range of 18.5–24.9 is associated with the lowest risk of weight-related disease. A BMI of 25–29.9 is Overweight, where health risks begin to increase measurably. Anything 30 or above is classified as Obese, with significantly elevated risk for cardiovascular disease, sleep apnoea, and metabolic syndrome.
BMI is a useful screening tool but it has well-documented limitations. It cannot distinguish between muscle and fat, so a bodybuilder with 8% body fat might register as "overweight." Conversely, an older adult who has lost muscle mass might have a "normal" BMI while carrying excess visceral fat.
For a more complete picture of health, use BMI alongside Waist-Hip Ratio and, ideally, a DEXA scan or bioelectrical impedance measurement for direct body fat percentage.
A ketogenic diet reduces BMI through several mechanisms that go beyond simple calorie restriction. When you cut carbs below 20–50g per day, your body depletes glycogen stores and enters ketosis — producing ketone bodies from fat as its primary fuel.
This metabolic shift suppresses appetite by reducing ghrelin (the hunger hormone) and increasing cholecystokinin (the satiety hormone). Studies published in the British Journal of Nutrition consistently show keto dieters spontaneously eat fewer calories without feeling deprived.
Insulin levels also drop sharply on keto. Since insulin is the primary fat-storage hormone, lower insulin means your body is much more able to release and burn stored body fat — directly reducing BMI over time.
Use our Keto Macro Calculator to find your exact daily carb, protein, and fat targets. Then check the carb counts for any food you're considering using the search bar above.
The best foods for lowering BMI on a ketogenic approach are those that keep carbs extremely low while providing satiety through protein and healthy fat. Check the net carb count on any food instantly:
High-protein, low-carb choices like chicken breast, eggs, and salmon provide the amino acids needed to preserve muscle during weight loss. Avocado and olive oil deliver healthy monounsaturated fats that support hormone production. Non-starchy vegetables like leafy greens, broccoli, and cauliflower provide fibre, vitamins, and minerals with minimal carbs.
The foods to minimise are those with the highest net carb density: bread, pasta, rice, sugary drinks, and most processed snacks. Browse our food database to check net carbs for any food before you eat it.