BMI & Ideal Weight Calculator

Body Mass Index (BMI) is the ratio of your weight to your height squared. It's the most widely used screening tool in the world, but it has one major flaw for people who train: it can't tell muscle from fat. Our calculator goes beyond plain BMI by also estimating your ideal body weight using three recognized formulas, and putting every result in honest, real-world context.

Here's the problem in practice: a lifter who is 5'9" (1.75 m) weighing 187 lb (85 kg) shows a BMI of 27.8 — flagged as "overweight" by the WHO. Yet at 12% body fat, that person is in excellent shape. We display several indicators rather than a single number precisely to avoid this kind of misreading.

How BMI is calculated

The formula is simple: BMI = weight (kg) ÷ height² (m). For someone weighing 70 kg at 1.75 m, that's 70 ÷ (1.75 × 1.75) = 22.9. The World Health Organization then sorts the result into categories: under 18.5 (underweight), 18.5–24.9 (normal), 25–29.9 (overweight), and 30 or above (obese).

These thresholds were established on sedentary populations in the 1970s and 1980s. They remain useful at the population level but lose meaning as soon as you apply them to someone muscular, very tall, very short, or elderly.

The three ideal-weight formulas explained

Rather than a wide range, ideal-weight formulas give a target number based on your height (and sometimes sex). None is "the right one" — they work best as complementary reference points.

• Lorentz formula: accounts for sex and generally gives the most realistic range for an average build.
• Devine formula: originally developed to calculate drug dosages, widely used in clinical settings, and tends to give slightly lower values.
• Hamwi formula: the oldest (1964) and quickest to compute, often used as a first estimate but it underestimates weight for taller people.

Why BMI misleads athletes

Muscle is roughly 18% denser than fat by volume. As you build muscle, your weight — and your BMI — goes up, even as your health improves. BMI can't tell the difference: it only "sees" total mass.

To truly assess body composition, three measurements are far more reliable than BMI: body fat percentage (via skinfold calipers, bioimpedance, or DEXA), waist circumference (a direct marker of visceral fat), and waist-to-hip ratio. If your waist stays under 40 in (94 cm) for men or 31.5 in (80 cm) for women, a high BMI caused by muscle is not a warning sign.