Health Calculations
BMI, calorie and fitness metrics — what each formula measures, and where its limits are.
10 calculations published in this category.
Weight Management
Body Composition
How To Calculate BMI
Body mass index divides weight by height squared and returns a single number that is easy to compute and easy to misread. The arithmetic is trivial. The difficulty is in knowing what the number cannot say about muscle, fat distribution, age and population.
BMI = weight in kilograms / (height in metres)^2
How To Calculate Body Fat Percentage
Body fat percentage splits the body into fat and everything else, and estimates the fat share from a few tape measurements. The US Navy circumference method is the most widely used of these estimates. Its arithmetic is short; its limits are worth understanding before you trust the number.
BF% = 495 / (1.0324 - 0.19077 x log(waist - neck) + 0.15456 x log(height)) - 450 for men
How To Calculate Ideal Body Weight
Ideal body weight is a formula, not a fact about health. It was written in 1974 to help clinicians dose drugs, and every version of it returns a slightly different number.
IBW = base + increment x (height in inches - 60)
How To Calculate Waist To Hip Ratio
Waist-to-hip ratio divides one circumference by another and returns a single unit-free number. The arithmetic takes seconds; the judgement lies in measuring at the right landmarks and in reading the result as a screening signal rather than a diagnosis.
WHR = waist / hip
Energy
Strength Training
Nutrition
How To Calculate Protein Intake
Protein needs are quoted as a rate, not a fixed number: grams per kilogram of bodyweight per day. That single multiplication hides most of the real questions, which are about which rate applies to you and how the daily total is best spread.
Protein (g) = bodyweight (kg) x grams per kg
How To Calculate Water Intake
Water intake is estimated from body weight: the common guideline is 35 millilitres for every kilogram you weigh, which is 2450 mL at 70 kg. The number is a starting point rather than a rule, because food, other drinks, heat and exercise all shift it.
mL = bodyweight (kg) x mL per kg