Health Calculators
Free calculators for BMI, calories, hydration, ideal weight, and macros. Metric and imperial supported.
Health tools — what they do and when to use them
BMI (Body Mass Index) is the most widely used screening number in health, but it has significant limitations worth understanding before acting on it. The formula — weight in kg divided by height in metres squared — produces a single number that correlates reasonably well with body fat at the population level. However, it cannot distinguish muscle from fat: a professional athlete and a sedentary person of identical height and weight will share the same BMI despite very different body compositions. It also ignores fat distribution, which matters more than total fat for cardiovascular risk. The BMI calculator here follows WHO categories (underweight below 18.5, normal 18.5–24.9, overweight 25–29.9, obese 30+) and supports both metric and imperial inputs. For a more actionable number, pair your BMI result with a calorie calculation to understand your actual energy needs.
The calorie calculator uses the Mifflin-St Jeor equation — the most validated formula for estimating BMR (Basal Metabolic Rate) — multiplied by an activity factor to give TDEE (Total Daily Energy Expenditure). A deficit of 500 kcal/day below your TDEE creates roughly 0.5 kg of fat loss per week; a surplus of the same size supports muscle gain. Hydration is often underestimated: the water intake calculator uses weight × 35 ml/kg as a baseline, adjusted for activity. The ideal weight calculator applies the Devine formula to give a healthy weight range for your height. The macro calculator then splits your calorie target into protein (2 g/kg of body weight), fat (approximately 25% of calories), and carbohydrates covering the remainder — numbers grounded in sports nutrition research for body composition goals.
All tools run in your browser — no sign-up, no data sent to any server. Browse all free tools at toolsmithpro.com.
About these health tools
These calculators use established medical formulas: BMI (kg/m²), Mifflin-St Jeor for daily calories, weight × 35 ml/kg for water intake, Devine formula for ideal weight, and standard macronutrient ratios (protein 2 g/kg, fat 25%, rest carbs). All tools support both metric (kg, cm) and imperial (lbs, ft/in, fl oz). US visitors auto-land on imperial; everyone else gets metric.