These calculators translate body measurements and training numbers into targets you can train and eat to: a daily calorie figure, a protein target, a heart rate zone, a realistic one-rep max. They are used by people starting a weight change, lifters planning a program, and runners pacing a race.
29 calculators in this category
Body Mass Index is weight scaled by height squared. It is a population screening tool, not a diagnosis: it cannot distinguish muscle from fat, so a heavily muscled athlete can read as overweight while someone with low muscle mass can read as healthy. Waist-to-hip ratio and body fat estimates from skinfold or circumference formulas give a fuller picture because they say something about where mass sits, and abdominal fat carries more metabolic risk than the same weight on the hips.
Treat any single number as one data point. Trends over weeks — a waist measurement falling while body weight holds steady — say more about progress than a one-off reading.
Basal metabolic rate is the energy your body spends at complete rest keeping organs running. Multiply it by an activity factor and you get total daily energy expenditure, the figure that actually governs whether you gain or lose weight. A deficit of roughly 500 calories a day is the conventional target for losing about half a kilogram a week, though real-world results vary because metabolism adapts and food logging is imprecise.
Macronutrient splits divide that calorie target into protein, fat and carbohydrate. Protein is usually anchored first, at roughly 1.6 to 2.2 grams per kilogram of body weight for people training hard, because it protects muscle during a deficit and drives repair during a surplus.
A one-rep max estimated from a set of five or eight reps uses a repetition formula rather than an actual maximal attempt, which is safer and repeatable. Percentages of that estimate then drive program design — strength work near 85% and above, hypertrophy work in the 65 to 80% range.
For conditioning, heart rate zones are set as percentages of maximum heart rate, and pace calculators convert a target finish time into the per-kilometre or per-mile pace you must hold. Both are guides, not rules: how a session actually feels should override a number on a watch when you are ill, hot or under-slept.
Start with basal metabolic rate to see your resting energy use, then total daily energy expenditure to include activity. Subtract a modest deficit from the TDEE figure and use a macro calculator to split that target into protein, fat and carbohydrate.
Predictive equations sit within roughly 10% of measured values for most people, which is close enough for a starting point but not a prescription. Use the number for two to three weeks, track actual weight change, and adjust the calorie target based on what your body actually did.
Mostly no. Adult BMI categories, energy equations and body fat formulas are validated on non-pregnant adults. Children use age-and-sex percentile charts instead, and energy needs in pregnancy follow clinical guidance — speak to a clinician rather than relying on an adult calculator.
Different equations exist. Mifflin-St Jeor, Harris-Benedict and Katch-McArdle each fit different populations, and Katch-McArdle needs a body fat figure. Pick one, stay with it, and judge it by the weight change you observe rather than switching between equations.