Health & Fitness Calculator

How This Calculator Works

FitMetrics turns a handful of simple measurements into a complete picture of your metabolic health — body composition, calorie needs, macronutrient targets, heart rate zones, and a realistic weight-loss projection. Everything is calculated instantly in your browser using published, peer-reviewed formulas. Nothing you type is ever uploaded, stored, or shared.

What you enter

At minimum, the calculator needs your age, biological sex, height, and weight. Adding three tape measurements — waist, neck, and (for women) hip circumference — unlocks body fat percentage and body composition typing. The more measurements you provide, the more of the results cards appear. You can work in imperial (lbs/inches) or metric (kg/cm); the toggle at the top converts everything for you.

What it calculates

• Body Mass Index (BMI) — your weight relative to height, using the standard WHO categories. A fast screen, but it can't tell muscle from fat, which is why we pair it with the measures below.
• Basal Metabolic Rate (BMR) — the calories your body burns at complete rest, via the Mifflin-St Jeor equation, the most accurate predictive formula for modern populations.
• Body Fat Percentage — estimated from your circumference measurements using the U.S. Navy method, a tape-measure technique validated against clinical reference methods to within about ±3–4%.
• Waist-to-Height Ratio (WHtR) — a strong, simple predictor of cardiometabolic risk that captures central (belly) fat directly. The rule of thumb: keep your waist under half your height.
• Body Composition Type — classifies your build using the Institute for Functional Medicine flow diagram, combining BMI, waist size, waist-to-hip ratio, and body fat into a single phenotype.
• Protein & Macronutrient Targets — daily protein scaled to your activity and age, plus fat and carbohydrate splits for balanced, low-carb, keto, and other dietary approaches.
• Heart Rate Zones — sex-specific maximum heart rate (Tanaka for men, Gulati for women) and your Zone 2 fat-burning range, rather than the dated and imprecise "220 − age" rule.
• Weight-Loss Projection — a 52-week curve that recalculates your BMR each week as you lose weight, showing the natural plateau that every real diet produces instead of an unrealistic straight line.

How to read your results

These numbers are estimates based on population-level research, not clinical measurements — they're built for self-monitoring and for spotting trends over time, not for diagnosis. Measurement technique matters most: take your circumference readings snug but not compressing, at the same point each time, and recheck monthly under the same conditions. Where the calculator shows a calorie deficit, note the minimum floors (about 1,200 kcal/day for women and 1,500 kcal/day for men) below which it isn't safe to eat without medical supervision.

The evidence behind every number

Every formula and threshold on this page has a published source. The About page documents each one in detail — the equations, the studies that validated them, and their known limitations — and our blog goes deeper on topics like interpreting your waist-to-height ratio, why Zone 2 training matters, and how to measure your body circumference accurately.

FitMetrics is not a medical device and does not provide medical advice. All calculations are estimates and may not reflect individual variation. Consult a qualified healthcare provider before making significant changes to your diet or exercise program.