Health tools apply established formulas to body measurements or training inputs and return estimates such as body mass index, daily calorie needs, body fat percentage, one-rep max strength, heart rate zones, and pregnancy due dates. They are informational aids, not medical advice.
Health calculators come up around routine tracking and planning.
Health inputs are personal: weight, body measurements, training load, menstrual dates. Keeping them in the browser tab means no extra copies sit on a remote service. The math is closed-form and runs instantly on any device.
Server-based fitness and health platforms add value through tracking over time, synchronization with wearables, and integration with coaching teams. They are not a replacement for one-off calculations and they bring their own privacy considerations.
The formulas implemented here are all published and peer-reviewed. Running them locally means the result is reproducible: anyone with the same inputs will get the same output, which is exactly what a calculator should provide.
| Tool | Primary use | Formula / options |
|---|---|---|
| BMI calculator | Weight category | WHO ranges, metric/imperial |
| Calorie calculator | TDEE and targets | Mifflin-St Jeor + activity |
| Body fat calculator | Body composition | US Navy circumference |
| One rep max | 1RM estimate | Epley + Brzycki |
| Lean body mass | Lean and fat mass | Boer formula |
| Ideal weight | Healthy range | Hamwi, Devine, BMI |
| Heart rate zones | Training zones | Tanaka, 5 zones |
| Due date | Pregnancy due date | LMP or conception |
Start a structured fat-loss block. Use BMI calculator to confirm the starting category, run calorie calculator for a maintenance and a loss target, and track body composition with body fat calculator every two weeks.
Plan a strength cycle. Estimate working maxima with one rep max, then schedule a percentage progression for the next eight weeks. Pair recovery cardio with heart rate zone calculator to keep easy days easy.
Track a pregnancy. Run the LMP through due date calculator for the estimated date, then use date calculator to count down remaining weeks at each trimester boundary.
BMI is weight in kilograms divided by height in meters squared. It correlates loosely with body composition at the population level but does not account for muscle mass, frame size, or fat distribution.
BMR (basal metabolic rate) estimates the calories burned at rest. Multiplying by an activity factor gives TDEE, the total daily energy expenditure. A 500-calorie daily deficit produces roughly one pound of fat loss per week.
The US Navy method for body fat uses tape measurements at the neck, waist, and (for women) hips. It is accessible without calipers and produces estimates close to a DEXA scan when measurements are accurate.
Strength estimators rely on rep-to-1RM relationships. The Epley formula adds about 3.3 percent per additional rep over 1, the Brzycki formula uses a slightly different linear relationship. Either is reasonable up to roughly 10 reps; beyond that, the link between rep count and true 1RM degrades and the estimates diverge.
Heart rate zone systems split the working range into intensity bands. Zone 1 is recovery, zone 2 is aerobic base, zone 3 is tempo, zone 4 is threshold, and zone 5 is anaerobic. Most endurance plans spend the majority of time in zones 1 and 2, with intervals dipping into zones 4 and 5 during dedicated quality sessions.
Reviewed and tested May 25, 2026.