Health & Fitness Calculators
Body composition, energy and training
What these numbers can and cannot tell you
Body composition tools estimate. BMI is a ratio of weight to height squared, designed in the 1830s to describe populations rather than individuals, and it cannot distinguish muscle from fat — which is why a trained athlete and a sedentary person of the same weight and height get an identical score. The Navy body fat method uses tape measurements to get closer to the thing you actually care about, and the ideal weight calculator shows four different clinical formulas side by side precisely because they disagree with each other.
Energy tools are more reliable but still bounded. A calorie target comes from a basal metabolic rate equation multiplied by an activity factor, and both halves carry error: BMR formulas are fitted to population averages, and almost everyone misjudges their own activity level. Treat the number as a starting point to adjust from over a few weeks, not a figure to hit exactly.
Training, cycles and hydration
The training tools work from measured performance rather than estimates, so they are the firmest numbers here. A one-rep max calculated from a set you actually completed is close enough to program from, and heart rate zones derived from a measured maximum are more useful than zones derived from the 220-minus-age rule, which has a standard deviation of about twelve beats per minute.
The cycle tools — due date, ovulation — apply the standard clinical conventions: a 280-day gestation from the last menstrual period, and ovulation roughly fourteen days before the next period rather than fourteen days after the last. Both assume a regular cycle, and neither is a substitute for a scan or a test. Everything on these pages is calculated in your browser and nothing you enter is transmitted, which matters more in this category than in any other on the site.
Health & Fitness calculators — common questions
Is BMI accurate for me?
It is accurate as a rough screen and unreliable as an individual verdict. BMI does not measure body fat, does not account for muscle mass or frame size, and reads differently across ethnic groups — several health bodies use lower thresholds for South Asian populations. If your BMI sits near a category boundary, the body fat and ideal weight calculators will tell you more than the number alone.
How many calories should I eat to lose weight?
The calorie calculator gives your maintenance requirement and then targets around it. A deficit of roughly 500 calories a day corresponds to about half a kilogram a week, which is the rate most clinical guidance treats as sustainable. Very aggressive deficits cost muscle as well as fat and are harder to hold, which is why the tool caps how far it will go below your basal rate.
Which formula do the calculators use?
Each tool names its source on its own page. The calorie calculator uses Mifflin-St Jeor, currently the best-validated BMR equation for the general population. Body fat uses the US Navy circumference method. Ideal weight shows Devine, Robinson, Miller and Hamwi together rather than picking one. Where a formula has known limits, the page says so.
Is my data stored?
No. Every calculation runs in JavaScript in your browser. Your weight, height, cycle dates and training numbers are never sent to a server, never written to a database, and are gone when you close the tab.