Which calculator for which situation
- Body composition. BMI as a quick screen, but read it alongside waist-to-height — the ratio that sees abdominal fat, which BMI cannot. Body fat and lean mass go deeper; ideal weight returns a range, not a number.
- Energy and diet. BMR and calories estimates total daily expenditure; calorie deficit converts that into a rate of loss; calories burned measures the cost of a specific activity.
- Hydration and routine. Daily water and sleep cover the two habits with the largest effect on energy and recovery.
- Reproductive health. Fertile window and pregnancy due date, both carrying the warning that a calendar estimate is not a contraceptive method.
- Clinical reference and safety. Body surface area and medication dose for calculation reference; blood alcohol to understand the mechanism — never to decide whether you can drive.
What every health formula has in common
All the equations here are population averages. They were built from groups and describe group behaviour well — not necessarily yours.
Metabolic rate equations typically land within 10% either way of direct measurement. Maximum heart rate formulas carry a standard deviation of 10 to 12 beats between individuals of the same age. The Widmark formula used for blood alcohol is more sensitive still to individual differences.
That does not make the estimates useless: they give you the order of magnitude and a starting point. It makes it useless to treat them as measurements. Real calibration comes from tracking over weeks.
When the limit matters more than the number
On some topics the correct reading is not the figure but the boundary of the tool. The blood alcohol and fertile window pages open with a prominent warning precisely because misreading them carries serious consequences — and because the popular version of those calculations (coffee sobers you up, ovulation is on day 14) is wrong.
Frequently asked questions
Does this replace seeing a doctor? Never. These are informational tools. Blood pressure, glucose, HbA1c and a lipid panel measure directly what anthropometric estimates only hint at.
Why do two calculators give different results for the same thing? Because they use different equations, each validated on a different population. Every page states which formula it applies — compare the method, not just the number.
Do they work for children? Generally no. Children and adolescents are assessed on age- and sex-specific percentile curves, not adult ranges.
Is my health data stored? No. Every calculation runs in your browser, and weight, height and measurements are never sent to a server.