Why this site prints more than one number
Ask seven published formulas what a 225 lb set of five implies about your one-rep max and they answer 253, 253, 256, 262, 263, 264 and 268 lb. Ask three respected equations what an 80 kg 30-year-old burns at rest and they answer 1,780, 1,787 and 1,854 kcal. Every calculator you have used picked one of those and printed it as though the others did not exist.
That is the gap this site exists to close. Each page names the equation behind its headline figure, runs the alternatives beside it, and states the range. Where a method has a published error bar — ±3 percentage points for the Navy body-fat method, for instance — the error bar is part of the answer, not a footnote. Where an equation stops working, the page says so: above ten reps the one-rep max formulas drift and disagree about the direction, so we tell you rather than quietly returning a confident number.
The formula behind the calculator above
The tool at the top of this page is the one-rep max estimator, and it runs seven published regressions on your set: Epley (1985), Brzycki (1993), Lombardi (1989), O'Conner (1989), Wathan (1994), Lander (1985) and Mayhew (1992). It reports the median of the seven and prints the full range, because no published comparison has established a clear winner across lifts, populations and rep ranges. At one rep the answer is the weight itself, special-cased — Epley at a single would otherwise tell somebody who just lifted a true max that it was 3% heavier than the bar they lifted.
Limitations of that estimate
All seven were fitted on sets of roughly two to ten reps taken close to failure by trained lifters, and they drift outside that band. A set stopped three reps short of failure gives the same input and a very different truth. Strong, experienced lifters are frequently under-estimated because they hold a higher percentage of max for a given rep count than the regression average. Treat the result as a programming starting point, not as a number to attempt cold — and warm up properly with a spotter or safety bars if you ever do test it.
What it does not do
No BMI categories, no body-fat health thresholds, no diagnosis, and nothing that pretends a web page can assess you. These are training and planning estimates, not medical advice, and not a substitute for a professional who can actually see you. Anything that belongs with a doctor or a registered dietitian is named as such on the page where it comes up.
The full account of every formula, every assumption and every rounding decision is on how these calculators work. It is also the page to read first if one of our numbers disagrees with one you got elsewhere.