Which tool do I need?
Programming your lifts? The One-Rep Max Calculator. Most strength programs prescribe weights as percentages of your 1RM — "5 sets of 5 at 75%" — but actually testing a true max is risky, tiring, and unnecessary. Take a recent hard set (say, 100 kg for 5), enter it, and the calculator estimates your max four ways and hands you the whole percentage chart, so every prescribed percentage becomes a number you can load on the bar.
Tracking body composition without a lab? The Body Fat Calculator. The scale can't tell muscle from fat, and bathroom-scale "body fat" readings swing with hydration. The Navy tape method needs only height, neck and waist (plus hips for women), and lands close enough to be genuinely useful for tracking a trend over months.
Cutting or bulking and want to know what you're actually gaining or losing? Same tool — the lean/fat mass split is the number to watch. Weight down 3 kg means little on its own; lean mass held while fat mass dropped is the actual goal, and re-measuring monthly shows which one moved.
Wondering which 1RM formula to trust? The calculator shows all four side by side precisely because they disagree — Epley and Brzycki track closely up to about 10 reps, Lombardi runs low, O'Conner conservative. The spread between them IS the honest answer: an estimate from a 5-rep set carries a few percent of uncertainty either way.
Both tools are open about their limits, because a fitness calculator that pretends to lab precision is selling something. Every estimate here states its error range: the Navy method is typically within about 3–4% of a DEXA scan but can miss by more on very muscular or very lean builds, and rep-based 1RM estimates get unreliable past 10–12 reps. The numbers are for tracking your own trend — consistent measurement beats precise-looking one-offs.