Countworthy

Fitness Calculators

Two free calculators for training by the numbers — how strong you are, and what you're made of. Published formulas, stated error margins, computed in your browser.

One-Rep Max Calculator

Estimates your one-rep max from the weight and reps of a set taken to fatigue, using Epley, Brzycki, Lombardi and O'Conner, and shows the range across the four with their average. A table gives the load at six percentages of the estimate, from 95% to 70%.

Body Fat Calculator

Estimates body fat percentage from a tape measure using the circumference equations Hodgdon and Beckett published for US Navy men and women in 1984. You get the estimate with its standard error (3.52 points for men, 3.72 for women, against underwater weighing), a lean/fat mass split, and where each measurement is taken.

Which tool do I need?

Programming your lifts? The One-Rep Max Calculator. Strength programs often prescribe weights as percentages of your 1RM, such as "5 sets of 5 at 75%". Enter a set taken to fatigue (say, 100 kg for 5) and the calculator estimates your max four ways and gives the load at 95%, 90%, 85%, 80%, 75% and 70% of the average.

Estimating body composition without a lab? The Body Fat Calculator. The 1984 Navy equations need only height, neck and waist (plus hip for women). The page states the equations' standard error and makes no claim about tracking change over time, because the studies it draws on did not test that.

Wondering which 1RM formula to trust? The studies the calculator page summarizes do not settle it, so it shows all four side by side: Epley runs a few percent above Brzycki until they meet at 10 reps, Lombardi gives the highest estimate at 2 to 5 reps and the lowest past 10, and O'Conner is the lowest from 6 to 10. The spread between them (about 4% at 5 reps) is how far the formulas disagree with each other, not how far any of them is from a tested max.

Both tools state their limits. The body fat page gives the 1984 equations' standard error against underwater weighing (3.52 percentage points for men, 3.72 for women) and reports a 2022 study in which a version of the equations read lower than DXA on average for men and higher for women. The one-rep max page shows the range across four formulas and notes that studies summarized in a 2003 thesis suggest predictions are less close above 10 reps.