About This Site
Why this site exists and who maintains it.
This site started when I began building calculators like these as mobile apps. The idea worked, but testing them was slow: every version had to be installed separately. That led to the idea of a version you could open from a link. It ended up as liftnumbers.com simply because that name was free and the better ones were not — and it works the same on a phone as on a computer, which matters, because in the gym a phone is usually the only device you have with you.
The other reason was wanting to learn more about programming. The site is built without libraries or frameworks, because I wanted to understand what the code actually does rather than assemble it from ready-made parts.
Once the tools were good enough that I was using them myself in the gym, sharing them felt like a good idea. They are all free and require no sign-up.
What you'll find here
Seven calculators for working out training loads: a max estimate from a set you actually did, training loads as percentages, plate loading in both directions, RPE equivalents, Prilepin’s chart, strength standards and a 5/3/1 cycle.
One principle ties them together: they share the same maths. The max estimate, the RPE chart, the percentage chart and the 5/3/1 AMRAP estimate are all derived from the same average of five formulas. The same set therefore gives you the same estimate whichever tool you enter it into – and you never have to carry numbers by hand from one page to another.
On accuracy and sources
The formulas come from published strength training literature: Epley (1985), Brzycki (1993), Lombardi (1989) and Wathan (1994). They are estimates, and their accuracy varies with the exercise, your training background and the rep count. Each page states plainly what its numbers are based on and where the limits are — including when something is a rule of thumb rather than measured data.
Everything is calculated in your browser. The weights, reps and bodyweight you enter are never sent to a server and never stored anywhere.
This site is maintained by Matti Kenttä. Feedback and corrections are welcome – particularly if you find an error in a calculator.