How the calculator works

The estimated one rep max is calculated using the Epley formula, one of the most studied 1RM estimators in strength research. Take the weight you lifted, multiply your reps by 0.0333, add 1 and multiply the two together. A set of 5 reps at 60kg returns an estimated 1RM of around 70kg. The formula is accurate within a few percent for sets in the 1 to 10 rep range. Outside that range it loses accuracy.

The next session suggestion uses standard intermediate progression rules. For a strength goal the calculator suggests a 2.5 to 5kg jump on lower body lifts and a 1 to 2.5kg jump on upper body lifts. For a volume goal it holds the weight steady and asks you to add a rep. For hypertrophy it suggests a smaller load increase in a higher rep range.

How to use progressive overload

The calculator is a starting point. The principle is simple. To get stronger you need to do slightly more work over time. More weight, more reps, more sets or cleaner execution. The body adapts to a steady climb. It rebels against a sudden jump.

For a full explanation of progressive overload as a principle, read the progressive overload guide. For a complete 12-week plan that uses the principle properly, see the 12-week template. Both are written to sit alongside this tool.

When the suggestion will be wrong

Calculators do not know your sleep, food, life or training history. If the suggested next weight feels off, trust the room. The two most common reasons the calculator is wrong:

Your last set was not actually a working set. If you stopped at 5 reps but could have done 8, the 1RM estimate is low. Push the rep count up to where it would have stopped if you had gone to failure (or close to it) for a more accurate read.

You are deep into a training block and ready for a deload. Late-block sessions show stalled progress because fatigue has accumulated. The fix is a deload week, not heavier weights. Read the deload week guide for the structure.

Where STRNTH handles this automatically

Inside the STRNTH app the progressive overload logic runs for you. Each session reads your last performance, suggests the next target weight and tracks the climb across the programme. Stall on a lift for four sessions and an overload nudge appears with a specific increase. Deload weeks are scheduled into every programme. The STRNTH Index gives you one number for overall strength that updates after every session.

If you want the principle automated rather than a calculator you load by hand, that is what the app is for. Browse the programmes to find one for your level.