The short answer
Choose a weight that lets you complete your target number of reps with good form while keeping one to three reps in reserve. Find it with a few progressively heavier warm-up sets. If you know your 1RM, a set of 8 is usually around 75 to 80 % of it. Then let your log decide: top of the rep range on every set means a little more weight next time.
Start from the reps, not the weight
There is no right weight in absolute terms, only a right weight for a given number of reps. Read your programme first: 3 × 5 and 3 × 12 call for very different loads on the same exercise. The weight is whatever lets you hit those reps with clean technique.
Use reps in reserve as your guide
At the end of a set, ask yourself how many more clean reps you could have done. For most working sets, one to three in reserve is a good target. If you could have done five or more, the weight is too light for the rep target. If you fail before the target or your form breaks down, it is too heavy.
Find your weight with warm-up sets
On a new exercise, start very light and do a few sets of decreasing reps while adding weight: for example 10 reps easy, then 6, then 3. When a set of 3 starts to feel moderately hard, you are close to a working weight for 8 to 10 reps. Write it down: next session you start from there.
If you know your 1RM
Percentages give a first estimate. As a rough guide: about 85 % of your 1RM for 5 reps, 75 to 80 % for 8, 70 % for 10 to 12. People differ, and the gap is wider on machines and isolation exercises, so treat the percentage as a starting point and adjust with reps in reserve.
Adjust from one session to the next
Double progression is the simplest rule: stay with the same weight while you add reps within your range, then add a small amount once you reach the top on every set. Around 2.5 kg on upper-body lifts and 5 kg on lower-body lifts is common for barbells; with dumbbells, the next pair up is often a big jump, so add reps first.
Put it into practice
About this guide
STARK editorial guide. Examples explain how to use the library and record training; they are not individual coaching.
Training principles: ACSM — Resistance Training Guidelines (2026).
The session examples come from the STARK program catalogue linked above.