The short answer
The right dumbbell is heavy enough to make the last repetitions feel challenging while you can still keep good control and range of motion. The best load will vary by exercise: you may use heavier weights for a lower-body movement than for a shoulder exercise. If your form changes or you cannot complete the planned repetitions, reduce the load.
Choose by the movement, not by a number
There is no universally correct dumbbell weight. The right load depends on the exercise, your experience, the planned repetitions and how you feel that day. A pair that challenges your legs may be too much for a shoulder raise. It is normal to use different weights for different exercises.
Start lighter than you think you need for the first set. Rehearse the movement through a comfortable range, then ask: can I control every repetition, and do the last few feel like work? For a general session, finish with the sense that you could still do a few good repetitions. If you have to swing, rush, shorten the range or hold your breath to finish, reduce the load.
Use a small trial set
Try a weight for the lower end of your planned repetition range. If it feels easy and your movement stays steady, increase by the smallest available amount on the next set. If the first repetition is already difficult or your form changes, step down. Leave enough rest between sets to judge the next one fairly.
Your capacity can vary with sleep, stress, illness and the exercise order. That is useful information, not a failure. ACSM’s recent guidance supports individualizing resistance training; it does not prescribe one load for everyone. Sharp pain, dizziness or unusual symptoms are reasons to stop and seek appropriate advice.
A practical example
If your plan calls for 10 squats, choose a weight that lets you complete all 10 with steady movement. If you could easily do several more, try a slightly heavier weight next set. If you have to rush or shorten the movement, go lighter.
Try this next
Record the dumbbell weight and how the final set felt, then use that note next time.
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