RIR calculator: Find the right weight for your next set

Reps in reserve (RIR) is how many more good reps you could have done when you stopped a set. If you stop with two clean reps left, that set was 2 RIR.

The last two reps slow down, but your form still looks the same as rep one. You could grind out two more, but they would be ugly. That is where most of your sets should end. Not sure if you are at 2 or 3? Pick the higher number. Most people have about one more rep in them than they think.

For most people building muscle, stopping when you could do 0 to 3 more is a good range. "Challenging," which is about 2 more, is right for most sets. Beginners should stay closer to 2 or 3 while they learn the movement.

Sets that end close to failure, around 0 to 3 reps left, build muscle about as well as sets taken all the way to failure. The difference is what is left for the rest of your week. You recover faster, your form holds up, and you can do more good sets. If you are fitting training around work and family, that recovery is what keeps you consistent. When recovery is lower, like during a diet or a heavy-stress month, leaving a rep or two in the tank helps you hold on to your strength.

RPE (rate of perceived exertion) rates a set from 1 to 10. On the lifting version of the scale, RPE 10 is a set where you could not do another rep, and each point below that is one more rep left in the tank. RIR counts those same leftover reps directly. 3 RIR is RPE 7 (steady), 2 RIR is RPE 8 (challenging), 1 RIR is RPE 9 (hard), and 0 RIR is RPE 10. The scale is based on Zourdos et al., 2016, Journal of Strength and Conditioning Research.

Your reps plus your reps in reserve tells us roughly how many reps you could have done to failure. From that, it estimates your one-rep max, looks up what percentage of that max fits the reps and effort you picked for your next set, and rounds to the weight jumps your equipment allows.

Any exercise where you add weight in steps, like squats, deadlifts, presses, rows and machines. For dumbbells, enter the weight of one dumbbell. It is less useful for bodyweight moves.

It is the smallest step between the weights you have, so suggestions match real dumbbells and plates. Most dumbbells go up by 5 lb. On a barbell, a 2.5 lb plate on each side adds 5 lb. When that step is big for the weight you are lifting, the calculator suggests adding a rep or two before moving up.

It is an estimate. It is most reliable with sets of about 3 to 10 reps and an honest answer to how many more you could have done. With more reps than that, the estimate gets rougher, and the calculator says so.