People come up to me and ask, “Hey Pat, how many reps should I do?”.

Of course no one asks me that, but if they did, I would say, “Do one more than you think you can do”. They would say that my answer is not helpful – and they’d be right – but I didn’t tell them the whole truth.

There are lots of weight lifting programs that specify the number of sets and reps to do, where a set is a number of reps you do without a rest. A common rep scheme specifies 3 sets of 10 reps or 3 by 10. Do 10 reps, rest and then repeat the 10 reps until you have done 3 complete sets. Another well known scheme is 5 by 5, do 5 reps 5 times. With these programs you know how may sets and reps to do, but what weight should you use – a trickier question. If you pick a weight that is too light then you do more reps than required and if you pick a weight that is too heavy then you do fewer reps than required.

Some time ago I found a formula that estimated the weight you can lift for any number of reps (it works best in the 1 to 10 range).

The maximum weight you can lift is the weight with which you can only do 1 rep. This is your 1 Repetition Maximum or 1RM. You can also have a 10RM, which is the weight you can lift no more than 10 times or you could have a 5RM or any RM number.

If you want to start a 5 by 5 program you need to estimate your 5RM. The basis of the formula is that each rep is worth about 2.8% of the load. If you want to lift one more rep then you must decrease the load by 2.8%, if you want to decrease by one rep then you can increase the load by 2.8%.

Here is the formula to predict your 1RM (your maximum load) from a submaximal load and the number of reps you can complete with that submaximal load. Let’s say you can bench press 125 lbs 10 times.

1RM load = 125 / ((1.0278 – (0.0278 * 10)), where 125 is the submaximal load lifted and 10 is the number of times you lifted 125. The answer, your 1RM, is 166.71. If you can bench 125 ten times then you can likely bench 167 once.

If you know your 1RM then you can use a similar formula to predict your RM for any number of reps. For example, if your 1RM is 167 and you want to know your 3RM the formula would be:

167 * (1.0278 – (0.0278 * 3), where 167 is your 1RM and 3 is the number of target reps. The answer is 158, your 3RM weight. First calculate your 1RM and then you can estimate any RM load.

In my current weightlifting program the number of reps changes every workout and these two formulae allow me to get a good estimate of the weight I should be lifting – they are surprisingly accurate. While they do estimate the target weight, the idea is beat the prediction. If I can only lift my RM weight the exact number of times I am suppose to then I am not making any progress. Beating the prediction by adding a rep means I am making progress – nice! Do one more than you think you can do.

P.S. See the attached Excel spreadsheet for an example of how this works: RM Predictor