Finding out the maximum amount of weight you can currently lift, or your one-rep max, is exhilarating, but it can also be risky and dangerous. Instead of testing it out in the real world yourself, you can get a fair estimation another way: Lift between five to eight reps, remember the amount of weight you lifted and plug them into this calculator. Image by Fort Meade.
Let's say I did 5 reps of 68kg front squats. I plug those numbers into ExRx.net's calculator and get 77kg as my estimated one-rep max. It's fairly close. The calculator also breaks this one-rep max estimate further down into varying percentages, all the way down to 50 per cent of your one-rep max. This is helpful because some weight training programs call for lifting at 85 per cent of your 1RM, for example. With this, you'll know exactly what weight to do that still challenges you, and you'll be able to know if you're getting stronger if you can gradually do more reps, or do it more easily at the same percentage.
When you test, it doesn't have to be between five to eight reps, but you should be confident that you can hit that rep range with the heaviest weight you can manage, rather than going hard and potentially hurting yourself with a one-rep max. A real one-rep max test can be good if you know what you are doing and have people there to keep an eye on you, should anything go south.
Predicting One-Rep Max [ExRx.net]