5x5s have always been some of my favorite workouts! It's the combination of load and volume that makes for an incredibly intense workout. You should choose weights heavier that you would choose for, say, 3 sets of 12 reps...but keep in mind that you have 5 sets so you'll have to leave some room in your strength tank to get through all five (don't go your heaviest at the beginning and then fatigue out). Choose a weight that's around 85-90% of your 1-rep max if you know it (your 1-rep max is the heaviest weight you can lift for one single rep on an exercise). BUT one of my favorite reasons for 5x5s is the confidence. For example, if there are squats or pull ups on your workout that day, it's much easier to think about doing 25 total reps if they are broken down into 5 reps at a time (compared to trying to do 10 or 12, for example). I love these!
