The amount of weight you should squat depends on your goals, your skill and your energy levels. If you have only recently started squatting, you need to keep the weight light and develop proper technique. If you wish to gain lean muscle mass, you must squat with more weight. If you wish to squat as much as possible or plan on competing in powerlifting, there remain periods where you must squat as heavy as you can without getting hurt. Consult a healthcare practitioner before beginning any strength training program.
If You Are Just Starting Out
If you have only recently started squatting, you do not need to worry about weight. The squat remains one of the most skilled and technically demanding lifts you can perform, and like all skills, you must practice it. The only way to build proper technique is to use proper technique -- heavy weights make you worry more about standing up than improving your skill. Even if you must start with a broomstick to practice the correct technique, do not advance to an actual barbell until you can train through a full range of motion without rounding your back or leaning forward.
For Large Muscles
To gain size, you need to stress your muscles, which means you must work them hard. Experiment with different repetition ranges to see what works well for you. Avoid high-repetition squats, as fatigue can cause you to become sloppy and increase your risk of injury. If you train in the five to eight repetition range, select a weight that allows you to complete all of your repetitions with correct form. Never squat to the point where you collapse under the weight. Even though you should use spotters and lift inside a squat rack, you should not risk injury for one more repetition. If you wish to increase your training volume, perform another set after you have recovered.
For Maxing Out Power
To truly develop power, you need to train with extremely heavy weights. After you determine the maximal amount you can squat for a single repetition, train with at least 90 percent of this weight. Learning to squat heavy requires more than muscular size, it requires the ability to recruit a significant number of fibers, which you achieve through the use of near-maximal weights. When training in this range, perform only single repetitions, and rest up to three minutes in between each repetition.
To Generate Explosiveness
If you wish to increase your force, you need to squat with lighter weights, using no more than 70 percent of your single-repetition maximum, possibly as little as 50 percent. Developing the ability to generate force requires you to accelerate the bar as quickly as possible on the way up -- stand up fast as if you were trying to jump. This training percentage remains the percentage you would use to increase your vertical jump or explosive power.