The 1RM is the maximum weight you can lift in a particular exercise with one repeat. Because this weight can not be measured precisely, it is estimated by the 1RM sub maximal test. The principle is that you choose a weight for a given exercise and try so as many reps until your can't hold it anymore. Through the tables below you can calculate what your 1RM is. Repetitions count only if the technique is performed perfectly.
Imagine doing 30 kilograms on a Leg Press, and after 10 reps you are finished. Based on this, you can calculate your 1RM for the Leg press. 10 reps is 75% of 1RM (see table). So 75% of your 1 RM is 30 kilograms. If 75% to 30 kilograms, then 100% 40 kilograms (30/75 * 100)