You can measure how hard you are exercising based on your "rate of perceived exertion." This way of measuring exertion is great because it is scalable. If you are in a group exercise class, and the instructor asks you to warm up, you'll know you're in the "light activity level" if you could keep that pace and hold a conversation at the same time. But light activity to someone who has been, say, doing Kettlebells class for a long time will be different than it is for someone who is new to physical exercise. So when you're working out in a group, you can use the RPE as your measure of benefit and not worry about trying to keep up with that really fit person in the front row.
RPE is a good way to gauge your effort. It's also the way that the Flex Fit Powerbuilding Program prescribes intensity. If the workout says to do squats or deadlifts at a RPE of 7, that you should lift heavy enough for your your heart rate to go up and breathe heavily.
This chart originally appeared in the Exercise is Medicine Patient Notebook