We can’t tell you exactly how many pounds to lift or how steep a hill you should climb to reach a moderate or vigorous level of exercise, because what is easy for one person might be strenuous for another. It’s different for different people.

We can, however, provide some advice based on scientific research: Listen to your body. The level of effort you feel you are putting into an activity is likely to agree with actual physical measurements. In other words, if your body tells you that the exercise you are doing is moderate, measurements of how hard your heart is working would probably show that it really is working at a moderate level. During moderate activity, for instance, you can sense that you are challenging yourself but that you aren’t near your limit.

One way you can estimate how hard to work is by using the Borg Category Rating Scale. It was named after Gunnar Borg, the scientist who developed it. The numbers on the left of the scale don’t indicate how many times or how many minutes you should do an activity; they help you describe how hard you feel you are working.

For endurance activities, you should gradually work your way up to level 13 — the feeling that you are working at a somewhat hard level. Some people might feel that way when they are walking on flat ground; others might feel that way when they are jogging up a hill. Both are right. Only you know how hard your exercise feels to you.

Strength exercises are higher on the Borg scale. Gradually work your way up to level 15 to 17 — hard to very hard — to build muscle effectively. You can tell how hard an effort you are making by comparing it to your maximum effort. How hard does your current effort feel compared to when you are lifting the heaviest weight you can lift?

Once you start exerting more than a moderate amount of effort in your muscle-building exercises, your strength is likely to increase quickly.

As your body adapts and you become more fit, you can gradually keep making your activities more challenging. You might find, for example, that walking on a flat surface used to feel like you were working at level 13 on the Borg scale, but now you have to walk up a mild hill to feel like you are working at level 13. Later, you might find that you need to walk up an even steeper slope to feel that you are working at level 13.

The Borg scale is simple to use. But if your level of effort doesn’t match the numbers you see on the Borg scale — for example, if you think you are doing the exercises correctly, but you aren’t progressing or you are exhausted by your effort — check with a fitness professional. These experts are likely to understand the science that went into developing the Borg scale, and they can teach you how to match your level of effort with the right number on the scale.