Your maximum heart rate (MHR) is important because it helps you determine your exercise intensity. For years, we've been using the formula 220 – age to calculate your MHR. From here, you can calculate different heart rate "zones" to exercise in by multiplying your MHR by the percentages that follow: 50 to 70 percent (MHR x .5 to .7) for an easy workout, 70 to 85 percent (MHR x .7 to .85) or so for a moderate workout and 85 to 95 percent (MHR x .85 to .95) for an intense workout or interval training. But, like every formula, the 220 – age formula is just an estimate—and more and more research is showing it's not a very good one.

The only way to truly know your max heart rate is by testing it in a laboratory. Since this isn't practical for most people, we want to give you better tools to help determine your exercise intensity. A combination of the following should help you figure out where you are when you're working out and where you need to be.

1. THE TALK TEST This is a super-easy way to figure out your intensity. If you can sing, you're working at a very easy level. If you can maintain a conversation with a friend, you're generally working at a moderate level. If you can say a sentence or so at a time and maintaining a conversation is more challenging, you're approaching a somewhat hard level. Finally, if you can only get out a word or two at a time and conversation isn't possible, you're working at a very hard intensity (like if you were doing intervals).

2. RATE OF PERCEIVED EXERTION (RPE) We use this gauge frequently in Shape. Like the talk test, it's very easy to apply to your workout. While there are a couple of different scales that researchers use, we like the 1–10 scale, where 1 is laying in bed or on the couch. You're not making any effort. RPE 3 would be the equivalent of an easy walk; RPE 4–6 is moderate effort; RPE 7 is somewhat hard, and RPE 8–10 is the equivalent of sprinting for the bus. You can only sustain a 9–10 for a very short time.

3. HEART RATE Keeping in mind that most heart rate formulas have a wide margin of error, one formula that seems to be more accurate, according to Jason R. Karp, an exercise physiologist and running coach in San Diego, is 205.8 – (.685 x age). E.g., if you're 35, your max heart rate using this formula would be 182.

Use a combination of the above methods to determine your exercise intensity and you'll get a better, more effective workout each and every time.