If your curious about the effectiveness of your workouts, one way to test that is by using your target heart rate zone (THR). Knowing your THR zone can help you pace yourself so you burn calories without tiring out too quickly.

The most common way to find out your THR is to use a formula, though it's important to remember that formulas aren't always 100% accurate. Also, certain medications can affect heart rate so check with your doctor about monitoring exercise intensity if you fall into this category.

Your first step is to find your resting heart rate (RHR) which is a measure of your basic fitness level.

Before you get out of bed in the morning, take your pulse for 1 full minute, counting each heart beat to find your beats per minute (bpm). To take your pulse, use one of the following methods:

1. Place your index and middle fingers directly under your ear, then slide your fingers down until they are directly under your jawbone, pressing lightly.
2. Place your index and middle fingers over the outside of your opposite wrist, just below the base of your thumb.

For a more accurate measurement, take your pulse for 3 mornings and take an average. A normal RHR for adults is between 60-100, although exercisers and athletes may have lower RHR (the lower it is, the more fit you are). If your RHR is over 100, you should call your doctor to get checked out.
You've found your resting heart rate (RHR) above. Now use this number to find your target heart rate zone using the Karvonen formula, a measure of your heart rate reserve. Using this formula, we'll calculate a THR zone between 50% and 85% of your heart rate reserve. To calculate the low end of your THR zone, 50% of your heart rate reserve, you'll use the following formula:

206.9 - (0.67 x age) = Max Heart Rate (MaxHR)
MaxHR - (resting heart rate) = Heart Rate Reserve (HRR)
HRR x 50% = training range %
training range % + resting heart rate = low end of THR zone

The following example shows the low end of a THR for a 35 year old person with a resting heart rate of 60 bpm:

206.9 - (0.67 x 35) = 183.45
183.45 - 60 = 123.45
123.45 x 50%= 62
62 + 60 = 122 beats per minute

For this person, their low end THR starts at 122 bpm, which would be a light, warm up pace. Make a note of your own heart rate and we'll then go on to calculate the high end of your THR zone.

You used the Karvonen Formula to calculate the lower end of your THR zone. Now you'll use the same formula to calculate the higher end. Once again, all you need is your age and your resting heart rate:

206.9 - (0.67 x age) = Max Heart Rate (MaxHR)
MaxHR - (resting heart rate) = Heart Rate Reserve (HRR)
HRR x 85% = training range %
training range % + RHR = high end of your THR zone

The following example shows the high end of a THR for a 35 year old person with a resting heart rate of 60 bpm:

206.9 - (0.67 x 35) = 183.45
183.45 - 60 = 123.45
123.45 x 85%= 105
105 + 60 = 165 beats per minute

You should now have two numbers that will range somewhere between 120-180 beats per minute, depending on your age and resting heart rate. Your next step is to figure out how to use these numbers in your workout to make sure you're working at the right intensity.

The heart rate range you've calculated represents 50-85% of your heart rate reserve, but where do you work within that range?. The answer may include working at a variety of levels for different workouts. The ranges you can choose are categorized by intensity:

Low Intensity - 50-60%
Medium Intensity - 60-70%
High Intensity - 75-85%

Each level of intensity draws on different energy systems and focuses on different goals such as building endurance (e.g., working at a medium intensity) or increasing your anaerobic threshold and burning more calories (e.g., working at a high intensity).

You may hear that working at the low end of your THR zone will burn more fat. While this is technically true, this isn't the best intensity to work at if you're trying to lose weight. Instead, you want to work at a higher intensity to burn more overall calories.

Now that you have your THR zone, you need to decide how you'll keep track of it. The easiest way is with a heart rate monitor. Most come with a chest strap and a watch that allows you to see a continuous reading of your heart rate. 

If that isn't an option, you can always periodically take your pulse during your workout by counting the beats for 6 seconds and adding a zero to get your beats per minute.

In the first step I mentioned that these heart rate calculations aren't always accurate and one way to get around that is to use both your heart rate and your Perceived Exertion.

There's been some question about the accuracy of heart rate formulas, specifically the idea that 220 is actually maximum for all of us. For this reason, it's important to use these numbers as a starting point and combine them with other methods to get a more accurate number for you. Here's how:

1. Use your heart rate monitor during a normal workout and note your heart rate during your warm up.
2. Take your intensity to a level that feels comfortable to you and use a mental Perceived Exertion Scale to rate where you are--it should be around a 4-5. Make a note of your heart rate at this intensity.
3. Now increase your intensity (go faster, add resistance or find a hill and hustle up) until you're at a 7-8 on the Perceived Exertion Scale. Make a note of your heart rate.
4. Now, increase your intensity even more by going as hard and fast as hard as you can until you feel you're at a 9. Note your heart rate.

Using these numbers, you can get an idea of how your heart rate correlates to your intensity and you can tweak your THR zone to find a new low end (which will match 4-5 on the Perceived Exertion Scale). This low end becomes homebase for each workout. Using the heart rate from the highest intensity (matching 9 on the Perceived Exertion Scale) you have a new a high end--a heart rate you can achieve only for a brief period of time. This new THR zone will help you work even more efficiently.

The key with cardio, whether you're trying to lose body fat or stay fit, is variety. Choosing different activities at varying lengths and intensities will help ensure that you work your body at all levels of endurance. Here are some ways you can use your THR zone to create a variety of workouts.

1. With interval training, you'll alternate between low intensity exercise (like walking or jogging) and high intensity (sprinting or hill climbs) to burn calories and build endurance. Your intervals can be of any length, e.g. 3 minutes easy, 1 minute hard. Use your heart rate monitor along with Perceived Exertion to make sure you're working hard enough during your work sets.

2. Endurance training. Building endurance requires longer time spent exercising. For that reason, these workouts might be longer and slower, e.g. 45 minutes at a heart rate that matches level 4-6.

3. Burning calories. To burn the most calories, you want to work at a higher intensity. As a result, these workouts might be shorter and more intense than endurance training. For these workouts, you want to keep your heart rate at a level 6-8 throughout your workout.

Practice using your heart rate monitor during different workouts to get a sense of how hard you're working and make sure you're training effectively. If you don't have a heart rate monitor, move on to the next step for more ways to monitor your exercise intensity.

If you don't have a heart rate monitor, there are other ways to monitor your exercise intensity. One of those is the Talk Test, which is just what it sounds like--the ability to talk during your workout can help you determine how hard you're working. The American College of Sports Medicine states: "People who are able to comfortably converse during exercise are likely working out at an acceptable intensity." If you can speak comfortably, you're probably somewhere around the low-middle range of your THR zone (or a level 4-5 on the Perceived Exertion Scale). Experts generally suggest that you shouldn't be breathless during your workouts. However, if you're doing interval training or a short, high intensity workout, being somewhat breathless is what you're looking for. Obviously, if you feel dizzy or lightheaded, you should slow down or stop exercising.

As previously, the Perceived Exertion Scale is another way to track the intensity of your workouts. Perceived Exertion is a mental scale from 1-10 (1 being the easiest workload and 10 being the absolute hardest) that helps you determine how hard you're working. Your Perceived Exertion level, like the talk test, will depend on the type of workout you're doing, your fitness goals, your fitness level. You can also use a Borg Scale which offers a more detailed rating scale.