Target Heart Rate = ((max HR − resting HR) × %Intensity) + resting HR
Now to calulate your heart you just plug in your numbers. Your resting heart rate can be assessed by simply counting counting your pulse for one minute upon waking up. So for an example of a 23year old with a average resting heart rate of 60bpm. The work to determine the this individuals correct target heart rate range would look like this:
MHR = 220 - (0.67 x 23) = 205bpm
Target Heart Rate Zone
Low end of Zone = [(205 - 60) x 65%] + 60 = 154bpm
High end of Zone = [(205 - 60) x 90%] + 60 = 191bpm
Target Heart Rate Zone: 154bpm - 191bpm
So if you want to get the most out of your cardiovascular training and improve your hearts strength you must be able to measure your intensity or stress you are placing on the body by maintain your heart rate in the 65% - 90% zone. So investing in a heart rate monitor would be a very wise and useful move. You can pick-up heart rate monitors from your local sporting goods store there are usually two kinds a watch with a montior that straps around your chest just below your nipples or just a watch that you were againist your wrist that has a built in monitor that takes your pulse straight from your wrist.