This calculator is using the Karvonen Formula which factors in your Max Heart rate and you Resting Heart Rate (HRrest) to calculate Target Heart Rate (THR):

THR = ((HRmax − HRrest) × %Intensity) + HRrest

Where HRmax is your Maximum heart rate. We have a HRmax calculator on this site

Where HRrest is your resting heart rate. To measure your resting heart rate you should take your heart rate just before you get of bed in the morning. It is best to doing this at least three times then take the average.

The Zones are as follows:

The Red Zone (Maximum Effort) --- this is 90 - 100% of maximum heart rate: Although this zone burns the highest number of calories, it is very intense. Most people can only stay in this zone for short periods. You should only train in this zone if you are in very good shape and are doing intense speed workouts.

Anaerobic Zone (Performance Training) --- this is 80 - 90% of maximum heart rate: Benefits of this zone include an improved VO2 maximum (the highest amount of oxygen one can consume during exercise) and thus an improved cardiovascular system, and a higher lactate tolerance ability which means your endurance will improve and you'll be able to fight fatigue better. This is a high intensity zone burning more calories, 15 % from fat.

Aerobic Zone (Endurance Training) --- 70 - 80% of maximum heart rate: The aerobic zone will improve your cardiovascular and respiratory system and increase the size and strength of your heart. This is the preferred zone if you are training for an endurance event. More calories are burned with 50% from fat. Given Training you should be able to stay in this zone for long periods hence endurance.

Fitness Zone (Fat Burning) --- 60 - 70% of maximum heart rate: This zone provides the same benefits as the maintenance zone, but is more intense and burns more total calories in the same time. The percent of fat calories is still 85%. This is probably the best zone for people just starting a fitness program.

Maintenance Zone (Warm up) --- 50 - 60% of maximum heart rate: The easiest zone which is used as the warm up zone. This zone has been shown to help decrease body fat, blood pressure and cholesterol. It also decreases the risk of degenerative diseases and has a low risk of injury. 85% of calories burned in this zone are fats.

Having a good heart rate monitor is invaluable in keeping track of you training. You even get ones like the Polar RS800 which can record you heart beat during exercise which you can download and analyze onto you computer.