This heart rate zones calculator uses your Maximum Heart Rate and your Minimum Heart Rate to calculate your Heart Rate Reserve – this is how much your heart rate can vary. It then calculates your heart rate zone boundaries as percentages of that Heart Rate Reserve.
The Maximum Heart Rate should have been measured in a Maximum Heart Rate Stress Test. The Minimum Heart Rate should have been measured while standing still, first thing in the morning.
In the absence of availability of a blood lactate response test, this heart rate zones calculator is the best option for determining your training zones.
Remember that whatever values this heart rate zones calculator tells you, listening to your body is always the most important thing, so if you are struggling (effort is higher) to achieve a particular heart rate for a normal session pace, perhaps you are coming down with something, so always spend time thinking about what the physical effort related to any particular heart rate would normally be, rather than blindly following your heart rate monitor. This will help in races immensely and in interval training, with less distractions from constantly looking at your watch. You shouldn’t need a watch to tell you that you are working really, really hard!
Other reasons that your heart rate might vary from normal are atmospheric conditions, dehydration and altitude, which can all have big impacts on your ability to hold a certain pace as you’d expect. Again, what’s that underlying perceived effort – that’s what you really need to duplicate each time.