Cycling heart rate zones calculator

Zones built from maximum heart rate rest on a number almost nobody has measured properly. The formula everyone quotes — 220 minus your age — has a spread of about twelve beats either way, so for a 45-year-old the honest answer is "somewhere between 163 and 187". Zones built on that are guesswork wearing a lab coat.

Threshold heart rate is measured, not guessed, and you get it from a test you were going to ride anyway: the average heart rate over the last 20 minutes of a 20-minute FTP test. If you know only one of the two numbers, know this one.

Heart rate lags power by one to three minutes, and then keeps drifting upward for the rest of the ride even when the power does not move. Two consequences worth knowing:

• On short intervals, trust the watts. In a 3-minute VO₂max effort your heart rate only arrives in the right zone near the end. Chasing it means going too hard.
• On long endurance rides, watch the heartbeats. Same power, heart rate ten beats higher than usual, is the clearest early sign that something is wrong: heat, dehydration, fatigue, or an infection you do not know about yet.

That gap between the two is information, not a fault. Power says what you are doing; heart rate says what it is costing.

Then heart rate is your main guide, and it works — with two adjustments. Give every interval an extra minute before you judge whether the effort is right, and treat anything under three minutes as a perceived-effort session instead, because your heart rate will never catch up with it.

A word of warning about this site specifically: the plan is written in watts and assumes a smart trainer. You can ride it on heart rate, but the short, sharp sessions will not translate cleanly and the ramp test will not work at all.