Heart Rate Zone Calculator

Percentage of max versus Karvonen

Percentage of maximum takes zones as straight fractions of your max. Simple, and it ignores your resting rate — so a very fit person with a resting rate of 45 and an unfit person with a resting rate of 75 get identical zones.

Karvonen uses heart rate reserve instead:

Zone HR = resting + (max − resting) × intensity%

This accounts for your resting rate and produces higher, generally more realistic zone boundaries. It is the better default when you know your resting heart rate.

Note the two methods give noticeably different numbers — for a max of 185 and a resting rate of 55, zone 2 is 111–130 by percentage of max and 133–146 by Karvonen. Neither is wrong; they are different conventions, so pick one and stay with it.

Zone 2, and why most people get it wrong

Zone 2 is where most endurance volume should sit — conversational, sustainable for a long time, and the intensity that builds aerobic base with the least fatigue cost.

The commonest error is running zone 2 too hard, which turns easy sessions into moderately hard ones. That accumulates fatigue without the benefit of either easy or hard training.

The practical test beats the number: you should be able to hold a full conversation in complete sentences. If you are breathing in a rhythm that interrupts speech, you are above zone 2 regardless of what the watch says.