Heart rate reserve is your max heart rate minus your resting heart rate: the full range of beats your heart can add on top of rest. If your max is 185 and your resting rate is 55, your reserve is 130 beats. Training zones built from this range (the Karvonen method) reflect your personal fitness in a way that raw percentages of max never can.
Why it matters
Percent-of-max has a blind spot: it ignores where your heart starts. Two people with a max of 185 get identical zones from the standard method, even if one rests at 48 and the other at 75. Their hearts are living completely different lives. Reserve fixes that, because resting heart rate drops as you get fitter, and the formula picks it up automatically. Same effort level, personalized numbers. It's also why "70 percent" means different things in different apps: 70 percent of max and 70 percent of reserve can sit 15 or more beats apart.
How to use it in training
The Karvonen formula: target HR = resting HR + (reserve × intensity). Say resting is 55, max is 185, and you want a 60 to 70 percent easy zone. That's 55 + (130 × 0.6) = 133 on the low end, and 146 up top.
Get your resting number from a few mornings of measurement before getting out of bed, or trust your watch's sleeping average. Get max from a field test if you can, a good formula if you can't. Then recalculate every few months. As fitness improves, resting HR falls, your reserve grows, and your zones quietly shift. That drift is progress you can measure.
Related terms
Go deeper
For the research behind heart rate formulas and why the estimates miss, read Max heart rate formula research.