What is heart rate reserve (HRR)?
The difference between your maximum and resting heart rate. Expressing intensity as a percentage of that reserve tracks oxygen uptake far better than a percentage of maximum heart rate.
Heart rate reserve is simply maximum heart rate minus resting heart rate. To find a target heart rate at a given intensity, take that percentage of the reserve and add your resting value back: target = resting + (reserve × percentage). This is the Karvonen method.
It is preferred over raw percentage-of-maximum for a concrete reason: %HRR tracks the percentage of oxygen uptake reserve closely, whereas %HRmax does not. Because your heart is already beating at rest, 50% of maximum heart rate corresponds to almost no work at all, while 50% of reserve corresponds to genuinely moderate effort.
Resting heart rate makes the difference, and it varies a lot. Two runners with the same maximum but resting rates of 45 and 65 will get quite different targets from the same percentage, which is exactly right — the one with the lower resting rate has more reserve to work with.
The thresholds this site uses are expressed in these terms: LT1 sits around 65–75% of reserve, LT2 around 80–88%. Measure resting heart rate first thing in the morning before getting up, averaged across several days, and treat any formula-derived maximum as a rough placeholder until a hard race or test gives you a real one.
Key numbers
| Formula | HRmax − HRrest | the reserve |
|---|---|---|
| Target HR | rest + (reserve × %) | Karvonen method |
| LT1 / LT2 | 65–75% / 80–88% HRR |
Work it out for yourself
HR pacing simulator — Build your zones from your own max and resting heart rate.
Frequently asked questions
Is 220 minus age a good estimate of maximum heart rate?
It is a population average with a standard deviation of about 10–12 beats, so for any individual it can be wrong by a lot. Use it only as a placeholder. A maximal effort at the end of a hard race, or a supervised test, gives a far more useful number.
Why use heart rate reserve instead of percentage of maximum?
Because percentage of reserve corresponds closely to percentage of oxygen uptake reserve, while percentage of maximum does not. Using %HRmax systematically overstates how hard low-intensity work is.
Sources
- Swain & Leutholtz (1997). Heart rate reserve is equivalent to %VO₂ reserve. Medicine & Science in Sports & Exercise.
- Karvonen, Kentala & Mustala (1957). The effects of training on heart rate. Annales Medicinae Experimentalis et Biologiae Fenniae.
Related explainers
Glossary: HRR (Heart Rate Reserve)