The formula
How to calculate heart rate reserve
Heart rate reserve is the range between your resting and maximum heart rates — the working span your heart has available. Training zones based on it adapt to your fitness in a way that percentages of maximum heart rate do not.
The Karvonen method adds the resting rate back after applying the intensity percentage. That matters: 70% of a 185 maximum is 130 bpm, but 70% of reserve for someone resting at 58 is 147 bpm — a meaningfully harder session.
Fill in the following:
- Maximum heart rate (bpm)
- Resting heart rate (bpm) — measured on waking, before getting out of bed
- Target intensity (%)
No submit button: type and the answer moves. Your inputs end up in the link, so the page can be shared already filled in.
None of the fields above need specialist equipment to fill in — the inputs are the kind of measurements that can be taken at home with a tape measure or scale, which is exactly why this style of calculation is widely used as a first, approximate check rather than a diagnostic one.
Why heart rate reserve matters
The formula behind heart rate reserve comes from published research rather than being invented for this page, and it is presented here exactly as it appears in the literature it is drawn from — the value of a calculator is applying it correctly and instantly, not reinventing it.
Because the calculation depends on a formula rather than a device, it is useful anywhere a scale, a tape measure or a stopwatch is available, without needing specialist equipment — which is also exactly why it should be treated as an estimate rather than a clinical measurement.
None of the figures produced on this page are intended to replace a proper assessment, and nothing here should be read as a diagnosis or a treatment recommendation. What a calculator like this is good for is preparation — arriving at an appointment with an actual number already in hand, rather than a vague sense of a change, tends to make that conversation more useful for everyone involved.
A calculator like this one is frequently used to prepare a specific number for an upcoming appointment, so that a description like "I think it has changed" can be replaced with an actual figure and a date it was measured on. Clinicians and trainers generally find a concrete number, however approximate, more useful to work from than a general impression.
Worked example
Work through the defaults on this page:
- Maximum heart rate: 185 bpm
- Resting heart rate: 58 bpm
- Target intensity: 78 %
That gives:
- Heart rate reserve: 127 bpm
- Target heart rate at that intensity: 157 bpm
- Top of zone 2, at 70% reserve: 147 bpm
Those starting figures exist only to show the calculation working, not as a target or a reference value to compare yourself against — replace them with your own measurements in the calculator above and the same formula applies exactly as shown.
Reading the result
As fitness improves, the resting rate falls and the reserve widens, so zones calculated this way automatically shift upward. That is the main argument for using reserve rather than a flat percentage of maximum.
Where this goes wrong. Measure the resting rate properly — on waking, lying still, before caffeine. A daytime "resting" reading is often 10–15 bpm high, which pushes every zone with it.
As with any general-purpose formula, the result here is an estimate derived from population averages, not a measurement specific to you — a reasonable starting point for a conversation with a qualified professional, not a replacement for one. Nothing on this page is medical advice.
For most people, yes, because it accounts for individual resting heart rate. It requires an accurate resting measurement, which is the part people get wrong.
There is no target. A wide reserve generally reflects good aerobic fitness through a low resting rate, but maximum heart rate is largely genetic and falls with age.
It returns heart rate reserve. With 185 bpm maximum heart rate, 58 bpm resting heart rate and 78 % target intensity, that comes to 127 bpm. Change any field and the figure moves with it.
It uses a published formula applied exactly as defined, so the arithmetic itself is precise — but the formula is a population-level estimate, and individual results can vary from it for reasons the formula has no way to account for. Treat the output as a reasonable estimate rather than a measured or diagnostic value.
For most people, yes — a single reading is a snapshot, while the same measurement repeated consistently over weeks or months shows a trend, which is generally more informative than any one figure taken in isolation.
Small differences between calculators usually come down to which published formula or reference dataset each one uses — several equations in this area exist in more than one accepted version. This page states the specific formula used in the section above, so it is possible to check directly whether another source is using the same one.