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.
The inputs, one by one:
- Maximum heart rate (bpm)
- Resting heart rate (bpm) — measured on waking, before getting out of bed
- Target intensity (%)
Results appear immediately — there is nothing to submit. Changing a field rewrites the link, so you can share the exact scenario you are looking at.
Consistency in how the underlying measurements are taken matters more than precision to the last decimal — measuring at the same time of day, in the same state of rest or hydration, makes readings taken weeks apart genuinely comparable in a way that a single very precise reading on its own is not.
Why heart rate reserve matters
This kind of calculation gets used both out of general interest and ahead of a specific conversation — with a trainer, a midwife or a GP — where having an actual number ready, rather than a description, makes that conversation more concrete and specific.
This tends to get checked before or after a change in routine — a new training block, a change in diet, a different stage of a pregnancy — as a way of putting a figure on something that would otherwise only be felt rather than measured.
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
Here is the calculation with the starting values:
- 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.
The headline figure is 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.