Article — Karvonen Formula Calculator
Karvonen Formula Calculator: Heart Rate Reserve Training Zones
The Karvonen formula calculates a target training heart rate from heart rate reserve (HRR). THR = Resting HR + HRR times intensity percentage, where HRR = Max HR minus Resting HR. For a 35-year-old runner with a resting HR of 65, max HR is 185, HRR is 120, and the 70 percent training target lands at 149 bpm.
Finnish exercise physiologist Martti Karvonen described this method in 1957 in Annales Medicinae Experimentalis et Biologiae Fenniae. It remains the basis for the heart rate zones used by the American Heart Association, ACSM, and most polar coaching software. The calculator on this page applies the original equation and breaks the result into five training zones.
What is the Karvonen formula?
The formula is a simple linear scaling of heart rate reserve. Take the difference between the maximum heart rate the body can hit and the rate it idles at when fully rested. Multiply that range by the percentage of effort you want. Add the resting rate back in. The number that falls out is the heart rate that produces that intensity inside that specific person.
Compared with the older percent-of-max-HR rule, Karvonen does more than scale the top of the range. It anchors the bottom at the resting rate, which is where a body actually starts during exercise. A trained athlete with a resting heart rate of 50 and an untrained colleague at 80 share a max HR yet need very different target rates for the same intensity.
Karvonen's 1957 study only had six male subjects and was designed as a quick training-load proxy. It outlasted half a century of fancier alternatives because the math is portable to a wristwatch and the assumption set holds up surprisingly well.
Karvonen formula math
The default max HR formula is 220 minus age. The standard error is about 10 to 12 bpm, so a measured max HR from a treadmill ramp test is preferable when available. The newer Tanaka equation, 208 minus 0.7 times age, slightly narrows the error band, especially in adults past 40.
Max HR 220 - ageHRR Max HR - Resting HRTHR RHR + HRR × (intensity / 100)Z2 fat-burn 60-70% HRRZ4 threshold 80-90% HRRHeart rate reserve itself is a useful number. A young, untrained 25 year old with RHR 75 has HRR 120. A 25-year-old endurance cyclist with RHR 45 has HRR 150. The cyclist has more bandwidth, which translates directly into the ability to spend more time at intensities that build aerobic capacity without spiking near maximum.
Karvonen heart rate zones
The Karvonen calculator splits the HRR range into five training zones. Each is tied to a physiological substrate and a training purpose.
- Zone 1 50 to 60 percent HRR. Warm-up, recovery rides, easy walks.
- Zone 2 60 to 70 percent HRR. Aerobic base, fat oxidation, long slow distance.
- Zone 3 70 to 80 percent HRR. Tempo, aerobic capacity, talk-broken-sentence.
- Zone 4 80 to 90 percent HRR. Lactate threshold, race pace, sustainable hard.
- Zone 5 90 to 100 percent HRR. VO2 max, HIIT, very short intervals.
Most endurance plans concentrate volume in Zone 2 and intensity in Zone 4. A common rule for runners and cyclists is 80 percent of time in Zones 1 and 2, 20 percent in Zones 4 and 5, with Zone 3 used sparingly. Spending too much time in Zone 3 stresses the body without producing the adaptations of either extreme, a phenomenon coaches call the gray zone.
How to use the Karvonen calculator
Enter your age. The calculator will set max HR to 220 minus age. Then type your resting heart rate. The widget computes HRR and prints all five zones with their bpm ranges. Switch to manual mode if you have a treadmill-tested max HR from a lab or sports clinic; that figure is the single biggest accuracy upgrade you can give the formula.
The two stat cards beneath the zone list pull out the Zone 2 fat-burn band and the Zone 4 threshold band, which are the targets most commonly written into training plans.
Recheck your resting HR every few months. As fitness improves, RHR drops, sometimes by 5 to 10 bpm in the first season of structured training. Your Karvonen zones shift with it.
Karvonen vs. percent of max HR
Percent of max HR ignores resting heart rate. A 70 percent of max HR target reads the same for everyone of a given age. Karvonen ties the bottom of the formula to a real, measured number and produces a target that scales with fitness. The two methods agree closely at high intensities and diverge at low ones, where percent-of-max under-targets unfit users.
For the same 35 year old with max HR 185 and RHR 65, a 70 percent max HR target is 130 bpm, while a 70 percent HRR target is 149 bpm. The Karvonen number is the one that produces a tempo-pace heart rate. The lower percent-of-max number sits closer to easy aerobic, not threshold.
Karvonen and resting heart rate
Resting heart rate is measured first thing in the morning, before standing up or drinking coffee. A 60-second count over the carotid is fine; a chest strap is better. Normal range for adults is 60 to 100 bpm. Endurance-trained athletes routinely sit between 40 and 60, and a few elite marathoners measure below 35.
Day-to-day variability is normal: an extra glass of wine, poor sleep, low-grade infection, or training residue all raise RHR by 5 to 10 bpm. A morning reading 10 bpm above baseline is a coach's classic flag for over-reaching.
Beta-blockers, calcium channel blockers, and some antiarrhythmics suppress heart rate response. Anyone with cardiovascular disease, on those medications, or new to vigorous training should clear Zone 4 and Zone 5 work with a physician first.
Common Karvonen mistakes
The first mistake is plugging in a measured resting HR taken after coffee, after the gym, or after a stressful meeting. Anything that elevates baseline shrinks HRR and produces underestimated zones. The second is forgetting that 220 minus age has plus-or-minus 12 bpm built in. A 50 year old whose true max HR is 180 will get pushed too hard by a calculator that assumes 170.
The third is treating zones as hard walls. They are guides. A run that drifts 3 bpm above Zone 2 is still a Zone 2 run. Athletes who chase tight numbers tend to over-correct, slow down, lose effort, and miss the training stimulus the zone was meant to deliver.