Health & Medicine · Fitness · Cardio & Endurance
Heart Rate Reserve Calculator
Calculate Heart Rate Reserve (HRR) and target training zones using the Karvonen method based on age, resting heart rate, and intensity.
Calculator
Formula
THR is target heart rate in bpm. HR_max is maximum heart rate (220 minus age). HR_rest is resting heart rate in bpm. Intensity is the desired training intensity expressed as a decimal (e.g. 0.70 for 70%). HRR itself equals HR_max minus HR_rest.
Source: Karvonen M, Kentala E, Mustala O. The effects of training on heart rate. Ann Med Exp Biol Fenn. 1957;35(3):307-315.
How it works
Heart Rate Reserve is defined as the difference between your estimated maximum heart rate (220 minus your age) and your resting heart rate. This reserve represents the total working range of your heart during exercise. The Karvonen formula then applies a desired intensity percentage to this reserve and adds back the resting heart rate to find a true, individualised target: THR = ((HR_max - HR_rest) x Intensity) + HR_rest.
Because two people of the same age can have very different resting heart rates, this method yields more accurate training zones than simply taking a flat percentage of maximum heart rate. An athlete with a resting HR of 45 bpm and a sedentary individual with 80 bpm will need to exercise at very different absolute heart rates to achieve the same physiological stimulus.
Applications include programming cardio intervals, monitoring recovery runs, pacing marathon efforts, designing cardiac rehab protocols, and calibrating wearable device alerts. The five standard zones produced by this calculator map to active recovery, aerobic base building, aerobic endurance, lactate threshold work, and VO2 max training respectively.
Worked example
Example: 35-year-old runner with a resting HR of 58 bpm targeting 70–80% intensity.
Step 1 — Estimate maximum HR: 220 − 35 = 185 bpm.
Step 2 — Calculate HRR: 185 − 58 = 127 bpm.
Step 3 — Lower target (70%): (127 × 0.70) + 58 = 88.9 + 58 = 147 bpm.
Step 4 — Upper target (80%): (127 × 0.80) + 58 = 101.6 + 58 = 160 bpm.
This runner should aim to keep their heart rate between 147 and 160 bpm to train in the aerobic endurance zone. Using a flat 70–80% of max method would give 130–148 bpm — a meaningfully lower and less accurate range for someone with a low resting heart rate.
Limitations & notes
The maximum heart rate formula 220 − age is a population average with a standard deviation of roughly ±10–12 bpm; highly trained athletes and older adults often deviate significantly. For the most accurate results, a graded exercise test or field test should be used to determine true HRmax. Resting heart rate should be measured first thing in the morning after waking, before getting out of bed, averaged over several days. This calculator does not account for medications such as beta-blockers that suppress heart rate, medical conditions affecting cardiac output, altitude, heat, or hydration status — all of which can alter the heart's response to exercise. Individuals with heart disease or other serious medical conditions should consult a physician before using heart rate targets for exercise programming.
Frequently asked questions
What is a good Heart Rate Reserve number?
There is no single 'good' HRR value, but higher HRR generally indicates better cardiovascular fitness. Elite endurance athletes may have an HRR above 150 bpm (e.g. max HR 185, resting HR 35), while a sedentary adult might have an HRR around 90 bpm. An increasing HRR over time is a positive sign that aerobic conditioning is improving.
Why is the Karvonen method better than using a flat percentage of max HR?
A flat percentage (e.g. 70% of 185 bpm = 130 bpm) ignores resting heart rate entirely. Because resting HR reflects your baseline cardiovascular function, subtracting it before applying the intensity percentage ensures the zone is anchored to your actual working range. Two people with the same max HR but different resting HRs will train at appropriately different targets using the Karvonen method.
How do I measure my resting heart rate accurately?
Measure your resting heart rate first thing in the morning before you get out of bed, after at least 5 minutes of quiet lying still. Count your pulse for 60 seconds (or 30 seconds and multiply by 2). Repeat this on 3–5 consecutive days and use the average. Avoid caffeine, alcohol, stress, or vigorous activity in the hours before measurement. Many modern smartwatches can also provide an overnight resting HR average.
What training zone should beginners focus on?
Beginners should focus primarily on Zone 2 (60–70% HRR), the aerobic base zone. Training here builds mitochondrial density, improves fat oxidation, strengthens the cardiovascular system, and is sustainable for long durations without excessive fatigue or injury risk. Research from Stephen Seiler and others suggests that endurance athletes performing the most total training volume do so predominantly in this lower-intensity zone.
Can I use this calculator for cardiac rehabilitation?
The Karvonen method is widely used in cardiac rehabilitation programs, and clinicians often prescribe exercise at 40–60% HRR for deconditioned or post-cardiac event patients. However, cardiac rehab participants should always have their target heart rate ranges set and monitored by a qualified healthcare provider. This calculator provides a general estimate and is not a substitute for medically supervised exercise prescription.
What if my resting heart rate changes over time as I get fitter?
You should recalculate your HRR and training zones whenever your resting heart rate changes significantly (typically a drop of 5 bpm or more). As aerobic fitness improves, resting HR often decreases, which increases your HRR and means your old zone targets may now be set too low. Reassessing every 4–8 weeks during structured training is good practice.
Does age-predicted max HR (220 minus age) apply to everyone?
No. The 220 − age formula was derived from a meta-analysis of many subjects and represents a statistical average with substantial individual variation (standard deviation of about 10–12 bpm). Older adults, highly trained athletes, and people with certain medical conditions often have maximum heart rates well above or below the prediction. For precision training, a maximal exercise test performed under supervision provides a much more accurate HRmax.
Last updated: 2025-01-30 · Formula verified against primary sources.