Heart Rate Zone Calculator
Compute a target training heart rate using a Karvonen-style formula with resting heart rate, maximum heart rate estimated as 220 minus age, and a combined intensity factor (training zone × fitness level). Returns bpm.
Last updated: May 2026
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About this calculator
The calculator uses Target HR = restingHR + (HRmax − restingHR) × trainingZone × fitnessLevel, where HRmax is approximated as 220 − age. This is a variant of the Karvonen formula, which normally uses Target HR = restingHR + heart-rate reserve × intensity with intensity as a single number between 0.5 and 1.0. Here the calculator multiplies two intensity factors (trainingZone option 0.6–0.9 and fitnessLevel option 0.5–0.85), giving a combined effective intensity ranging from 0.30 (low fitness × low zone) to 0.77 (high fitness × high zone). Edge cases: the 220 − age formula for HRmax has substantial individual variability (standard deviation ~10–12 bpm), so for serious training a measured HRmax (graded exercise test or peak from hard intervals) is preferred. The dual-multiplier design here departs from standard Karvonen — most resources interpret 'zone' alone as the intensity factor, so multiplying by fitnessLevel reduces the target to often quite low values. For example, a 35-year-old with restingHR 65, fitnessLevel 0.6, zone 0.7 gets target = 65 + (185-65) × 0.7 × 0.6 = 65 + 50.4 = 115 bpm — well below the standard Karvonen 70% interpretation which gives 65 + 120 × 0.7 = 149 bpm. Verify against your coach's intended interpretation before using these numbers for prescribed workouts.
How to use
Example 1 — moderate-intensity workout, recreational athlete. Age 35, restingHR 65 bpm, fitnessLevel 0.6 (intermediate), trainingZone 0.7 (moderate). Step 1: HRmax = 220 − 35 = 185 bpm. Step 2: HRR (reserve) = 185 − 65 = 120 bpm. Step 3: combined intensity = 0.7 × 0.6 = 0.42. Step 4: target HR = 65 + 120 × 0.42 = 65 + 50.4 ≈ 115 bpm. Verify: 115 bpm is on the low side for 'zone 2' aerobic work for a 35-year-old (standard Karvonen at 70% gives ~149 bpm). The dual-multiplier formula here produces lower targets than typical Karvonen interpretation; treat the number as conservative starting guidance rather than an exact prescription ✓. Example 2 — vigorous interval workout, well-trained athlete. Age 28, restingHR 50 bpm, fitnessLevel 0.85 (advanced), trainingZone 0.9 (high). Step 1: HRmax = 220 − 28 = 192 bpm. Step 2: HRR = 192 − 50 = 142 bpm. Step 3: combined intensity = 0.9 × 0.85 = 0.765. Step 4: target HR = 50 + 142 × 0.765 ≈ 50 + 108.6 ≈ 159 bpm. Verify: standard Karvonen at 90% would give 50 + 142 × 0.9 ≈ 178 bpm; this dual-multiplier result of 159 bpm corresponds to roughly 77% of HRR, more like high zone 3 / threshold than true VO₂max zone 5 work ✓. Compare to your own observed performance HR at similar perceived effort.
Frequently asked questions
What is the Karvonen formula and how does this calculator differ?
The standard Karvonen formula (1957) is Target HR = restingHR + (HRmax − restingHR) × intensity, where intensity is a single decimal between 0.5 and 1.0 representing the percentage of heart-rate reserve (HRR) you want to train at. It uses HRR rather than HRmax alone because it accounts for individual fitness — a fit person with low resting HR has a larger reserve and proportional intensity targets work better than percentage-of-HRmax alone. This calculator's twist is multiplying two intensity inputs (trainingZone and fitnessLevel), which produces lower effective intensities than the standard interpretation. To match standard Karvonen, leave fitnessLevel at 1.0 (or treat trainingZone × fitnessLevel as your true intended intensity). The 220 − age estimate of HRmax is itself a rough approximation; alternatives like Tanaka (208 − 0.7 × age) and Gulati (women: 206 − 0.88 × age) give slightly different estimates, especially for older athletes where 220 − age systematically under-estimates HRmax.
How accurate is the 220 − age estimate of HRmax?
It's a population-average estimate with substantial individual variability — standard deviation is about 10–12 bpm, meaning roughly 1 in 3 people's true HRmax differs from 220 − age by more than 10 bpm in either direction. The formula also has systematic biases: it over-estimates HRmax in young adults under 25 (true HRmax often closer to 195 than 200) and under-estimates in older adults over 60 (true HRmax often higher than 160 for a 60-year-old). More accurate formulas include Tanaka (208 − 0.7 × age, lower bias overall), Gulati for women (206 − 0.88 × age), and HUNT (211 − 0.64 × age). For serious training prescription, a graded exercise test on a treadmill or bike with ECG and gas exchange gives the gold-standard measurement. Practical alternatives: do a hard 5-minute all-out effort at the end of a maximal interval workout — the peak HR you achieve approximates your true HRmax within a few bpm. Update your zones whenever you measure significantly different values.
How do heart-rate zones relate to perceived exertion and training adaptations?
Zones are conventionally numbered 1–5 (sometimes 1–7) corresponding to different intensities and physiological adaptations. Zone 1 (50–60% HRR): very easy, recovery, fat metabolism. Zone 2 (60–70% HRR): aerobic base, builds mitochondrial density and capillarisation — most coaches recommend ≥80% of training time here for long-term endurance. Zone 3 (70–80% HRR): aerobic threshold, builds lactate clearance. Zone 4 (80–90% HRR): lactate threshold, the highest sustainable intensity for 30–60 minutes — improves race-pace economy. Zone 5 (90–100% HRR): VO₂max intervals, 3–5 minute efforts, maximises aerobic ceiling. Polarised training (mostly zone 2 plus some zone 4–5, very little zone 3) is supported by research for endurance athletes. Perceived exertion correlates with zones: zone 2 should feel 'easy enough to nose-breathe and converse'; zone 4 'hard, can speak only short phrases'; zone 5 'maximal, can't talk'. Use HR and RPE together — both are subject to noise from heat, dehydration, sleep, illness.
What are the common mistakes when using heart-rate training zones?
The biggest mistake is treating 220 − age as your true HRmax and not adjusting after measuring otherwise — many athletes' real HRmax differs by 15–20 bpm, making all subsequent zones wrong. The second is using HRmax-percentage zones (e.g., '80% of HRmax') instead of HRR-percentage zones (Karvonen) — these give different targets, especially for fit people with low resting HR. The third is ignoring heart-rate variability factors: heat, dehydration, caffeine, illness, and overtraining can all raise your HR for a given workload, making prescribed zones impossible to hit; back off on intensity rather than push through. People also forget that beta-blockers, thyroid medication, and other drugs alter HR response, sometimes by 20–40 bpm. New HR-monitor users often see noise from chest-strap interference or wrist-optical errors and conclude they're 'in the wrong zone' when the monitor is just inaccurate. Finally, religiously training only by HR ignores RPE and pace, which together give a more complete picture and protect against the inevitable bad-HR days.
When should I not use this calculator?
Do not use it for medical heart-rate targets or stress testing — those use graded exercise protocols with continuous ECG and physician supervision, not simple formulas. It is not appropriate for cardiac patients, people on beta-blockers or other HR-altering medications, or anyone with arrhythmia — clinical exercise prescription requires professional input. Do not use it for athletes whose true HRmax has been measured and differs significantly from 220 − age; use measured HRmax in the Karvonen formula directly. The dual-intensity multiplier design produces lower targets than standard Karvonen interpretation, so do not use this calculator's output for prescribed workouts without cross-checking against your coach's intended interpretation. It is unsuitable for swimming, where HRmax is typically 10–15 bpm lower than running due to water cooling and posture effects; use a swim-specific HRmax. Avoid relying on a single zone calculation as you age or change fitness levels; re-test HRmax annually or after major fitness changes. For pacing, RPE and pace (or power) typically integrate better with cardiac drift than pure HR-based zones.