What is Zone 2?
Zone 2 is steady-state aerobic exercise held at a heart rate low enough that you can hold a conversation but high enough to build the aerobic engine that endurance sports run on. Physiologically, it sits below your first ventilatory threshold - the effort where breathing first has to speed up faster than the exercise intensity. Zone 2 training grows mitochondrial density, capillary networks, and fat-burning capacity. For a mountain athlete, that translates to more hours of climbing you can sustain before the engine runs out of fuel.
How to calculate Zone 2 heart rate: three methods
There are three standard ways to calculate Zone 2 heart rate. They differ in how personal - and therefore how accurate - the number is.
1. Simple (age-based). Max heart rate is estimated as 220 minus your age. Zone 2 is 60 to 70 percent of that estimated max. For a 35 year old, max is 185, Zone 2 lands roughly at 111 to 130 bpm. Easy to use, weakest accuracy: the formula's error can be 20 to 30 bpm for some individuals. Fine as a first pass; not what you plan a training block around.
2. Karvonen (heart rate reserve). Uses the difference between your maximum and resting heart rates. Zone 2 = ((max HR - rest HR) x intensity) + rest HR, with intensity at 60 to 70 percent. This accounts for individual differences in resting heart rate (which reflects aerobic fitness) and gives a more personal target. Still limited by the age-based max HR estimate.
3. LTHR-based (Friel five-zone). The most accurate for endurance athletes. Your lactate threshold heart rate is measured directly from a field test - not estimated. Zone 2 is 85 to 89 percent of LTHR (Friel). Because LTHR is a real physiological anchor, LTHR-based zones can be 10 to 20 bpm different from the age-formula estimate for the same person. This is the method the training-science literature uses when accuracy matters.
How to calculate Zone 2 cardio: what the numbers mean
"Zone 2 cardio" and "Zone 2 heart rate" describe the same thing. When you calculate Zone 2 cardio, you are setting the heart rate range within which the majority of your aerobic sessions should sit. Whether you use the age formula or LTHR, the target is the same: sustained steady effort where you build the engine without accumulating fatigue. The range is not narrow - Zone 2 is a band, not a point. If your session drifts up to the top of the range on climbs and settles back down on flats, that is normal and correct.
Zone 2 heart rate range: reading the output
The calculator above returns your Zone 2 heart rate range as a bpm band with a lower and upper bound. Interpret it as follows.
- Lower bound: the floor. Effort below this is easier than Zone 2 (recovery pace or slower).
- Upper bound: the ceiling. Effort above this pushes you into Zone 3 (tempo), which trains a different system and accumulates more fatigue.
- Sitting in the middle: the sweet spot for building the aerobic engine.
- Drifting up on climbs: expected. Do not try to keep heart rate perfectly still on undulating terrain. Manage effort, not bpm.
Why LTHR-based Zone 2 is more accurate than 220 minus age
The 220 minus age formula was designed as a rough population estimate. Its standard deviation is about 10 to 12 bpm, which means your true maximum heart rate can be 20 to 30 bpm above or below the estimate. Trained endurance athletes are usually higher; less-active adults usually lower. If you plan a training block around the age formula, you might spend months training at the wrong intensity without knowing it - either too easy to build fitness or hard enough to drift into Zone 3 and blunt aerobic adaptations.
Lactate threshold heart rate is different because it is measured from your own physiology, not estimated. It reflects the specific point where lactate production exceeds clearance in your body, and Zone 2 sits at 85 to 89 percent of that anchor. Two athletes with the same age can have LTHRs 20 bpm apart, and both would train at the wrong Zone 2 if they used the age formula alone.
How to find your LTHR: the 30-minute test
To find your lactate threshold heart rate, perform a 30-minute individual time trial at the hardest effort you can sustain for the full duration. Record heart rate throughout. Take your average heart rate over the last 20 minutes of the test - that is your LTHR estimate. The test works on a treadmill, in the field, or on a bike (each modality has its own LTHR). Repeat every 8 to 12 weeks as fitness changes, and every time you enter a new training phase.
Once you have LTHR, plug it into the calculator above (LTHR method) and the tool returns your Zone 2 band at 85 to 89 percent of that value.
Zone 2 for mountaineering and long mountain days
Zone 2 is the aerobic foundation of every mountaineering plan. A summit day is 8 to 15 hours of Zone-2-to-Zone-3 effort, and the size of your aerobic engine is what decides how much of it you finish with reserves for the descent. Most mountaineering training programmes recommend 3 to 5 Zone 2 sessions per week of 60 to 180 minutes each, with the long weekend session progressing toward the duration of your goal peak's summit day.
If you want the deeper method behind this - polarised distribution, weekly load, and how it connects to summit readiness - read the heart rate zones for mountaineering guide. For where Zone 2 fits in a complete training week, see Zone 2 training for mountaineering. And to test how your current fitness stacks up against your objective, run the mountaineering readiness calculator.
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Common questions
How do you calculate Zone 2 heart rate?
The accurate way is LTHR-based: Zone 2 is 85 to 89 percent of your lactate threshold heart rate (Friel five-zone model), measured from a 30-minute field test. This is the method the endurance-training literature uses when precision matters, and it is what this calculator defaults to. There are two quicker but weaker estimates: Karvonen, which uses heart rate reserve for a more personal target, and the simple age-based formula (60 to 70 percent of 220 minus age), which is fine as a first pass but carries a 10 to 20 bpm error.
How do I find my Zone 2 heart rate?
Enter your age into the calculator above for a starting estimate. For the accurate answer, do a 30-minute time trial at the hardest effort you can sustain for the full duration, record your average heart rate over the last 20 minutes - that is your LTHR - then take 85 to 89 percent of it. That is your Zone 2 heart rate range.
What is the Zone 2 heart rate range for my age?
On the age-based formula, Zone 2 is 60 to 70 percent of (220 minus age). At 25 that is roughly 117-137 bpm; at 35 it is 111-130 bpm; at 45 it is 105-123 bpm; at 55 it is 99-116 bpm. These are estimates - the age formula's standard deviation is 10-12 bpm, so an LTHR test can shift your true range by 15-20 bpm. Use the age numbers as a first pass, then confirm with LTHR.
What is Zone 2 cardio?
Zone 2 cardio is steady-state aerobic exercise held at a heart rate low enough to hold a conversation but high enough to build the aerobic engine. It sits below the first ventilatory threshold. It builds mitochondrial density, capillary networks, and fat-burning capacity - the foundation of long-duration performance from marathon running to a 12-hour mountain summit day.
How long should Zone 2 sessions be?
For endurance and mountain athletes, 60 to 180 minutes. A common weekly pattern is 3 to 5 Zone 2 sessions per week, ranging from 60 to 120 minutes each, with one longer session at the weekend that progresses toward the duration of your goal event. The Seiler polarised model has roughly 80 percent of training time in low intensity (Zone 2 and below), which is why the volume matters.
Is 220 minus age accurate for Zone 2?
Only as a rough estimate. The formula has a standard deviation of 10-12 bpm, so your true max heart rate can be 20-30 bpm off. Trained endurance athletes usually run higher than the formula predicts; sedentary adults often lower. For a rough Zone 2 target it is fine; for a plan you follow for months, do a 30-minute LTHR field test and use the LTHR-based method instead.
Does Zone 2 work at altitude?
Absolute heart rate numbers shift at altitude - resting heart rate rises, maximum heart rate drops slightly, and the same perceived effort corresponds to a lower absolute bpm than at sea level. A sea-level LTHR calculation will overshoot your target at altitude. At altitude, guide effort by perceived exertion ("talk pace") or re-test LTHR at training altitude if you will be there long enough to make it worthwhile.