Zone 2 is the top of your aerobic base: the hardest steady effort you can hold while your body is still fueled mainly by fat. It is the most valuable training intensity for almost every endurance and mountain athlete, and it is the one most people get wrong. The reason is simple. Most Zone 2 training advice starts from a formula (220 minus your age) instead of being based on your physiology. This guide explains what Zone 2 really is, why the standard formula misleads you, and how to find your true Zone 2 with a field test you can do today.
One of the smartest things you can do when you begin endurance training is determine your training zones. This handy calculator is a great starting point.
What is Zone 2 heart rate training?
Zone 2 is low-to-moderate aerobic exercise at an intensity your body fuels primarily by burning fat, the band of effort that sits just below your aerobic threshold (AeT). At this intensity you are working, but you could hold a conversation in full sentences and sustain the effort for an hour or more. It is steady, controlled, and repeatable day after day.
Training in Zone 2 develops the engine that powers all endurance performance by increasing the number and efficiency of your mitochondria (the structures in muscle cells that produce energy with oxygen), building more of the small blood vessels that deliver that oxygen to your working muscles, and trains your body to spare carbohydrate by burning more fat at any given pace. These are the adaptations that let you go longer, recover faster, and absorb the harder work that builds race fitness.
The popular world calls this “Zone 2.” We have long called it aerobic base training. The idea is the same. What matters is that you understand that a useful Zone 2 is defined by your metabolism right now, not by a percentage of a number you guessed from your age.
Want to stop guessing? A coach and a structured plan take the uncertainty out of finding and training your zones. Explore Training Groups, where your personalized dashboard reads your aerobic threshold for you week to week, automatically. Personal Coaching is best for those wanting one-to-one guidance.
Heart rate zones, anchored to your thresholds
To keep it simple, we use a four-zone system. What makes it accurate is that it is anchored to two measured points in your own physiology: your aerobic threshold (AeT) and your anaerobic threshold (AnT, also called lactate threshold).
Defining the Heart Rate Zones
By anchoring this system to two important metabolic markers (AeT and AnT/LT), this simple 4-zone system does a very good job of personalizing intensities to your current metabolic response. RPE stands for Rating of Perceived Exertion, which is fancy lingo for how hard you think you’re exercising on a scale of 1-10.
Zone 1 / RPE 2-4
Heart Rate: AeT-20% to AeT-10%
Rate of Perceived Exertion (RPE): 2-4, very easy to easy
Training Effect/Purpose: Recovery
Metabolism: Aerobic-fat
Muscle Fiber Recruitment: ST
Training Method: Continuous 30 min to several hoursThink walk/jog pace. You could tell a whole story without pausing for breath. Breathing is comfortable and constricted.
Zone 2 / RPE 5-6
Heart Rate: AeT-10% to AeT
Rate of Perceived Exertion (RPE): 5-6, moderate for those with high AeT, easy for those with low AeT
Training Effect/Purpose: Aerobic capacity, economy
Metabolism: Aerobic-fat dominates, maximum fat utilization
Muscle Fiber Recruitment: Most ST
Training Method: 30 minutes-several hoursYou can hold a conversation but would need to take a breath every four or five sentences. This is the zone where most base training lives.
Zone 3 / RPE 7-8
Heart Rate: AeT to Lactate Threshold
Rate of Perceived Exertion (RPE): 7-8 Medium to comfortably hard
Training Effect/Purpose: Aerobic capacity, anaerobic capacity, lactate shuttle, economy
Metabolism: Glycolytic/anaerobic begins to dominate
Muscle Fiber Recruitment: All ST + some FT
Training Method: Interval 10–20 min, continuous to 60 minThis is hard work. Talk test gets you about three or four words before you need a breath. The upper end of this zone is your anaerobic threshold, and it is an effort you could sustain for a maximum of about an hour.
Zone 4 / RPE 9-10
Heart Rate: Lactate Threshold to LT to maxHR
Rate of Perceived Exertion (RPE): 9-10 Hard, max sustainable
Training Effect/Purpose: Maximal aerobic power, strength/speed endurance, economy, technique
Metabolism: Both aerobic and anaerobic capacities maxed out
Muscle Fiber Recruitment: All ST + most FT
Training Method: Interval 30 sec–8 minIf you can only get out one four-letter word, you are here. An effort sustainable for no more than a few minutes at the upper end.
Reading the table you can see that your zones are built up and down from your aerobic threshold. Find your AeT accurately and every zone falls into place. Get it wrong, and every zone inherits the error.
How to calculate Zone 2 heart rate
The method you will see almost everywhere has two steps. First, estimate maximum heart rate as 220 minus your age. Second, take 60 to 70 percent of that number and call it Zone 2. For a 40-year-old that produces a maximum of 180 and a Zone 2 of roughly 108 to 126 beats per minute.
The problem is in the first step. Maximum heart rate varies widely between people of the same age, commonly by 10 to 20 beats per minute and sometimes more, so two healthy 40-year-olds can have true maximums 30-plus beats apart. Any error in your estimated maximum is then carried into your Zone 2 range and often widened by the percentage step.
There is a deeper issue. Zone 2 is a metabolic state, not a heart rate. Zone 2 is the point below which your body is still mostly burning fat and clearing lactate as fast as it produces it. That crossover happens at a zone of intensity specific to you and to your current fitness. A population-average formula cannot find it reliably, but a simple test on your own body can.
Use the formula if you need a number today. Then replace it with your real one as soon as you can.
The Heart Rate Drift Test
The heart rate drift test is the most reliable self-test for athletes. The principle is straightforward: hold a steady, easy pace for a sustained effort of around an hour and watch what your heart rate does relative to that pace.
If your heart rate stays nearly flat, drifting less than about 5 percent across the effort, you were working below your aerobic threshold. If it climbs more than that while your pace holds steady, you were above it. By repeating the test at slightly different efforts you can locate the line precisely. The full protocol, including how to set it up and read the result, is in our guide to the heart rate drift test.
Continuous AeT Monitoring
Here is how that weekly reading works, and why we built it the way we did. Every test on this page gives you a snapshot: a number that holds for six to ten weeks while your fitness keeps moving in between. Continuous AeT closes that gap. It refreshes the top of your Zone 2 each week from the ordinary training you already do, so your ceiling tracks your fitness instead of waiting for the next test day.
It works on the same principle as the heart rate drift test. Across your steady aerobic runs, it follows the relationship between your pace and your heart rate. When your heart rate holds flat at a given pace, you were below your aerobic threshold. When it climbs while your pace stays the same, you were above it. Reading this from real training, rather than from a single controlled test, means accounting for the things that distort everyday data: heat that lifts heart rate no matter how easy the effort, hills that change the pace-to-heart-rate relationship, and short runs that end before heart rate has settled. What the dashboard publishes each week is one aerobic threshold reading, drawn from your own runs rather than from a formula or a single day’s test.
We built it inside Training Groups first, and on purpose. Training Groups are where a new reading meets a wide range of real athletes on real terrain in real weather, well before it reaches our one-to-one coached athletes. Recently one athlete’s reading came back lower than the easy pace he had trusted for years, and he said so. We scored his data by hand, the way a coach would, and his instinct held up: his summer reading had been pulled down by heat and hilly terrain, not by lost fitness. That kind of honest, messy, real-world data is what shows us where the method holds and where it needs to be sharper, and the athlete who pushes back is the engine of that work rather than a complication in it.
That points to the rule at the center of how Continuous AeT works: it proposes a number, but the final word belongs to you, whether you are reading your own dashboard or going over it with a coach. The felt sense that tells you an easy pace is easy is data too, and the reading is built to make room for it rather than to overrule it. When the number looks wrong, you say so, and a manual drift test settles it.
Used this way, Continuous AeT keeps the work in this guide current. Your Zone 2 ceiling stops drifting out of date between tests, your easy days stay genuinely easy, and the patient base work that raises your aerobic threshold shows up where you can see it, as a number that climbs week to week.
Why is my Zone 2 pace so slow?
This is the most common frustration with Zone 2, and the answer is more useful than “be patient.”
If your heart rate shoots into Zone 3 or 4 at what feels like an easy jog, or your Zone 2 pace is so slow you have to walk, your aerobic system is underdeveloped relative to your anaerobic system. We call this Aerobic Deficiency. It is common, it is diagnosable, and it is fixable.
Here is how to recognize it. In a well-developed athlete, the aerobic threshold (AeT) sits close to the anaerobic threshold (AnT), within about 10 percent. When the gap is larger than that, when your AeT is more than 10 percent below your AnT, your body shifts to burning sugar and producing lactate at a low heart rate, which is why easy running feels hard and your sustainable easy pace is slow. This is the “10 percent test.”
Aerobic Deficiency usually develops from training too often in the moderate-to-hard middle, the “grey zone,” and too rarely in the genuinely easy aerobic range. The fix is not more intensity. It is a block of patient aerobic base work, most of it at or just below your true AeT, until the aerobic threshold climbs back toward the anaerobic one. Pace at a given heart rate improves over weeks and months, and the slow easy running that frustrated you becomes noticeably faster at the same effort.
Athletes with significant Aerobic Deficiency can often talk, and feel fine, well above their true aerobic threshold, so effort-based checks read high for them. The heart rate drift test or the Continuous AeT monitoring give them a more reliable number.
How long should you train in Zone 2?
Most of your weekly endurance training should be genuinely easy aerobic work in Zone 1 and Zone 2. For most athletes building a base, that means the majority of training time spent below the aerobic threshold, with sessions long enough to matter (commonly 45 minutes and up, building toward the long sessions your goal demands).
Two principles matter more than any exact percentage. The first is consistency: the aerobic adaptations come from repeated, frequent, sustainable sessions over months, not from a handful of heroic efforts. The second is restraint: the value of Zone 2 comes from keeping it easy, which means letting your pace be slow when your heart rate says so, especially early on.
Is Zone 2 enough?
Zone 2 is the foundation of endurance fitness. It is not the whole house.
A complete program is built mostly on easy aerobic work, with a smaller, well-timed amount of higher-intensity training added once the base can support it. The mistake to avoid is the opposite of too much intensity: it is living in Zone 3, the “grey zone,” where the effort is too hard to recover from easily and too easy to drive the top-end adaptations of true hard work. Much of the popular “Zone 2 versus high intensity” debate misses this. The question is not which single zone is best. It is building a deep aerobic base first, then layering intensity onto it deliberately.
A practical signal for when to add intensity is the 10 percent test described above. When your aerobic threshold has climbed to within about 10 percent of your anaerobic threshold, your base is ready to support harder work.
How do you know your Zone 2 is improving?
You will see it in the numbers before you feel it in a race. As your aerobic base develops, your pace at a given heart rate increases: you run, ride, or climb faster at the same easy effort. Your heart rate drift at a fixed easy pace shrinks. And your tested aerobic threshold rises over time. Retest with the field test or drift test every 6 to 10 weeks and update your zones, because training to an old, lower AeT holds you back once you have outgrown it.
For athletes who would rather not retest by hand, our Training Groups dashboard includes Continuous AeT, so your Zone 2 ceiling updates as your fitness changes instead of drifting out of date between manual tests.
Zone 2 for mountain and endurance athletes
For a runner chasing a personal best, a deep aerobic base means a faster sustainable pace. For a mountain athlete, it is more fundamental still. Long days in the mountains, big vertical gain, and time at altitude are aerobic events first. The athlete with the larger, more efficient aerobic engine moves faster for longer, fatigues later, recovers sooner, and copes better with thin air. Building Zone 2 is not a wellness sidebar for this audience. It is the literal foundation of mountain performance, which is why every one of our training plans starts by building it.
Frequently asked questions
What is Zone 2 heart rate training? Zone 2 is steady aerobic exercise at the top of your fat-burning range, just below your aerobic threshold. You can hold a full conversation and sustain the effort for an hour or more. It builds the mitochondrial and cardiovascular base that underpins all endurance performance.
How do I calculate my Zone 2 heart rate? A common starting estimate is 60 to 70 percent of your maximum heart rate, with maximum estimated as 220 minus your age. This is only a rough guide, because true maximum heart rate varies widely at any age. A field test of your own aerobic threshold gives a far more accurate Zone 2.
Why is my heart rate so high, or my pace so slow, in Zone 2? Most often because your aerobic system is underdeveloped relative to your anaerobic system, a fixable condition called Aerobic Deficiency. A block of patient aerobic base work at or just below your true aerobic threshold raises your easy pace over weeks and months.
How do I find my Zone 2 without a lab test? Use the heart rate drift test. Hold a steady, easy pace for about an hour and watch your heart rate. If it drifts less than about 5 percent while your pace holds steady, you were below your aerobic threshold, the top of Zone 2. If it climbs more than that, you were above it. Repeating the test at slightly different efforts lets you locate the line precisely.
How long should I train in Zone 2 each week? Most of your endurance training should be easy aerobic work, in sustainable sessions repeated consistently over months. Consistency and restraint matter more than hitting an exact weekly percentage.
Is Zone 2 training enough on its own? It is the foundation, not the entire program. Build a deep aerobic base first, then add a smaller, well-timed amount of higher-intensity work once your base can support it.
How do I know my Zone 2 is improving? Your pace at a given easy heart rate increases, your heart rate drift shrinks, and your tested aerobic threshold rises. Retest every 6 to 10 weeks and update your zones. If you train in one of our Training Groups, Continuous AeT does this reading for you every week, so your zones stay current without a manual retest.