The five zones
Each zone has a specific physiological purpose. Training in the right zone at the right time is how structured endurance training works.
Zone 1 sits at 50–60% of your heart rate reserve. Your body is operating well below the aerobic threshold, running almost entirely on fat as fuel, with minimal cardiovascular stress. Lactate stays very low.
The primary use of Zone 1 is active recovery: warm-ups, cool-downs, easy movement between hard sessions. Many runners skip Zone 1 entirely and treat Zone 2 as their easy zone. Both have a place. Zone 1 is for days when the goal is restoration, not adaptation.
Zone 2 runs from 60–70% of heart rate reserve. This is the most important training zone for endurance runners. It sits just below the aerobic threshold, where fat oxidation is maximized and the slow-twitch muscle fibers that carry you through long efforts are being specifically developed.
The adaptations from Zone 2 training are foundational: mitochondrial density increases, cardiac stroke volume improves, fat metabolism becomes more efficient.[2] These changes take weeks to build but they are the engine everything else depends on. 75–80% of your total training volume should be here.[1]
Zone 3 covers 70–80% of heart rate reserve. It sits between the aerobic threshold and the anaerobic threshold. This is the most commonly used zone among recreational runners. It is also the zone most responsible for stalled progress.[3]
In Zone 3, you are working hard enough to accumulate significant fatigue. But the intensity is not high enough to drive the threshold and VO2Max adaptations that hard training sessions target. You pay the cost of intensity without getting the return. The result is tired legs and slow improvement.
The grey zone problem
Most recreational runners spend the majority of their training time in Zone 3. It feels productive. It is not. Too hard to build aerobic base. Not hard enough to develop threshold capacity. If your easy days leave you tired the next morning, you are running them in Zone 3.
Zone 4 is 80–90% of heart rate reserve. This is the lactate threshold zone: the upper limit of sustainable aerobic effort.[4] At this intensity, lactate production starts to rise noticeably. A well-trained runner can hold Zone 4 for 20–40 minutes continuously. An untrained runner may only sustain it for 8–12 minutes.
Zone 4 training raises your anaerobic threshold over time. This is step 3 in the Run Mastery framework: you build it on top of the aerobic base, not before it. Threshold work on an undeveloped base produces minimal adaptation and significant fatigue.
Zone 5 is 90–100% of heart rate reserve. Maximum effort. Your aerobic system is operating at or near its ceiling, and you are producing lactate faster than you can clear it. Efforts in Zone 5 are short: intervals of 2–8 minutes are typical. Race finishes and hill sprints pull you here.
Zone 5 is step 4 in the framework. It only delivers meaningful adaptation when Zone 2 base and Zone 4 threshold capacity are already established. Jumping to VO2Max work without the foundation mostly produces injury and fatigue.
The intensity map
The five zones form a continuous spectrum from recovery to maximum effort. Two key thresholds divide it: the aerobic threshold between Zone 2 and Zone 3, and the anaerobic threshold between Zone 4 and Zone 5.
Other zone models
The 5-zone model is the most widely used by coaches and GPS devices. Several other systems exist and you will encounter them in training plans, research literature, and sports science contexts. They divide the same physiological continuum differently.
3-zone model
Used extensively in sports science research. The three zones are defined by ventilatory thresholds rather than percentages: Zone 1 (below VT1), Zone 2 (between VT1 and VT2), and Zone 3 (above VT2). In practice, Zone 1 maps roughly to Zones 1–2 in the 5-zone model, Zone 2 maps to Zone 3, and Zone 3 maps to Zones 4–5. This model is useful for research analysis but less practical for daily training guidance.
A1 / A2 model (Scandinavian athletics)
Common in Swedish and Norwegian elite training environments. The labels are aerobic, with intensity coded by number: A1 covers easy aerobic work below the aerobic threshold (roughly Zones 1–2), A2 covers moderate aerobic work between the thresholds (roughly Zone 3), and A3 and AN cover threshold and near-maximal efforts (roughly Zones 4–5). The A-notation is still used in many Scandinavian club training plans.
Norwegian 4-zone model (lactate-based)
Used in elite Norwegian endurance training and based on measured blood lactate rather than heart rate percentages. Zone 1: 1–2 mmol/L. Zone 2: 2–4 mmol/L. Zone 3: 4–6 mmol/L. Zone 4: above 6 mmol/L.[5] This model requires actual lactate testing but gives the most physiologically accurate threshold placement. Many elite athletes now combine lactate testing with heart rate monitoring to anchor their zones precisely.
A1 / A2 Model (Scandinavian)
Norwegian 4-Zone (lactate)
The zone labels differ, but the physiology does not. Every model is describing the same underlying intensity curve. The 5-zone model is used throughout this site because it gives the most useful resolution for daily training decisions.
How to set your zones correctly
The calculator above uses the Karvonen heart rate reserve method,[6] which is the most accurate formula-based approach available. Max HR is estimated using the Tanaka formula (208 minus 0.7 times age).[7] Zone boundaries are calculated as percentages of heart rate reserve, the range between your resting and maximum heart rate.
Getting more accurate zones
Formula-based zones are good enough for most runners to start training with intent. For more precision, a field test gives better results. Run a 30-minute time trial at maximum sustainable effort. Take 95% of your average heart rate during that effort as an estimate of your lactate threshold (Zone 4 ceiling). Build zones upward and downward from there.
The most accurate method is a lab-based lactate or VO2Max test, where actual blood lactate levels are measured at different intensities. This is how elite athletes set their zones. It is available at most sports performance clinics and is worth doing if you are training seriously for a marathon or half marathon.
Zones shift with fitness
As your aerobic base develops, your zones do not change in terms of heart rate percentage. What changes is the pace you can hold within each zone. A Zone 2 run that required 6:30 per kilometer when you started may only require 5:50 per kilometer after 10 weeks of consistent base training. This is the adaptation. The zone is the same. The pace improves.
Zones within the framework
Establishing your heart rate zones is step 1 in the Run Mastery 5-step framework. Every session in every other step references your zones. Without defined zones, intensity is guesswork. Easy days drift too hard. Hard days are not hard enough. The entire structure collapses into the grey zone.
Step 2 builds your aerobic base in Zone 2. Step 3 adds Zone 4 threshold work on top of that base. Step 4 introduces Zone 5 intervals once threshold capacity is established. Step 5 tests your progress with a benchmark effort. Zones make every step measurable. They are also what turns a generic schedule into a real plan, whether you are following a 5K training plan or a 10K training plan.
Sources
- Seiler S, Tønnessen E. Intervals, thresholds, and long slow distance: the role of intensity and duration in endurance training. Sportscience. 2009;13:32–53.
- Holloszy JO. Biochemical adaptations in muscle. Effects of exercise on mitochondrial oxygen uptake and respiratory enzyme activity in skeletal muscle. J Biol Chem. 1967;242(9):2278–2282.
- Seiler KS, Kjerland GØ. Quantifying training intensity distribution in elite endurance athletes: is there evidence for an optimal distribution? Scand J Med Sci Sports. 2006;16(1):49–56.
- Faude O, Kindermann W, Meyer T. Lactate threshold concepts: how valid are they? Sports Med. 2009;39(6):469–490.
- Tønnessen E, Sylta Ø, Haugen TA, Hem E, Svendsen IS, Seiler S. The road to gold: training and peaking characteristics in the year prior to a gold medal endurance performance. PLoS ONE. 2014;9(7):e101796.
- Karvonen MJ, Kentala E, Mustala O. The effects of training on heart rate: a longitudinal study. Ann Med Exp Biol Fenn. 1957;35(3):307–315.
- Tanaka H, Monahan KD, Seals DR. Age-predicted maximal heart rate revisited. J Am Coll Cardiol. 2001;37(1):153–156.
