Heart Rate
Zones.

01 / Basics

What zones actually are

Your body does not shift smoothly from easy to hard. It moves through distinct metabolic states, each defined by which fuel sources are being used, how your cardiovascular system is being loaded, and what physiological adaptations are being triggered. Heart rate zones are a map of those states.

The 5-zone model divides the intensity spectrum from complete rest to maximum effort into five bands. Each band has a different purpose, a different cost, and a different return on training time invested.

Why intensity distribution matters

Each zone stresses a different energy system. Zone 2 builds your aerobic engine through mitochondrial adaptation. Zone 4 raises your lactate threshold. Zone 5 develops VO2Max. These adaptations do not overlap cleanly.

Training in the wrong zone for a given session does not produce a blend of adaptations. It mostly produces fatigue without a proportional return. Zone 3, the most common default for recreational runners, sits between the aerobic and anaerobic thresholds. It is too hard for aerobic base adaptation. It is not hard enough for threshold development. It is the most common reason runners plateau.

03 / Zones

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]

Five things easy Zone 2 running does that hard running cannot:

• More mitochondria. More and bigger cellular engines, so you produce more energy at every pace.
• Better fat oxidation. Your body learns to burn fat and spare its glycogen, so you hold pace late instead of fading.
• A stronger heart. Higher stroke volume means more oxygen delivered with every beat.
• A denser capillary network. More capillaries around your muscles means more oxygen reaches them.
• Durability. A deep aerobic base lets you absorb hard sessions and add volume without breaking down.

05 / Practice

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.

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.