For general cardiovascular fitness, you should spend most of your cardio time in Zone 2 (60–70% of your max heart rate), with shorter bursts in Zones 3 and 4 mixed in weekly. There’s no single “right” zone for cardio because the best zone depends on your goal, whether that’s burning fat, building endurance, or improving your peak fitness. But the evidence strongly favors a specific distribution: roughly 80% of your training at low intensity and 20% at high intensity.
The Five Heart Rate Zones
Heart rate zones are expressed as percentages of your maximum heart rate. A quick estimate of your max is 220 minus your age, so a 35-year-old would have an estimated max of 185 beats per minute. The zones break down like this:
- Zone 1 (50–60% of max): Very light effort. Walking, warm-ups, and recovery days. You can hold a full conversation easily.
- Zone 2 (60–70% of max): Light to moderate effort. Comfortable jogging, cycling, or swimming where you can still talk in sentences. This is the foundation of aerobic fitness.
- Zone 3 (70–80% of max): Moderate effort. You’re breathing harder and can only speak in short phrases. This is the intensity most people default to on a steady run.
- Zone 4 (80–90% of max): Hard effort at or near your lactate threshold. Talking is difficult. Tempo runs and sustained interval efforts live here.
- Zone 5 (90–100% of max): All-out effort you can only sustain for short bursts, like sprints. This pushes your body to its peak oxygen capacity.
Why Zone 2 Gets So Much Attention
Zone 2 training has become enormously popular in fitness and longevity circles, often described as the optimal intensity for improving how your cells produce energy and burn fat. There’s real physiological logic behind it: working at this low intensity trains your body to use fat as fuel more efficiently and builds the aerobic base that supports every other kind of exercise.
That said, a 2025 narrative review published in PubMed concluded that current evidence does not actually support Zone 2 as the single optimal intensity for improving mitochondrial or fat-burning capacity. The review found that prioritizing higher exercise intensities (above Zone 2) is critical for maximizing heart and metabolic health benefits, especially if you’re working out for shorter periods. In other words, Zone 2 is valuable, but it’s not the whole picture. If you only have 30 minutes a few times a week, spending all of it in Zone 2 may leave significant fitness gains on the table.
The Best Zone for Fat Burning
You’ve probably seen “fat burning zone” labels on treadmills, typically pointing to a low-moderate intensity. This isn’t entirely wrong. A study of 36 trained runners found that maximal fat oxidation occurred at about 54% of peak oxygen uptake, which corresponds roughly to 60–80% of maximum heart rate. That range overlaps heavily with Zones 2 and 3.
Here’s the practical takeaway, though: the so-called fat burning zone (roughly 68–87% of max heart rate in that study) and the aerobic fitness zone (59–76% of max) overlap so much that training for fat loss and training for cardiovascular fitness are not mutually exclusive. You can accomplish both with the same workout. Higher-intensity exercise burns more total calories per minute, so even though a smaller percentage of those calories come from fat, the total fat burned can be comparable or higher. Don’t pick your zone based on the fat-burning label alone.
The 80/20 Approach
If you look at how elite endurance athletes actually structure their training, a clear pattern emerges. Research published in Frontiers in Physiology found that well-trained endurance athletes consistently follow a polarized distribution: 75–80% of their training at low intensity (Zones 1 and 2) and 15–20% at high intensity (Zones 4 and 5), with very little time in the moderate Zone 3.
Prospective studies have confirmed this isn’t just elite habit. Recreational athletes who adopted a polarized distribution showed greater improvements in endurance performance compared to those who spent most of their time at moderate (Zone 3) intensity. The logic is straightforward: easy days need to be genuinely easy so your body can recover, and hard days need to be genuinely hard to trigger meaningful cardiovascular adaptations. The middle ground, where many people spend most of their gym time, is less effective than either extreme.
For a practical weekly plan, that might look like three or four easy Zone 2 sessions (longer duration, conversational pace) and one or two sessions that include intervals in Zones 4 or 5. The American Heart Association recommends moderate-intensity activity at 50–70% of max heart rate and vigorous activity at 70–85%, which aligns well with this split.
How to Find Your Zones
The simplest method uses the formula 220 minus your age to estimate max heart rate, then multiplies by the zone percentages. A 40-year-old with an estimated max of 180 bpm would target 108–126 bpm for Zone 2 and 144–162 bpm for Zone 4.
A more personalized approach is the Karvonen method, which factors in your resting heart rate. You subtract your resting heart rate from your max to get your heart rate reserve, then multiply that reserve by the zone percentage and add your resting heart rate back. This accounts for your current fitness level, since a lower resting heart rate shifts your zones upward. For example, a 40-year-old with a resting heart rate of 60 bpm would have a heart rate reserve of 120. Zone 2 at 60–70% would be 132–144 bpm using this method, noticeably higher than the basic formula.
Neither formula is perfect. Individual max heart rates can vary by 10–20 beats from the 220-minus-age estimate. If the numbers feel wildly off from your perceived effort, trust how your body feels over the math.
When Heart Rate Zones Don’t Work
Certain medications make heart rate an unreliable guide. Beta blockers, commonly prescribed for high blood pressure, slow your heart rate and can prevent it from rising normally during exercise. As the Mayo Clinic notes, you might never reach your calculated target heart rate no matter how hard you push. In that case, a perceived exertion scale is more useful. The general rule: most cardio sessions should feel somewhat hard, meaning they take real effort but you could keep going. If you can’t talk at all, you’re likely overdoing it. If you could sing, you’re probably not working hard enough.
Heat, humidity, caffeine, sleep quality, and stress all affect heart rate independently of exercise intensity. A heart rate of 150 on a cool morning and 150 on a hot, humid afternoon represent very different levels of physiological strain. Using perceived effort as a secondary check keeps your training honest on days when your heart rate behaves unexpectedly.