Short answer: Zone 2 — roughly 60-70% of your max heart rate — burns the highest percentage of fat as fuel of any zone. It's not the zone that burns the most total calories per minute, but it's the one that trains your body to use fat efficiently, which is what most people actually mean when they ask this question. Here's why, and how to find yours.
Training smarter doesn't mean training less. It means matching your effort to what you're actually trying to achieve.
What Are Heart Rate Zones?
Heart rate zones are ranges of your heart rate, expressed as a percentage of your maximum heart rate (MHR), that correspond to different training effects. Your body responds differently depending on which zone you're working in — burning different fuel sources, building different systems, and creating different adaptations.
Estimating your max heart rate: the simplest formula is 220 − your age. So if you're 30, your estimated max heart rate is 190 bpm. This isn't perfectly precise for everyone, but it's a solid starting point for calculating your zones. We'll use this 30-year-old / 190 bpm example throughout the zone breakdowns below — just swap in your own MHR to find your personal numbers.
What About "Zone 0"?
Some fitness trackers and apps label a "Zone 0" below the standard five zones — typically your true resting heart rate, or an extremely light range under about 50% of MHR, below where Zone 1 usually starts. It's not a formal training zone in most sports-science literature, which is why it's inconsistent from one device to another. If your tracker shows "Zone 0," it generally just means your heart rate is at or near rest — day-to-day background activity rather than an intentional training effort. It's not something to train "in," but it's a useful marker that you're genuinely recovering rather than under any load.
The Five Zones
Jump straight to the zone you're looking for:
Zone 1 — Very Light (50–60% of MHR)
Recovery-pace effort — a gentle walk, light stretching, easy movement. You should be able to hold a full conversation without any effort. Plays a real role in recovery days and warm-ups by promoting blood flow without adding fatigue.
Zone 2 — Light / Aerobic Base (60–70% of MHR)
The zone endurance athletes obsess over. At this intensity your body primarily burns fat for fuel and builds your aerobic base — the foundation that makes everything else more sustainable. Most people's biggest mistake is skipping this zone because it doesn't feel hard enough to "count."
Zone 3 — Moderate (70–80% of MHR)
The "comfortably hard" zone. Improves cardiovascular efficiency, but spending too much time here without also training Zones 2 and 4 can lead to a plateau — often called the "gray zone" for exactly that reason.
Zone 4 — Hard (80–90% of MHR)
Where you build genuine speed and lactate threshold — your body's ability to clear metabolic byproducts efficiently at higher intensities. Structured HIIT intervals usually live here, in short bursts followed by recovery.
Zone 5 — Maximum Effort (90–100% of MHR)
All-out effort, sustainable for only seconds to a couple of minutes. Builds raw power and anaerobic capacity but comes with the highest fatigue cost. A tool to use sparingly and intentionally, not a default pace.
Why Most People Get This Wrong
The common mistake is spending almost all training time in Zone 3 — hard enough to feel productive, but not targeted enough to build a specific adaptation efficiently. A more effective structure, used by many endurance coaches, follows roughly an 80/20 split: about 80% of training time in Zone 1–2 and 20% in Zone 4–5. Counterintuitive, but it produces better long-term results than constantly training in the moderately-hard middle ground.
How to Actually Use This
You don't need an expensive device to get started, though a heart rate monitor makes this far more precise than guessing by feel. If you don't have one, try the "talk test":
- Can singZone 1
- Can hold a full conversationZone 2
- Can speak in short sentencesZone 3
- Can only say single wordsZone 4
- Can't talk at allZone 5
Start by simply noticing which zone your usual workouts fall into. If everything lands in Zone 3, that's your sign to deliberately add more genuinely easy sessions and a couple of genuinely hard ones, rather than staying stuck in the same moderate gear every time.
Training with intention — knowing why a workout is easy or hard, rather than just doing what feels routine — is what actually separates steady progress from spinning your wheels.