Heart rate training zones are the ranges gym equipment and fitness apps display as "fat burn," "cardio," and "peak," and every one of them is derived from a single starting number: your estimated maximum heart rate. Understanding where that starting number comes from, and how imprecise it actually is, explains both why the zones are useful for structuring a workout and why they shouldn't be treated as exact medical thresholds.
Where '220 minus age' comes from
The most widely used formula for estimated maximum heart rate is 220 minus your age in years. It originated from research in the 1970s that was never intended to become the universal standard it did, it was based on a relatively limited dataset, and the actual variation between individuals of the same age can be substantial, sometimes 10-20 beats per minute in either direction. More refined formulas exist, such as the Tanaka formula (208 minus 0.7 times age), which some research suggests is more accurate across a wider age range, but 220 minus age remains the default in most calculators and fitness trackers because of how simple and well-known it is.
How the five zones are derived as percentages
Once max heart rate is estimated, each training zone is defined as a percentage range of that number. A common breakdown is Zone 1 (roughly 50-60% of max, light recovery activity), Zone 2 (60-70%, fat-burning/endurance pace), Zone 3 (70-80%, aerobic/cardio), Zone 4 (80-90%, hard/anaerobic threshold), and Zone 5 (90-100%, maximum effort). For that same 40-year-old with a 180 bpm estimated max, Zone 2 would run from about 108 to 126 bpm. The Heart Rate Zone Calculator runs this math automatically once you enter your age, producing all five ranges rather than requiring manual percentage calculations.
It's worth noting that "fat burn" zone doesn't mean it burns more total fat than higher zones, it means a higher percentage of the calories burned at that specific intensity come from fat rather than carbohydrate. At higher intensities, a larger share of calories comes from carbohydrate, but total calorie burn (and often total fat burned) tends to be higher because you're working harder, which is why zone choice depends on training goals, not just fat-burning percentage.
Why the estimate matters less than consistency
Because estimated max heart rate carries real individual variation, the zones calculated from it are approximations, useful for structuring effort levels during a workout, not precise medical boundaries. Someone training seriously for a specific heart-rate-based program may benefit from a supervised max heart rate test, but for general fitness tracking, the formula-based zones remain a reasonable and consistent way to gauge relative effort over time, alongside metrics like VO2 max that estimate cardiovascular fitness more broadly.