Fitness trackers do not count every minute of exercise the same way. The intensity minutes metric only credits time spent at a heart rate above a specific threshold, and different heart rate zones earn different amounts of credit. This page walks through a real 53-minute workout, shows exactly how the zone breakdown turns into intensity minutes, and explains the rules that decide what counts.
The Workout: Zone Breakdown
Here is the heart rate zone summary from a single 53-minute session. Five zones are shown, each with its heart rate range, how much time was spent there, and the percentage of the total workout.
Heart Rate Zone Breakdown From a 53-Minute Workout
The two short segments at the bottom, Zone 1 and Zone 2, total just over a minute between them. They represent warm-up drift and brief recovery dips. The bulk of the workout happened in Zone 4 (threshold), with a significant stretch in Zone 3 (aerobic) and a meaningful effort in Zone 5 (maximum).
When Intensity Minutes Are Counted
Intensity minutes are counted whenever your heart rate sits in the moderate or vigorous range. On Garmin, Fitbit, and most other trackers that follow World Health Organization guidelines, this maps directly to heart rate zones:
- Zone 1 (Warm Up, below 60% max HR) – does NOT count. Heart rate is too low to produce cardiovascular adaptation.
- Zone 2 (Easy, 60-70% max HR) – does NOT count on most trackers. This zone builds aerobic base but falls below the moderate intensity threshold used by WHO guidelines.
- Zone 3 (Aerobic, 70-80% max HR) – counts as MODERATE. Every minute earns 1 intensity minute.
- Zone 4 (Threshold, 80-90% max HR) – counts as VIGOROUS. Every minute earns 2 intensity minutes.
- Zone 5 (Maximum, above 90% max HR) – counts as VIGOROUS. Every minute earns 2 intensity minutes.
The doubling rule for Zones 4 and 5 comes directly from the WHO recommendation that 75 minutes of vigorous activity produces the same health benefit as 150 minutes of moderate activity. Trackers implement this by crediting vigorous time at twice the rate.
Intensity Minutes Earned From This Workout
Applying the rules above to the workout data gives the following breakdown. Each zone’s raw time is multiplied by its credit rate to produce intensity minutes.
Intensity Minutes Earned From This Workout
Zone 4 dominates the intensity minute total, contributing 58.2 of the 89 minutes earned. This is the practical reason threshold work is so efficient for hitting the 150-minute weekly target: a single sustained session in Zone 4 produces a large chunk of the weekly requirement. Zone 5 adds another 15.7, and Zone 3 rounds out the total with 15.0.
The 63 seconds spent in Zones 1 and 2 produce nothing. This is the most important thing to understand about the metric: low-intensity movement simply does not register, no matter how much of it you do.
Why 53 Minutes of Exercise Became 89 Intensity Minutes
It can look strange that a 53-minute workout shows up as 89 intensity minutes on a tracker. The reason is the 2x multiplier on vigorous zones. In this session, roughly 37 minutes were spent in Zones 4 and 5 combined. That time was doubled, turning 37 real minutes into 74 credited minutes. Add 15 minutes of aerobic Zone 3 credit, and the total reaches 89.
This is why hard workouts feel so efficient on the intensity minute metric. A single threshold session can deliver more than half the weekly target. In contrast, a full hour of Zone 2 easy work would show up as zero intensity minutes, even though it carries real aerobic and recovery value.
Weekly Context: 150 Minute Target
The World Health Organization target is 150 intensity minutes per week. A workout like the one analyzed above would put you at 89 minutes, roughly 59 percent of the weekly goal, after one session. Two similar sessions per week would clear the target with margin to spare. For readers building a weekly plan, see our guide on how to get 150 intensity minutes per week.
For a broader look at what the metric measures and why 150 is the target, see intensity minutes meaning. And for a comparison across different weekly doses, read 0 vs 75 vs 150 vs 300 weekly intensity minutes.
Practical Takeaways
- Zones 3, 4, and 5 are the only zones that generate intensity minute credit.
- Zones 1 and 2 are valuable for recovery and endurance base, but they produce zero intensity minutes.
- Time spent in Zones 4 and 5 is credited at double rate, so vigorous workouts accumulate intensity minutes fast.
- A single hard 50 to 55 minute threshold session can earn 80 to 90 intensity minutes, more than half the weekly target.
- If your tracker shows fewer intensity minutes than expected, check how much time you actually spent at or above Zone 3, not total workout duration.
Frequently Asked Questions
Which heart rate zones count toward intensity minutes?
Zones 3, 4, and 5 count. Zone 3 (aerobic) earns 1 intensity minute per minute of exercise. Zones 4 and 5 (threshold and maximum) earn 2 intensity minutes per minute because they fall in the vigorous range. Zones 1 and 2 sit below the moderate threshold and earn zero credit regardless of how long you stay there.
Why does Zone 2 not count as intensity minutes?
Zone 2 is easy effort, typically around 60 to 70 percent of maximum heart rate. It is useful for recovery and for building an aerobic base, but it falls below the moderate intensity threshold the WHO uses to define health-relevant exercise. Intensity minute credit starts at roughly 70 percent of maximum heart rate on most trackers, which corresponds to the top of Zone 2 or the bottom of Zone 3.
How many intensity minutes did this 53-minute workout earn?
About 89 intensity minutes. Zone 5 contributed 15.7, Zone 4 contributed 58.2, and Zone 3 contributed 15.0. Zones 1 and 2 contributed zero. That is more than half the weekly 150 minute target from a single session, which shows why mixed threshold workouts are so efficient for hitting the recommended weekly dose.
Do intensity minutes start counting immediately when you enter a zone?
Older Garmin watches required 10 consecutive minutes in a qualifying zone before credit started accumulating. Most modern devices credit activity in shorter bouts as long as the average heart rate stays in the moderate or vigorous range. A brief spike into Zone 3 during an otherwise low-intensity walk may not register, but sustained time in any qualifying zone does.
Is Zone 4 or Zone 5 better for earning intensity minutes?
Both earn the same 2x credit, so neither is mathematically better per minute. Zone 4 is far more sustainable, which means you can typically accumulate many more minutes in Zone 4 than Zone 5 in a single workout. Zone 5 is brief by nature, since the body cannot hold that effort for long. For practical intensity minute accumulation, Zone 4 is usually the highest-yield target.