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How we calculate: every metric in ZonePit

ZonePit shows not just numbers but how they come about. For every metric, this page says what goes in, how it is calculated, how precise it is and where the method comes from.

Updated: October 6, 2026 · 15 min read

Principles

• One source: all measurements come from Apple Health. What is missing there, ZonePit does not make up.
• Comparison with yourself: recovery values are measured against your own normal range, not population values.
• Gaps stay gaps: if a value is missing, it drops out. Where ZonePit estimates, it says so.
• Uncertainty is visible: load shows its basis, readiness and sleep score show how reliable they are.
• On your iPhone: all calculations run locally.

Load

Input: power, speed or heart rate of a workout and your threshold (FTP, threshold pace or threshold heart rate).

Calculation: one hour exactly at threshold gives 100 points. For each sport, ZonePit uses the most precise measure available:

• Cycling with power (TSS): duration × normalised power × intensity, divided by FTP × 1 hour, times 100. Intensity is normalised power divided by FTP.
• Running (rTSS) and swimming (sTSS) with pace: duration in hours × intensity² × 100, for swimming intensity³, because water resistance rises more steeply with speed. Intensity is speed divided by threshold speed.
• Heart rate only (hrTSS): from the heart rate of every second relative to threshold heart rate.

Uncertainty: power is most precise, pace good, heart rate an estimate that gets less precise for intervals. Every workout shows its basis. If less than 60 % of a workout is covered by measurements, ZonePit calculates no load and says why.

Fitness, fatigue and form

Input: the daily load of all workouts.

Calculation: fitness is the exponentially weighted rolling average of daily load over 42 days, fatigue the same average over 7 days. Form is fitness minus the previous day's fatigue. The projection in your plan continues this as if you complete the planned sessions exactly; days without a session count as 0.

Uncertainty: 42 and 7 days are conventions, not values measured for you. The projection is a calculation from the plan, not a prediction of your performance: illness, sleep and readiness do not enter it.

FTP suggestion and best efforts

Input: rides with a power meter from the last 90 days with at least 80 % power coverage.

Calculation: best efforts are the highest average power over 5 seconds and 1, 5 and 20 minutes, only from sections without gaps. The FTP suggestion is 95 % of the best 20-minute value. Rides recorded twice count once.

Uncertainty: the suggestion is only as good as your hardest ride. If you never went all out for 20 minutes, it is too low. It is never applied automatically.

Zones

• Power: 7 zones after Coggan in percent of FTP (lower limits 0, 55, 76, 91, 106, 121, 151 %). Time in zones uses the FTP that applied on the day of the workout.
• Running: 7 pace zones in percent of threshold speed, derived from Joe Friel's zones.
• Heart rate: 5 zones with lower limits of 50, 60, 70, 80 and 90 %, either of heart rate reserve (Karvonen, needs resting heart rate) or of maximum heart rate.
• Maximum heart rate: the highest plausible measured value (100–230) from your workouts of the last 12 months. If there is none, Tanaka's formula: 208 − 0.7 × age. Your own value always takes priority.

Uncertainty: age formulas are off by 10 beats or more for many people. A measured value is better, your own test best.

Readiness

Input: HRV, resting heart rate, sleep score and your wellbeing.

Calculation: HRV and resting heart rate as a 7-day average against your 60-day band (mean ± 0.5 standard deviations), sleep against your 28-night average. The signals produce a level and a number from 0 to 100. Details are in the guide “Daily readiness”.

Uncertainty: ZonePit shows reliability (high, medium, low) by the number of available signals. Apple stores HRV as SDNN, most studies use rMSSD. The method is applied to your own trend, not validated on Apple values.

Sleep score

Input: sleep duration, bedtime and time awake during the night from Apple Health, your sleep goal.

Calculation: 0 to 100 points from three parts, weighted like Apple's:

• Duration (50): full points from your sleep goal, then linearly fewer down to 0 at 4 hours short. Default goal 8 hours, from age 65 7.5.
• Regularity (30): distance of your bedtime from your usual time over the last 14 days. Full points up to 30 minutes, none from 2 hours. Needs 5 nights.
• Interruptions (20): time awake after falling asleep. Full points up to 20 minutes, none from 60 minutes.

If a part is missing, the rest is scaled up to 100 and reliability drops. Levels: under 41 very low, under 61 low, under 81 ok, under 96 good, above that very good.

Uncertainty: ZonePit deliberately does not rate sleep stages (deep, REM). Watches estimate them quite imprecisely compared with a sleep lab. Apple's own sleep score cannot be read by other apps; ZonePit's value may differ from it.

Sleep debt and bedtime

Sleep debt: the sum of hours you fell short of your sleep goal over the last 7 nights, at most 10 hours. Extra sleep one night does not make up for another.

Bedtime: your usual wake-up time (median of the last 14 days, split into weekdays and weekends if they differ by more than an hour) minus sleep goal minus usual time to fall asleep (15 minutes without data). From 2 hours of debt 15 minutes earlier, from 4 hours 30 minutes earlier.

Uncertainty: time to fall asleep is approximated from time in bed. The compensation steps are our own choice, not study values.

VO₂max

Input and calculation: ZonePit takes Apple's estimate from Apple Health and shows it as a trend, compared with your own history. If Apple Health has no value, for example because you only cycle, ZonePit shows a rough estimate after Uth and colleagues: 15.3 × maximum heart rate ÷ resting heart rate (7-day average). As soon as Apple provides a value, it replaces the estimate.

Uncertainty: Apple estimates VO₂max from heart rate and pace during outdoor walks, runs and hikes. Single values can be off by several ml/kg/min. The fallback formula is rougher still, especially if your maximum heart rate only comes from the age formula. The trend over weeks is what carries meaning.

Training adaptation and training effect on HRV

Training adaptation: this week's load is compared with your own normal range of the last weeks (at least 4 weeks of history). Result: below, within or above. There is deliberately no risk score, because ratios such as the acute:chronic workload ratio are considered refuted as injury predictors.

Training effect on HRV: ZonePit compares your HRV on days after higher load with days after lower load the day before, with at least 5 days per group. This is an observation, not a cause-and-effect statement.

Below and above threshold

The “aerobic and anaerobic” card splits time in power zones: zones 1 to 3 are below threshold, from zone 4 (threshold) time counts as intense. The boundary is a simplification; in the body the transition is gradual.

Heart rate drift

For sessions of 20 minutes or more, ZonePit compares heart rate in the second half with the first. If power or pace is available, it is related to intensity (decoupling of heart rate and power). From a 5 % rise a hint appears, for example about fatigue, heat or too little fluid. This is a training value, not a medical assessment.

Movement and strength minutes

The WHO recommends 150 to 300 minutes of moderate activity per week for adults, plus strength training on at least two days. ZonePit counts your total training time against 150 minutes and your strength minutes against 30 to 60 minutes per week. It does not distinguish by intensity, because power, pace and perceived effort cannot be cleanly converted into each other.

Daily curve

Heart rate and HRV as hourly averages across the day, with sleep and workouts marked. It shows how your body moves through the day. ZonePit does not derive a stress score from it.

Steps

Calculation: daily total via Apple Health's statistics, which correctly merges iPhone and Apple Watch without counting twice. The default goal is 7,000 steps.

Why 7,000: large analyses show the biggest health benefit up to about 7,000 to 8,000 steps, after which it grows more slowly. The well-known 10,000 comes from an advertising campaign. You can change your goal.

Keep reading

Sources

• Andrew Coggan: Training Stress Score, Normalized Power and power zones, TrainingPeaks; Hunter Allen, Andrew Coggan: Training and Racing with a Power Meter.
• Eric Banister et al. (1975): A system model of training for athletic performance, Australian Journal of Sports Medicine 7.
• Joe Friel: pace and heart rate zones for run training.
• Tanaka H, Monahan KD, Seals DR (2001): Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology 37(1).
• Australian Institute of Sport: Heart Rate Variability: Best Practice Guide.
• Paluch AE et al. (2022): Daily steps and all-cause mortality: a meta-analysis of 15 international cohorts. The Lancet Public Health 7(3).
• Apple: Using Apple Watch to estimate cardio fitness with VO₂ max (white paper).
• Uth N, Sørensen H, Overgaard K, Pedersen PK (2004): Estimation of VO2max from the ratio between HRmax and HRrest – the Heart Rate Ratio Method. European Journal of Applied Physiology 91.
• World Health Organization (2020): Guidelines on physical activity and sedentary behaviour.
• Impellizzeri FM et al. (2020): Acute:Chronic Workload Ratio: Conceptual Issues and Fundamental Pitfalls. International Journal of Sports Physiology and Performance 15.

ZonePit is a training and wellness app, not a medical device.