Methodology · how we calculate
Methodology & Sources: How We Calculate
Every zone formula on TrainingZones is implemented as a pure, unit-tested function that cites a primary source. We do not improvise numbers: each model below is taken from the published research or coaching reference that defined it, and an automated test asserts it against known reference values.
How we calculate
Each formula is a small, deterministic function: the same inputs always produce the same zones, with no hidden tuning. Below is every model family on the site and the primary source it is built from.
| Model | Primary source |
|---|---|
| Max heart rate | Tanaka et al. 2001; Gulati et al. 2010 (women) |
| %HRmax & Karvonen (heart-rate reserve) | Karvonen et al. 1957; ACSM Guidelines |
| Lactate-threshold heart rate (LTHR) | Friel, Training Bible; Allen & Coggan |
| MAF aerobic heart rate (180 Formula) | Maffetone, The Big Book of Endurance Training |
| FTP & power zones | Allen & Coggan, Training and Racing with a Power Meter |
| Critical Power | Monod & Scherrer 1965; Jones et al. |
| VDOT & running paces | Daniels & Gilbert 1979; Daniels' Running Formula |
| Critical Swim Speed (CSS) | Wakayoshi et al. 1992 |
| Polarized / 80-20 distribution | Seiler & Kjerland 2006 |
| Rowing UT2–AN zones | British Rowing; Concept2 |
| Intensity zones I1–I5 | Norwegian Olympic federation; Seiler & Tønnessen |
| Ski-mountaineering VAM | Vertical ascent rate (metres climbed per hour) |
Accuracy & testing
Each formula ships with an automated test that asserts reference values straight from its source. For example, Tanaka's 208 − 0.7 × age must return 187 bpm at age 30, and Daniels' equations must reproduce the VDOT tables. If a refactor breaks a number, the test fails before the page ever ships. The formulas are computed, not improvised.
Limitations
Every formula here is an estimate. Age-based equations carry a standard deviation of roughly 10–12 bpm, and population averages cannot capture your individual physiology. Field tests and, above all, lab tests are more accurate. See lab vs field testing for how the three tiers of accuracy compare and when each is worth it.
Freshness commitment
We review every formula and source at least twice a year. We revisit the formulas and their sources at least twice a year to catch updated guidelines and new research, and we render an honest, visible review date on the methodology.
Last reviewed: 10 June 2026
Corrections welcome
If you spot a formula, source or value that looks wrong, tell us. Email hello@svenahrens.com with the page and the reference, and we will check it against the source and fix it.