Estimated FTP from Cycling Power
eFTP is a constrained estimate from cycling power-duration evidence, separate from the configured FTP.
For the Power zones walkthrough, worked graphs and exact confidence rules, read the Estimated FTP (eFTP) guide.
Evidence and model selection
These examples describe saved eFTP v1 estimates. New calculations combine indoor and outdoor cycling across recording devices and cap confidence at Medium. Previously saved estimates retain their original model inputs and confidence.
The model considers qualified efforts over the preceding 180 days, grouped by cycling power source. Evidence older than 90 days is stale. Target durations include 300, 600, 1,200 and 1,800 seconds. A multi-point critical-power fit needs at least three points spanning a short duration of at most 600 seconds and a long duration of at least 1,200 seconds, from at least two activities. Otherwise a five-/twenty-minute pair can be used, or a longest available anchor of at least 600 seconds with lower-confidence prior assumptions.
Formula and cap
For a critical-power fit, power P at duration t is modeled as P = CP + W′/t, with CP in watts and W′ in joules. The anchor is converted to a twenty-minute equivalent:
P20 equivalent = anchor power × (anchor seconds / 1200)^0.07
safety cap = 0.95 × P20 equivalent
eFTP = min(CP, safety cap), or safety cap for the single-anchor modelThe final estimate is rounded to whole watts. Values outside 50–2,000 W, invalid/nonpositive CP or W′, or insufficient evidence are rejected. The 0.07 exponent and cap are product assumptions, not proof that all athletes share a fixed power-duration relationship.
Verified example
A single twenty-minute anchor at 300 W has a P20 equivalent of 300 W and cap 285 W, so the single-anchor estimate is 285 W. It does not obtain multi-point confidence merely because this arithmetic is exact.
Decision and limitations
Source disagreement, recency, duration coverage and repeated evidence affect confidence and whether a suggestion is surfaced. The configured FTP is not an input to the estimate itself. Applying an estimate is a separate explicit decision in Power zones; viewing the number does not change training targets. This is cycling evidence, not a laboratory measurement or automatic coaching.
Sources and implementation scope
Clarke and Skiba (2013), performance models and critical power explains the research background. The formulas, windows, quality gates and examples above describe EffortSignal’s saved eFTP v1 calculation, checked September 14, 2026. Thresholds and rounding are EffortSignal rules.
Content checked September 30, 2026 · EffortSignal release 0.1.13