Indoor Trainer Power Correction

Power Correction Model

A trainer measures power at the rear wheel or hub — after the chain and cassette. An outdoor crank or pedal power meter measures before the drivetrain, so for the same effort it reads higher. This tool grosses the wheel reading back up to that crank-equivalent figure using two factors:

• Drivetrain loss: Roughly 2–4% of power is lost to chain and cassette friction between the cranks and the wheel (DC Rainmaker budgets 2–4%, more with a dirty chain). The wheel reading is divided by (1 − loss) to recover crank power.
• Trainer accuracy offset: Direct-drive smart trainers are typically ±2% and need no offset here; wheel-on trainers (~5%) and rollers (~8%) tend to under-read more, so the reading is grossed up by that amount.

Outdoor Equivalent = Indicated Power ÷ (1 − drivetrain loss) ÷ (1 − trainer under-read)

Temperature is treated as a calibration caveat, not a fixed per-degree correction: no manufacturer or independent source publishes a deterministic per-°C drift constant. Direct-drive units drift little once warmed up; wheel-on units are more temperature-sensitive through the tyre/roller contact, so calibrate at your riding temperature after a 15–20 minute warm-up.