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Indoor Trainer Power Correction
Estimate outdoor equivalent power from indoor trainer readings.
How We Calculate This
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.
Frequently Asked Questions
Indoor trainers measure power at different points in the drivetrain compared to outdoor power meters. Wheel-on trainers measure at the rear axle (after drivetrain losses), while crank-based power meters measure before those losses. Temperature also affects trainer resistance and calibration.
Direct-drive smart trainers from major brands (Wahoo, Tacx, Elite) are typically accurate to ±2% after proper calibration. Wheel-on trainers can vary by ±5% or more depending on tyre pressure, roller tension and temperature.
It can, but mainly on wheel-on trainers, where tyre compound and pressure change with temperature and alter the tyre-to-roller contact. Direct-drive smart trainers have no tyre interface and drift only slightly once warmed up (Saris). No manufacturer publishes a fixed per-degree drift figure, so rather than applying a numeric temperature correction this tool advises you to calibrate at your riding temperature after a 15–20 minute warm-up and re-run the spindown if conditions change.
Yes, especially for wheel-on trainers. Do a 10-minute warmup, then run the trainer calibration/spindown. This accounts for tyre pressure, temperature and roller tension changes. Smart trainers benefit from regular calibration too.
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Last updated: June 2026
All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.