How every result on this site is calculated, which assumptions sit behind it and where each model stops being reliable.
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At a steady speed, the power at the pedals equals the forces working against you, multiplied by speed and divided by drivetrain efficiency. Those forces are gravity on the combined rider and bike mass, aerodynamic drag, and tire rolling resistance. The cycling watts calculator, the speed calculator and the hill climb calculator all run this one engine, so a result on one page agrees with the others.
The aero watts and rolling resistance calculators each surface one term of the same equation, and the CdA calculator solves it in reverse for drag area. None of them adds a separate formula.
The training and performance calculators follow published conventions instead of the physics engine. Each is the right tool for its question, so they are not forced into one formula.
| Calculator | Model | Basis |
|---|---|---|
| Cycling Watts, Speed, Hill Climb | Classical mechanics: gravity, air drag, rolling resistance and drivetrain loss | Standard physics, as used by Analytic Cycling and similar bike-power tools |
| CdA, Aero Watts, Rolling Resistance | One term of the same power model, or that model solved in reverse | Same physics, no new coefficients |
| FTP | Test result times a protocol multiplier (0.95, 0.90 or 0.75) | Common field-test conventions |
| Power Zones, W/kg, TSS | Coggan percentages of FTP, the Allen and Coggan power profile, and the Coggan TSS formula | Allen & Coggan, Training and Racing with a Power Meter |
| Critical Power | Two-parameter work-time model | Monod & Scherrer (1965) |
| VAM | Elevation gain divided by climbing time in hours | Definition, with no modeling choice involved |
Full citations for every model are on the sources and references page.
Gravity is 9.80665 m/s². Air density is 1.225 kg/m³ at sea level and falls exponentially with elevation using a scale-height approximation. Riding-position CdA, wheelset savings, tire rolling resistance (Crr) and drivetrain losses come from the published figures listed in the table on the homepage, each with its source. Advanced mode lets you replace CdA and Crr with values you have measured, which is the single biggest accuracy gain available.
Under these conditions the physics model typically lands within a few percent of a power meter on steady solo rides. Stop-start riding, cornering and pack riding break the assumptions, and the numbers drift further from what a power meter would record.
Every calculator has unit tests that run its calculation function directly. Tests cover metric and imperial inputs, edge cases and invalid input such as zero speed or a negative result. Intermediate values are never rounded, so rounding only happens when a number is displayed.
Reference values for the newer calculators are worked out by hand from the equations before the code is written. The critical power check, for example, uses a 3-minute effort at 320 W and a 12-minute effort at 260 W, which gives a CP of exactly 240 W and a W′ of 14.4 kJ. Where no outside dataset exists, a round trip against an already-tested function is used, and we describe it that way instead of calling it independent validation.
The example calculations on each page are not typed in by hand. They call the same function the calculator runs and print its output, so an example cannot drift away from what the tool does. If a formula changes, every example changes with it.
When a keyword has no defensible method behind it, we say so on the page and do not invent a conversion factor to fill the gap. There are no made-up multipliers, and no programmatic pages that repeat one template with a different number. Questions about a specific method are welcome through the contact page, and our editorial policy explains how we handle corrections.
