Cycling Speed from Power Calculator

How it is calculated

Power reaching the wheel must balance the resisting forces at speed v (m/s): P × efficiency = (m·g·sinθ + Crr·m·g·cosθ) × v + ½·ρ·CdA·v³. P is pedal power in watts, m the total mass of rider, bike and kit in kg, grade (%) gives the slope angle θ, Crr is rolling resistance, CdA is drag area in m², and efficiency is the drivetrain share in %. Air density ρ is computed from altitude in metres using a standard atmosphere. The tool finds v by bisection and reports km/h, mph, pace per km, and the aero, rolling and gravity watts. The aero share is aero watts divided by the sum of the positive resistances, so it never exceeds 100 %. Blank or zero CdA and Crr, or an efficiency outside 1–100, fall back to 0.32, 0.005 and 97 %. Power over 3,000 W, mass over 400 kg, grades beyond ±30 % or a slope too steep for the watts give no result. There is no wind input.

Common mistakes

• Entering only the rider's weight. The field is the total of rider, bike, bottles and kit, because gravity and rolling resistance act on all of it.
• Mixing up pedal power with power from a hub or wheel meter. The tool deducts a drivetrain loss (97 % by default); for wheel power, set efficiency to 100.
• Using the default CdA for a very different position. Hoods, drops and a time-trial tuck differ a lot, and aero drag dominates on the flat.
• Expecting the same watts to give the same speed everywhere: on a 6 % grade the 250 W example falls from 36.4 to 15.9 km/h, and on a −6 % descent the rider reaches 61.5 km/h because gravity supplies about 803 W.
• Assuming wind is included. Only still air is modelled, so a headwind or tailwind outdoors will change the real speed.