Pick your gear and cadence, out comes your speed. Switch on advanced mode for the full power equation: aerodynamic drag, rolling resistance, climbing, and drivetrain efficiency. The watts number is the one that matters, it’s what you actually have to push for the finish time you want.
Three forces resist forward motion. The drivetrain costs you a bit on top.
Paero = ½ · ρ · CdA · v³, the cubic term means small speed gains cost a lot of watts. Dominant force above ~25 km/h. CdA 0.275 is reasonable for an in-tuck road position; 0.21 is good TT.
Proll = m · g · Crr · v, linear in speed and weight. Crr 0.0037 is decent road tire on smooth tarmac; 0.005–0.008 is gravel.
Pclimb = m · g · v · gradient, weight×gradient is gravity working against you. Drivetrain efficiency (~98% for clean modern systems) divides the total at the end.
Note: gradient is computed as average over the whole distance, useful for estimating average power on a course. For specific climbs, use the segment’s gradient and distance instead of the full ride. Air density is fixed at 1.225 kg/m³ (sea level, 15 °C); thinner air at altitude reduces aero power.
Most athletes obsess over FTP. Three things move the needle more for the same fitness:
Plug your numbers in, then play with the inputs. See where 10 watts comes from cheapest. That’s where to spend your training and money.
If you don’t have measured numbers, these are population averages by riding style.
Hands on hoods, head up, casual ride. Crr ~0.005. Most weekend riders sit here.
Race position, drops, hands narrow. Crr ~0.0040 with decent tires. Default for road racing.
Low arm pads, narrow shoulders, head tucked, aero helmet. Crr ~0.0035. The big gains live here.
Higher position, wider tires. Crr 0.006–0.012 depending on surface and pressure. Aero matters less; rolling matters more.
Clean, well-lubed modern drivetrain ≈ 98%. Dirty chain, worn cassette, cross-chained: drops to 95% or lower.
Just the gear ratio + development + speed table. Same engine, simpler view.
Open → Critical power (CP)Estimate your sustainable power from 3 maximal efforts. Use it as the “watts you can hold” benchmark.
Open → All calculatorsBack to the full catalog, 30+ endurance calculators across performance, fueling, swim, bike, run.
Browse →Field aero testing at the lab measures your CdA in your race position with your kit. Same protocol used by World Tour teams. The 10-15 watts you find here are the cheapest watts you’ll ever buy.
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