Watts → Speed Calculator
How fast can you go for a given power output? This calculator uses the fundamental physics of cycling – balancing your power against aerodynamic drag, rolling resistance, and gravity – to estimate your speed on flat or graded terrain.
On flat ground at speeds above 25 km/h, aerodynamic drag accounts for 80–90% of the resistance you face. This is why position and equipment aerodynamics matter so much for speed.
What this calculator does
Estimate your cycling speed from power using a simple physics model (aero drag, rolling resistance, and gradient).
Your sustained power output. Use your FTP for hour-long efforts, or lower for longer rides.
0 = flat. Negative = downhill. For steep climbs (>3%), use the Climb Time calculator instead.
Positive = wind in your face, negative = wind at your back.
Results
Power Comparison
How to use this result
- Compare different positions to see how CdA changes speed at the same power.
- Use the speed estimate to set pacing targets for solo efforts or time trials.
- If your speed feels off, check rolling resistance and wind inputs first.
CdA Reference
Pro TT position: 0.20–0.22 m²
Aggressive drops: 0.25–0.28 m²
Hoods position: 0.30–0.35 m²
Relaxed / hoods high: 0.35–0.40 m²
Upright / leisure: 0.45–0.55 m²
Understanding Cycling Speed
Aerodynamic Drag
Increases with the square of velocity. At 40 km/h, you need ~4× the power to overcome drag compared to 20 km/h. Your position and equipment are the biggest factors you can control.
CdA (Drag Area)
The product of drag coefficient (Cd) and frontal area (A). Reducing CdA by 10% can save significant power at race speeds – or go faster at the same power.
Rolling Resistance
Depends on tire type, pressure, road surface, and weight. At lower speeds (under 25 km/h), rolling resistance becomes a larger proportion of total resistance.
Wind Effects
A 15 km/h headwind at 30 km/h ground speed means you're pushing through 45 km/h of apparent wind – roughly doubling your aerodynamic drag compared to still air.
Physics: Power = velocity × (F_aero + F_rolling + F_gravity). F_aero = ½ρCdAv², F_rolling = mgCrr, F_gravity = mg·sin(θ). This calculator solves iteratively for velocity given power.