Enter riding conditions
Use average values for a steady effort, road surface, and route segment.
Example Data Table
Aero position, weather, and terrain can change the result quickly. These examples assume 85 kg total mass, 0.320 m² CdA, 0.005 Crr, 1.225 kg/m³ air density, and 96% drivetrain efficiency.
| Power | Gradient | Wind | Estimated speed | Use case |
|---|---|---|---|---|
| 150 W | 0% | 0 km/h | About 25 km/h | Relaxed endurance pace |
| 220 W | 0% | 0 km/h | About 31 km/h | Steady solo road effort |
| 220 W | 0% | 15 km/h headwind | About 25 km/h | Exposed windy road |
| 220 W | 4% | 0 km/h | About 15 km/h | Sustained climb |
| 220 W | -3% | 0 km/h | About 43 km/h | Controlled downhill section |
Formula Used
The calculator balances rider power against rolling, climbing, and aerodynamic resistance. It uses a numerical search because air drag changes with the square of air speed.
Frolling = Crr × m × g × cos(θ)
Fgrade = m × g × sin(θ)
Faero = 0.5 × ρ × CdA × (v + w) × |v + w|
θ = arctan(grade ÷ 100)
P is pedal power. v is ground speed. η is drivetrain efficiency. m is total mass. g is gravity. ρ is air density. w is signed wind speed. A positive wind is a headwind.
How to Use This Calculator
- Enter your average power for the route segment.
- Add combined rider and bicycle mass.
- Set the average road gradient and wind direction.
- Use CdA and Crr values that match your position and surface.
- Keep air density and efficiency near realistic local values.
- Press Calculate Speed and review the resistance breakdown.
- Change one input at a time to compare pacing choices.
Understanding Cycling Power and Speed
Power does not create one fixed speed
Power is work completed each second. Speed is the result after resistance is paid. The same 200 watts can feel fast on smooth flats. It can feel slow on a climb. Wind can also change the answer dramatically. This calculator estimates a steady speed for one set of conditions.
Aerodynamic drag grows quickly
Aerodynamic drag becomes the main challenge on fast roads. It increases with the square of relative air speed. Relative air speed includes your road speed and the wind. A small improvement in riding position can matter more than a small reduction in bicycle weight on level terrain. Lower handlebars, fitted clothing, and a tidy posture may reduce CdA.
Gradient changes the power demand
Climbing adds a force against forward motion. Total mass becomes important when the road points upward. A heavier bike, full bottles, or luggage increase climbing demand. Descents can produce very high estimated speeds. Real descents need careful braking, road awareness, and safe handling. Treat calculated downhill values as mathematical estimates, not riding targets.
Road surface affects rolling resistance
Rolling resistance describes energy lost where tires meet the road. Fresh, smooth pavement usually has a lower Crr. Rough chipseal, gravel, soft tires, and poor surfaces increase it. Tire width alone does not determine Crr. Pressure, casing, load, and surface texture matter. Choose a conservative Crr when the surface varies.
Wind requires honest inputs
Wind is often the largest unknown. Enter positive wind for a headwind. Enter negative wind for a tailwind. Gusts and changing directions are not represented perfectly. Use a route average when planning. For a loop, test the outbound and return legs separately. A headwind hurts speed more than an equal tailwind helps it.
Speed gains become harder on flat roads
On flat terrain, extra power does not translate into equal percentage gains in speed. Aerodynamic drag dominates resistance at speed. Each additional kilometre per hour needs more power. This is why a rider may gain only a little speed after harder effort. Reducing drag can be more efficient than adding watts. Test a lower CdA value to see the potential value of a better position, fitted clothing, narrow elbows, or tidy equipment. These changes help most in fast solo efforts.
Good inputs improve the estimate
Good inputs make the estimate more useful. Weigh yourself with bicycle and usual gear. Check local wind reports, but remember that buildings and trees change airflow. Choose the average grade for one climb or road segment instead of an entire mixed route. Recalculate each major section when conditions change. Keep a note of values that match your real rides. Over time, those comparisons can reveal a realistic CdA and Crr range. Use that personal range for training, pacing, and equipment decisions.
Use the result for pacing decisions
Try your normal endurance power first. Then test race power, a climbing segment, or a stronger headwind. Compare the resulting speeds and route times. This shows where extra watts matter most. It also highlights the value of drafting, better positioning, and smart route choices. Actual rides still vary with traffic, corners, stops, fatigue, and power fluctuations.
Frequently Asked Questions
No. It is a steady-state estimate. Traffic, corners, drafting, gusts, stops, riding position changes, and changing power can alter real speed.
Use the average power you can sustain for the selected route segment. A power meter value is best. Indoor trainer averages can also help when adjusted for outdoor conditions.
CdA is drag area. It combines body shape, position, clothing, bicycle setup, and frontal area. Lower CdA generally produces more speed at the same power on flat roads.
Headwinds raise relative air speed. Aerodynamic drag then rises rapidly because it depends on air speed squared. The power needed to hold speed can increase sharply.
Enter a negative wind value. For example, use -10 km/h for a 10 km/h tailwind. The calculator uses signed aerodynamic resistance.
Yes, but less than on climbs. Mass affects rolling resistance and acceleration. On fast flat roads, aerodynamic drag is often the dominant resistance.
Try 0.003 to 0.006 for quality pavement. Use higher values for rough roads, gravel, soft tires, or uncertain conditions. Surface quality changes it considerably.
Air density changes aerodynamic drag. Cooler, denser air creates more drag. Higher altitude or warmer air often has lower density and can slightly increase speed.
A clean, well-maintained drivetrain often uses 95% to 98%. Lower values can represent dirt, poor lubrication, or additional mechanical losses.
It can estimate solo speed. Group riding changes aerodynamic drag through drafting. Use a lower CdA only when you have a realistic estimate for your position in the group.
Routes include traffic lights, turns, gradients that vary, coasting, braking, temperature changes, and fatigue. Calculate segments separately for a more useful plan.
Safety note: Use estimates for planning, not for risky speed goals. Conditions, traffic, road quality, and personal control always come first.