Cycling speed calculator

How cycling speed is calculated

When you ride at a steady speed, the power you put into the pedals equals the power needed to overcome three forces: air resistance, rolling resistance and gravity. The calculator solves that balance for you. It is part of the free LeCoach cycling calculators.

P = (m·g·sin θ + Crr·m·g·cos θ + ½·ρ·CdA·v²) · v ÷ (1 − drivetrain loss)

In "speed from power" mode, the calculator finds the speed at which this equation equals your power. In "power for a speed" mode, it fills in the speed and returns the watts.

Where your watts go

The calculator shows how your power splits between the three forces at your speed. That split explains almost everything about how to get faster.

• On the flat, air resistance takes most of your power, usually 75 to 90 % above 30 km/h. Because air resistance rises with the square of speed, and the power to overcome it with the cube, going a little faster costs a lot more watts. Getting lower and narrower on the bike is often worth more than any equipment upgrade.
• On a climb, gravity takes over. At 7 % and 13 to 14 km/h, more than 80 % of your power goes into lifting rider and bike. Here weight and W/kg decide your speed. The watts per kg calculator shows how your power-to-weight compares.
• Rolling resistance is a smaller but constant share. Good tyres at the right pressure save more watts than most riders expect, especially at moderate speeds.

Choosing the right inputs

Position (CdA). The presets are typical values for an average-sized rider:

Taller and broader riders have a higher CdA, smaller riders a lower one. If you know your own CdA from a field test or wind tunnel, use the override in the advanced settings.

Tyres and surface (Crr). From about 0.0035 for fast race tyres on smooth tarmac, to 0.005 for training tyres on normal roads, 0.008 on gravel and 0.012 or more off-road.

Wind. Enter a headwind as a positive number and a tailwind as a negative one. A 15 km/h headwind on the flat slows a 200 W rider from about 32 to about 24 km/h.

Altitude and temperature. Thinner, warmer air means less drag. At 1,000 m and 20 °C air density is about 11 % lower than at sea level. You ride faster for the same power, although your own power output drops at altitude too.

Practical uses

• Pacing a long event. Enter the power you can hold for the distance, typically 70 to 80 % of FTP for a gran fondo, and see your expected time on each section. Our gran fondo training plan explains how to pace by power.
• Climb planning. Enter the gradient and length of a climb to see how long it will take at your threshold power, and what a few watts or kilos would save.
• Equipment decisions. Compare the time saved by a lower position, faster tyres or a lighter bike, for the courses you actually ride.
• Triathlon. Get a realistic bike speed for the triathlon pace calculator.

Common mistakes

• Entering average power from a ride with stops. The model assumes steady riding. Use the average while moving, or better, normalized power for a steady section.
• Forgetting the bike and kit. Total mass includes bike, bottles, shoes and helmet, typically 8 to 11 kg on a road bike.
• Expecting outdoor speed to match the trainer. Indoor apps use their own physics settings. Outdoor results depend on the real wind, road surface and how often you brake.
• Ignoring drafting. In a group, riding in the wheels can cut the power needed by 25 to 40 %. The calculator is for riding alone.

When the model is off

The physics are well established, but the inputs are estimates. Gusty wind, rough roads, corners and braking all slow you down in ways a steady-state model cannot see. Use the calculator to compare options and plan pacing, and expect real-world speed within a few percent on smooth, steady roads, and lower on technical courses.

LeCoach's coach can answer pacing questions from your own power data and FTP, for example what to hold on a long climb in your next event, and explain why.