Free calculator

Bike Calculator

Enter your power to predict your cycling speed — or enter a speed to see the watts it takes. For example, a 70 kg rider on a 9 kg road bike holding 150 watts on flat ground travels about 28.4 kph (17.7 mph). The model accounts for gravity, air resistance, and rolling resistance.

Power & speed

Power

watts

SpeedCalculated

kph

Drag to explore power 0–500 W

Rider & bike

Rider weight (kg) Bike weight (kg)

Tires

Riding position

More aerodynamic positions need less power for the same speed.

Route & conditions

Grade (%) Flat

Head wind (kph) Distance (km) Temperature (°C) Elevation (m)

Temperature and elevation change air density, which affects aerodynamic drag.

Prediction

28.4 kph

at 150 watts

Time for 30.0 km

1 h 03 min

Energy

2278 kJ (~544 kcal)

Est. weight loss

70 g

Power to weight

2.14 W/kg

More free tools

Compare two bike setups side by side Map gear ratios & gear inches Upload a GPX/FIT route for time & fueling

How it works

An engineering model balancing propulsive power against the three forces slowing you down: gravity (weight × grade), aerodynamic drag (position, wind, air density), and rolling resistance (tires). Assumes 95% drivetrain efficiency and ~24% metabolic efficiency for energy estimates.

Physics model based on bikecalculator.com by Curt Austin.

Train with structure

Turn your watts into a periodized plan built around your schedule.

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Frequently asked questions

How many watts do I need to cycle at 30 kph (18.6 mph)?

On flat ground with no wind, a 70 kg rider on a 9 kg road bike riding on the hoods needs roughly 172 watts to hold 30 kph. The requirement grows rapidly with speed because aerodynamic drag rises with the cube of velocity — holding 40 kph takes around 370 watts under the same conditions.

What affects cycling speed the most?

On flat roads above about 25 kph, aerodynamic drag is the dominant force — typically 70–90% of your total resistance — which is why riding position matters so much. On climbs steeper than about 5%, gravity takes over and total weight (rider plus bike) becomes the biggest factor. Rolling resistance from your tires is a smaller but constant cost at every speed.

How accurate is this bike calculator?

It uses the same engineering model as bikecalculator.com, which has been used by cyclists since 1997. It solves the physical balance between power and the three resistive forces (gravity, air drag, rolling resistance), assuming 95% drivetrain efficiency. Real-world results vary a few percent with road surface, clothing, drafting, and wind direction, but predictions are typically within 1–2 kph of measured speeds.

What is a good power-to-weight ratio for cycling?

Recreational riders typically sustain 2–3 W/kg for an hour, trained amateurs 3–4 W/kg, and elite racers exceed 5.5 W/kg. Power-to-weight matters most on climbs, where gravity dominates. To find your sustainable power, see our guide on what FTP is and how to test it.