Cycling Power Calculator
Cycling power calculator — estimate the watts needed to ride at a given speed and gradient using a physics model of rolling resistance, gravity, aerodynamic drag and drivetrain loss. See watts per kilo, calories per hour, and a breakdown of where your power goes. Runs entirely in your browser.
Cycling Power Calculator
How to Use the Cycling Power Calculator
Enter total mass
Add your weight plus the bike and kit, in kilograms.
Set speed and grade
Enter your target speed and the gradient.
Tune the details
Open advanced options to set Crr, CdA, air density and drivetrain loss.
Read the watts
See power, W/kg, calories and the force breakdown.
The Physics of Going Fast
Every watt a cyclist produces is spent fighting four things, and understanding their balance is the difference between guessing and pacing intelligently. This calculator models all four: the rolling resistance where tyre meets road, the pull of gravity on any gradient, the aerodynamic drag of pushing through the air, and the small but real friction lost in the chain and bearings. It computes the force from each, multiplies by your speed to get the power delivered at the wheel, then divides by drivetrain efficiency to estimate what your legs must actually produce. The result is the watts needed to hold a chosen speed — and, just as usefully, a breakdown showing where those watts go.
That breakdown is where the insight lives, because the balance shifts dramatically with conditions. On flat ground at speed, aerodynamic drag dominates almost everything else, and because drag rises with the square of air speed, the power to overcome it rises with the cube of speed — which is why nudging from 30 to 35 km/h feels so brutally hard and why riders obsess over their drag area, or CdA. Tilt the road upward, though, and gravity swiftly takes over as the largest term; on a steep climb you are mostly lifting your own mass, which is why watts per kilogram, not raw watts, is the metric that decides who reaches the summit first. The tool reports W/kg precisely so heavier and lighter riders can compare fairly.
The advanced parameters let you move from a rough estimate to something tailored. Rolling resistance depends on your tyres, pressure and the road surface; air density falls with altitude and rises in cold weather, changing how much drag you face; and drivetrain efficiency, typically around 97%, quietly skims a few watts off everything. The calorie estimate assumes a gross efficiency of roughly a quarter, reflecting that most of the energy you burn becomes heat rather than forward motion. The model assumes still air, so to approximate a headwind you can simply enter a higher speed. Treat the numbers as a well-founded physics estimate for pacing, goal-setting and gear choices rather than a substitute for a real power meter — and, as always, everything is computed in your browser, so your inputs never leave your device.
On the flat you fight the air; on a climb you fight gravity — which is why W/kg, not raw watts, wins mountains.
10 Facts About Cycling Power
On the flat, aerodynamic drag is the dominant force.
Air resistance rises with the cube of speed in power terms.
Going uphill, gravity quickly becomes the biggest cost.
Watts per kilogram is the key metric for climbing.
A lower, tucked position cuts CdA and saves big watts.
Pro riders sustain 5–6 W/kg for long efforts.
Better tyres and pressure lower rolling resistance.
Drivetrain friction wastes a few percent of your power.
Headwinds add to air speed, raising required power sharply.
This calculator runs in your browser — nothing is uploaded.
Frequently Asked Questions
- It models the four forces a rider must overcome: rolling resistance between tyres and road, gravity on any gradient, aerodynamic drag through the air, and the small loss in the drivetrain. It computes the force from each, multiplies by your speed to get power at the wheel, then divides by the drivetrain efficiency to estimate the power your legs must produce.
- Aerodynamic drag grows with the square of your speed, and because power is force times speed, the power needed to overcome air resistance grows with the cube of speed. That is why going from 30 to 35 km/h on the flat feels disproportionately hard: a small speed increase demands a large jump in watts.
- CdA is your drag area — the drag coefficient multiplied by your frontal area — and it captures how aerodynamic you and your bike are. A typical road rider on the hoods is around 0.32, dropping toward 0.25 in the drops or on aero bars. Because drag dominates on the flat and at speed, lowering CdA by getting into a tucked position is often the cheapest way to go faster for the same effort.
- Watts per kilogram normalises power by body weight and is the key metric for climbing, where you fight gravity. Recreational riders often sustain around 2–3 W/kg for an hour, strong amateurs 3.5–4.5, and professional riders 5–6 or more. Because heavier riders must produce more absolute watts to climb at the same speed, W/kg levels the comparison.
- On the flat, drag dominates and rolling resistance is small. As the road tilts up, the force of gravity along the slope grows quickly and soon becomes the largest term, which is why climbing is so demanding even at low speeds. The breakdown in this tool shows the climbing component growing as you increase the grade.
- The defaults are sensible starting points: a rolling-resistance coefficient around 0.005 for good road tyres on smooth tarmac, and air density of about 1.225 kg/m³ at sea level and 15°C. Rougher surfaces and lower tyre pressures raise rolling resistance; higher altitude and warmer air lower density, which reduces drag. Adjust them in the advanced options if you know your values.
- It is an estimate. The tool converts mechanical power to calories assuming a gross efficiency of around 24%, which is typical for cycling — most of the energy you burn becomes heat rather than forward motion. Individual efficiency varies, so treat the kilocalories-per-hour figure as a reasonable guide rather than a precise measurement.
- The model assumes still air, so the air speed equals your ground speed. A headwind effectively raises your air speed and therefore the power required, while a tailwind lowers it. To approximate a headwind, you can enter a higher speed than your actual ground speed; for precise wind modelling you would need a dedicated aero tool.
- This tool solves the common direction — the power needed to hold a chosen speed — which is what most riders want for pacing and goal-setting. The reverse, finding speed from a fixed power, requires solving a cubic equation and can be done by adjusting the speed until the power matches your target, which works well in practice.
- Completely free, with no account or limit. It works offline once the page has loaded and collects no data — your inputs never leave your device.
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