Cycling Wattage Calculator: Power, Speed, and W/kg
Enter your speed, weight, and riding conditions to find how many watts you need to sustain that pace. The calculator uses the same physics model that coaches and racing teams rely on, splitting total power into gravity, rolling resistance, and aerodynamic drag. You also get your power-to-weight ratio, a resistance breakdown bar chart, and an interactive power vs. speed curve so you can see how small changes in position or gradient reshape your effort.
Formula
Worked example
A 75 kg rider on a 9 kg bike rides at 30 km/h (8.33 m/s) on a flat asphalt road with no wind, in the hoods position: Fg = 0 N (flat), Fr = 9.807 x 1.0 x 84 x 0.003 = 2.47 N, Fa = 0.5 x 0.324 x 1.225 x 8.33^2 = 13.76 N, mechanical power = (0 + 2.47 + 13.76) x 8.33 = 135.0 W, gross power = 135.0 / (1 - 0.015) = 137 W, W/kg = 137 / 75 = 1.83 W/kg.
How cycling power is calculated
Your legs must overcome three physical forces to maintain a given speed: gravity (if the road rises), rolling resistance (tire-road friction), and aerodynamic drag (air pushing back). The total mechanical power is the sum of those three forces multiplied by your speed. Because the chain, cassette, and derailleur absorb a small fraction of energy (1.5% for a clean new drivetrain, up to 5% for a dry, worn chain), the gross output power at the pedals is slightly higher than the mechanical power at the wheel. The full formula is P = (Fg + Fr + Fa) x v / (1 - drivetrain loss).
Aerodynamic drag follows a square law: doubling your speed quadruples drag force, and the power needed to fight it rises with the cube of speed. On flat ground above roughly 20 km/h, aero drag is the dominant term. Below 20 km/h or on steep climbs, gravity and rolling resistance matter more.
What each input does to your power number
Grade: A 5% climb roughly doubles power demand for most recreational speeds. Even a 1% gradient adds a meaningful constant penalty over long rides.
Position: Moving from tops (CdA ~0.41 m^2) to aerobars (CdA ~0.27 m^2) cuts your drag area by a third. At 40 km/h, that saves about 45-55 W with no fitness change.
Wind: A 20 km/h headwind adds it directly to your effective air speed, making drag scale with (30 + 20)^2 = 2,500 instead of 30^2 = 900 - nearly tripling aero drag.
Surface: Slick tires on concrete have a rolling resistance coefficient (Crr) of about 0.002; knobby tires on sand can reach 0.05, a 25x difference that matters more at low speeds.
Altitude: Air density decreases roughly 1% per 85 m. At 2,000 m, aero drag is about 20% lower than at sea level - a significant free speed on mountain passes.
Power-to-weight ratio and what it means for your riding
Power-to-weight ratio (W/kg) normalises output for body size and is the key metric for climbing. On a 6-8% gradient, a rider producing 4.0 W/kg will comfortably drop one at 3.0 W/kg regardless of absolute power. On flat roads, absolute watts matter far more than W/kg because rolling and aero forces dominate and do not scale with body weight the same way gravity does.
Improving W/kg has two levers: raising power (the harder, longer route) or losing weight (faster per kilogram removed, but with diminishing returns below race weight). Functional threshold power (FTP) - the highest power you can sustain for roughly one hour - is the most widely used benchmark. Most training plans target FTP improvement, and coaches track W/kg at FTP as the primary fitness metric.
Calorie estimation and metabolic efficiency
The human body is roughly 20-25% mechanically efficient when cycling - meaning for every 100 kcal of food energy burned, about 24 kcal appears as useful pedal power. This calculator uses 24% efficiency to estimate metabolic cost: calories per hour = power (W) / 0.24 / 4184 x 3600. Actual efficiency varies with fitness, cadence, and terrain. More importantly, the calculation assumes all power goes to overcoming the three forces modelled here - accessories, stopping, and surges add to real-world burn. Use the calorie figure as a planning estimate, not a precise count.
Power-to-weight ratio categories
| W/kg (FTP) | Category | Typical rider |
|---|---|---|
| < 2.0 | Untrained | Newcomer to structured cycling |
| 2.0 - 2.5 | Recreational | Regular leisure rider |
| 2.5 - 3.2 | Trained | Committed club rider |
| 3.2 - 4.0 | Well-trained | Competitive amateur |
| 4.0 - 5.0 | Elite amateur | Category racer, top amateur |
| 5.0 - 7.0 | Professional | Pro continental or WorldTour rider |
| > 7.0 | World class | Grand tour climber, peak performance |
Approximate W/kg ranges at functional threshold power (FTP) for trained adult cyclists. Values vary by gender, age, and test protocol.
Frequently asked questions
How many watts do I need to ride at 30 km/h on flat ground?
For a typical 75 kg rider on a 9 kg bike, riding in the hoods position on flat asphalt with no wind, you need roughly 135-140 W to maintain 30 km/h. That works out to about 1.8 W/kg. The exact number shifts with your frontal area, tire choice, and how fresh the chain is.
What is a good watts per kilogram for a cyclist?
At functional threshold power (FTP), 2.5 W/kg is a trained recreational rider, 3.2-4.0 W/kg is a competitive amateur, and 5.0+ W/kg is professional level. World-class climbers have hit above 6.5 W/kg in race conditions. For most people, consistently riding 3.0 W/kg at threshold is a meaningful training target.
Why does cycling power increase so steeply with speed?
Aerodynamic drag is proportional to the square of your speed relative to the air. Because power equals force times velocity, the power needed to overcome aero drag grows with the cube of speed. Going from 30 to 40 km/h (33% faster) roughly doubles the power required on flat ground. This is why professional time trial riders optimise every gram of aero drag: even small CdA reductions are worth many watts at high speeds.
How does grade affect power?
Gradient adds a gravity term (Fg = mass x g x sin(theta)) that scales with your total weight and the slope. A 5% climb at 15 km/h triples power compared to flat ground for many riders, since the gravity term now dominates. Descending (negative grade) can produce a negative gravity force that partially or fully offsets rolling and aero drag, reducing or eliminating the need to pedal.
Does altitude change how hard I need to pedal?
Altitude reduces air density, which directly reduces aerodynamic drag. At 2,000 m the air is about 20% less dense than at sea level, so aero drag is 20% lower. On flat fast roads this can save 15-30 W. However, thinner air also means your body delivers less oxygen per breath, so your available power output drops too. The net physiological effect outweighs the aero benefit for most riders, making high-altitude riding feel harder even though the physics require fewer watts for a given speed.
What is CdA and how does riding position change it?
CdA is the product of the drag coefficient (Cd) and the frontal area (A) of the rider-bike system. It is the single most important aerodynamic parameter. Sitting upright on the tops gives a CdA of roughly 0.41 m^2; aggressive TT aerobars bring it to about 0.27 m^2. At 40 km/h, that 34% reduction in CdA saves approximately 50 W of aero drag, the equivalent of a significant fitness improvement.
How accurate is the calorie estimate?
The estimate assumes a cycling metabolic efficiency of 24% and steady-state effort. In practice, efficiency ranges from about 20% to 26% depending on cadence, fitness, terrain, and individual physiology. Sprints, stopping, and starts consume extra energy not captured here. Treat the number as a reasonable ballpark for planning nutrition, but not a precise count.
How can I use this calculator to improve my training?
Use it in two ways. First, set a target speed or event goal (a sportive, a time trial, a Strava segment) and use the calculator to find the wattage required. Then compare that to your current FTP to see how much of a fitness gap you need to close. Second, run what-if scenarios: what does losing 5 kg of body weight save on a mountain stage? How many watts does moving from hoods to aerobars save on a flat TT? These numbers help you prioritise your training and equipment investments.