Cycling Wattage Calculator
Enter your weight, bike weight, target speed and the road ahead — get the watts you need, split into aerodynamic drag, rolling resistance, gravity and drivetrain loss. Flip to reverse mode to find the speed a given wattage buys you.
Watts needed vs speed and gradient
Reference table for a 75 kg rider + 9 kg bike, in the drops (CdA 0.30), training tires (Crr 0.005), calm air, sea level. Downhill cells that reach 0 W mean you can coast.
| Speed (km/h) | -2% | 0% | 2% | 5% | 8% | 10% |
|---|---|---|---|---|---|---|
| 10 | 0 | 16 | 62 | 132 | 201 | 247 |
| 15 | 0 | 31 | 101 | 205 | 309 | 378 |
| 20 | 0 | 55 | 148 | 287 | 426 | 517 |
| 25 | 0 | 92 | 208 | 381 | 555 | 669 |
| 30 | 3 | 143 | 282 | 491 | 698 | 836 |
| 35 | 49 | 212 | 375 | 618 | 860 | 1021 |
| 40 | 117 | 302 | 488 | 766 | 1043 | 1227 |
| 45 | 208 | 417 | 626 | 939 | 1250 | 1457 |
| 50 | 326 | 558 | 790 | 1138 | 1484 | 1713 |
Reference: riding positions (CdA)
| Riding position | Typical CdA (m²) |
|---|---|
| Upright (commuter bike) | 0.40 |
| Riding on the tops | 0.35 |
| Riding on the hoods | 0.32 |
| In the drops | 0.30 |
| Aero hoods (elbows bent) | 0.27 |
| Time-trial / tri bars | 0.24 |
| Full aero tuck | 0.20 |
Values are representative ranges consistent with published wind-tunnel and field-test data (e.g. Martin et al., 1998). Your actual CdA depends on body size, clothing and bike setup — pick the row closest to how you ride.
Reference: tires (Crr)
| Tires / surface | Typical Crr |
|---|---|
| Racing slicks, smooth asphalt | 0.004 |
| Training tires | 0.006 |
| Gravel tires | 0.012 |
| MTB knobby tires, off-road | 0.020 |
Ranges follow independent lab rolling-resistance tests: fast racing slicks cluster near 0.004, training tires near 0.006, gravel near 0.012 and knobby MTB tires near 0.020.
The physics behind it: where your watts go
When you ride at a steady speed on a constant gradient, every watt your legs produce is spent fighting three resistive forces. Coaches and racing teams use the same model this calculator implements (validated for road cycling by Martin et al., 1998): the force you must overcome is the sum of aerodynamic drag, rolling resistance and the gravity component along the slope. Multiply that total force by your speed and you get the mechanical power at the wheel; divide by your drivetrain efficiency and you get the power your legs must produce.
The four pieces of the equation
1. Aerodynamic drag — the cube law. Drag force is ½ρ·CdA·v², where ρ is air density (1.225 kg/m³ at sea level), CdA is your drag area and v is your speed relative to the air — a headwind adds directly to it. Because power is force times speed, aerodynamic power grows with the cube of speed. That is why 30 km/h takes about 143 W for a 75 kg rider in the drops, while 40 km/h takes roughly 300 W: a 33% speed increase demands more than double the watts. Above ~25 km/h on flat ground, aero drag is 70–90% of everything you fight, which is why position (CdA) is the single biggest lever you own.
2. Rolling resistance — linear and honest. Rolling force is Crr·m·g: tire coefficient times system weight times gravity. It scales linearly with speed and weight, so good tires are free speed at any pace. Racing slicks (Crr ≈ 0.004) versus knobby MTB tires (Crr ≈ 0.020) is a five-fold difference in this term — at 30 km/h on flat ground that is roughly 136 W versus 247 W for the same rider.
3. Gravity — the climber's tax. On a slope the gravity force along the road is m·g·sin(θ), which for cycling gradients is almost exactly m·g·grade. Climbing power is therefore nearly proportional to weight times speed times gradient: this is why watts per kilogram (W/kg) is the currency of climbing, and why shaving a kilogram off the system matters far more at 8% than at 0%.
4. Drivetrain loss. Chain, jockey wheels and bearings eat a small cut — typically 1.5–3% (this calculator defaults to 1.5%, consistent with published chain-efficiency measurements). Wheel power divided by (1 − loss) gives pedal power.
Worked example 1: flat road at 30 km/h
Take a 75 kg rider on a 9 kg bike (84 kg system), riding 30 km/h on flat ground, calm air, in the drops (CdA = 0.30 m²), on training tires (Crr = 0.005), at sea level:
- Speed v = 30 / 3.6 = 8.333 m/s.
- Rolling: F = 0.005 × 84 × 9.80665 = 4.12 N → P = 4.12 × 8.333 = 34.3 W.
- Aero: F = ½ × 1.225 × 0.30 × 8.333² = 12.76 N → P = 12.76 × 8.333 = 106.3 W.
- Gravity on flat ground: 0 W.
- Subtotal at the wheel: 140.7 W; ÷ (1 − 0.015) for drivetrain loss → 142.8 W at the pedals.
- Power-to-weight: 142.8 ÷ 75 = 1.90 W/kg. Gross energy burn ≈ 512 kcal/hour (assuming 24% metabolic efficiency).
Worked example 2: 8% climb at 20 km/h
Same rider, now climbing an 8% grade at 20 km/h (5.556 m/s):
- Gravity: F = 84 × 9.80665 × 0.0797 = 65.68 N → P = 65.68 × 5.556 = 364.9 W — now 86% of the total.
- Rolling: 22.8 W. Aero: only 31.5 W (speed is low, so drag nearly vanishes).
- Wheel subtotal 419.3 W → 425.7 W at the pedals, or 5.68 W/kg.
Compare the two: on the flat, aero is three-quarters of your effort; on the climb, gravity is six-sevenths. Training and equipment choices should follow the terrain you actually ride.
Practical tips: spend watts where they count
- On flat and fast rides, fix position before buying watts. Dropping from upright (CdA ≈ 0.40) to the drops (0.30) cuts aero drag by 25% — at 35 km/h that is worth roughly 60 W, more than most training blocks add in a season.
- Tires are the cheapest watts in cycling. A slow tire can cost 15–25 W at 30 km/h versus a fast one. Check independent rolling-resistance lab tests rather than marketing claims.
- On climbs, weight rules. Every kilogram of system weight costs about g·v·grade watts — at 15 km/h up 8% that is ~3.3 W per kilo. Rider weight dominates bike weight, so body composition beats exotic frames.
- Pace climbs steadily. Because gravity power is linear in speed, surging above your sustainable W/kg burns matches you cannot get back; the model above shows exactly what a steady pace costs.
- Respect the wind. A 10 km/h headwind at 30 km/h raises the bill from ~143 W to ~227 W. In a bunch, drafting saves 25–40% — the physics equivalent of a strong tailwind.
- Altitude is real but small. At 2000 m the air is ~10% thinner (ρ ≈ 1.01 kg/m³), so our flat-30 example drops from ~143 W to ~124 W — but your body's oxygen delivery drops too, which this model does not cover.
Frequently asked questions
What is cycling wattage?1
Cycling wattage is the mechanical power you produce with your legs, measured in watts (1 watt = 1 joule per second). It is the most objective measure of cycling effort because, unlike heart rate, it is not affected by heat, fatigue, caffeine or excitement — 200 W is 200 W whether you feel fresh or destroyed.
How does this cycling wattage calculator estimate power?2
It sums the three resistive forces — aerodynamic drag (0.5 x air density x CdA x relative wind speed squared), rolling resistance (Crr x mass x gravity) and the gravity component on slopes (mass x gravity x gradient) — multiplies the total force by your speed, then divides by drivetrain efficiency. It is the same physics model used by coaches, racing teams and tools like bikecalculator.com.
How many watts does it take to cycle 30 km/h on flat ground?3
About 143 W for a 75 kg rider on a 9 kg bike in the drops with no wind at sea level. Rough benchmarks for the same rider: 25 km/h takes ~90 W, 35 km/h ~210 W and 40 km/h ~300 W — notice how the cost accelerates because aerodynamic power grows with the cube of speed.
Why does going faster cost so much more power?4
Aerodynamic drag force grows with the square of your speed, and power is force times speed, so aero power grows with the cube of speed. Doubling your speed from 20 to 40 km/h multiplies the aero watts roughly eight-fold. Above ~25 km/h on flat ground, aero drag is 70-90% of total resistance.
How does gradient change the watts needed?5
Climbing adds a gravity term of roughly mass x gravity x grade x speed. An 8% grade at 20 km/h costs our example rider about 426 W (5.68 W/kg), versus 55 W on the flat at the same speed. Each extra percentage point of gradient at 20 km/h costs roughly 46 W for an 84 kg system.
What is CdA and which value should I pick?6
CdA (drag area, in m^2) combines your frontal area and how slippery your shape is — it is the single most important number for flat speed. Typical values: upright commuter 0.40, tops 0.35, hoods 0.32, drops 0.30, aero hoods 0.27, time-trial bars 0.24, full tuck 0.20. These are representative ranges consistent with published wind-tunnel and field-test data; pick the row that matches how you actually ride.
How much do tires matter (Crr)?7
A lot more than most riders expect. Rolling-resistance coefficient Crr multiplies your full system weight, so it bites on every terrain. Racing slicks (Crr ~0.004) versus knobby MTB tires (Crr ~0.020) is the difference between ~136 W and ~247 W at 30 km/h flat for the same rider — a bigger gap than most wheel upgrades.
Does altitude change the watts required?8
Yes, modestly: thinner air means less drag. At 2000 m air density is about 1.01 kg/m^3 versus 1.225 at sea level, so the flat-30 example drops from ~143 W to ~124 W. Note this is purely aerodynamic — your body's ability to produce power also falls with altitude, which this model does not estimate.
What about headwind and tailwind?9
Wind enters the aero term squared via your speed relative to the air: a 10 km/h headwind while you ride 30 km/h makes the air hit you at 40 km/h. Our example rider's bill jumps from ~143 W to ~227 W. Enter headwind as a positive number and tailwind as a negative number.
How accurate is this compared to a power meter?10
Typically within about 5-10% for steady riding if your CdA and Crr guesses are honest — the same ballpark as virtual-power estimates from speed sensors. It cannot capture accelerations, cornering, rough-road losses or drafting. Treat it as a planning and what-if tool; for training zones, measure with a real power meter and use our cycling zones calculator.
What does W/kg mean and what is a good value?11
Watts per kilogram of rider body mass — the currency of climbing. Very rough sustainable-power benchmarks: under 2 W/kg untrained, 2-3 recreational, 3-4 trained amateur, 4-5 advanced, 5+ elite. Our 8%-climb example at 5.68 W/kg sits at the top of the trained-amateur range. Use the power-to-weight ratio tool to convert any measured watts into W/kg.
How is this different from your bike speed and cycling zones tools?12
This tool estimates the watts needed to hold a speed (physics model). The bike speed calculator does the inverse kinematics — speed, pace and time from distance. The cycling zones calculator maps a measured FTP into Coggan training zones. The power-to-weight ratio tool converts watts you already measured into W/kg, hp/ton and related units.
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