How fast is 200 watts?
It depends almost entirely on how much air you're pushing. The same watts on the same flat road, at four riding positions:
| Power | Upright / tops | Road, on the hoods | Road, in the drops | TT / clip-on aero bars |
|---|---|---|---|---|
| 150 W | 17.5 mph | 18.8 mph | 19.5 mph | 20.7 mph |
| 200 W | 19.5 mph | 20.9 mph | 21.8 mph | 23.2 mph |
| 250 W | 21.2 mph | 22.8 mph | 23.7 mph | 25.3 mph |
| 300 W | 22.7 mph | 24.4 mph | 25.4 mph | 27.0 mph |
Flat road, still air, 165 lb rider + 19 lb bike, quality road tires — computed by the calculator above. The spread across one row is the whole aerodynamics argument in a single line.
At 22.8 mph on the flat, roughly four of every five watts fight the air; rolling resistance takes most of the rest, and the drivetrain skims ~2.5%. Gravity only enters when the road tilts — and then it takes over almost completely. The breakdown bars in the calculator show the split for your exact ride, which is the right way to decide whether your next dollar buys aero, weight, or tires.
How many watts does it take to ride 20 mph?
| Steady speed | On the hoods | In the drops |
|---|---|---|
| 15 mph | 86 W | 78 W |
| 18 mph | 135 W | 122 W |
| 20 mph | 177 W | 159 W |
| 25 mph | 322 W | 287 W |
Same rider and flat road as above. Note the curve: 25 mph costs roughly double 20 mph — air resistance grows with the cube of speed, which is why the last few mph are always the expensive ones.
How much does a headwind slow you down?
More than intuition says — and it does not cancel out on the way back. The air term uses your speed through the air, so wind moves the whole curve. The same rider at 200 W on the hoods:
| Wind along the road | Speed at 200 W |
|---|---|
| 10 mph tailwind | 27.6 mph |
| 5 mph tailwind | 24.2 mph |
| Still air | 20.9 mph |
| 5 mph headwind | 18.0 mph |
| 10 mph headwind | 15.2 mph |
| 15 mph headwind | 12.8 mph |
Wind is the component along your direction of travel; a 10 mph crosswind at 45° counts as about 7 mph of headwind here. Flat road, 165 lb rider + 19 lb bike.
A 10 mph headwind takes 5.7 mph off, and the same tailwind adds 6.7 — the tailwind's gift is slightly bigger in mph. The out-and-back still loses, because you spend far more minutes in the slow half than in the fast one: at 200 W the round trip in a 10 mph wind takes about 7% longer than the same miles in still air. It's why windy days never average out, and why a group ride's value is measured in mph: sitting in a wheel removes 25–40% of the air term, the equivalent of a permanent tailwind you can't buy.
Does gravel or a trail really cost more watts?
| Surface | Crr | Speed at 200 W | Watts to hold 15 mph |
|---|---|---|---|
| Paved road (quality road tire) | 0.0045 | 20.9 mph | 86 W |
| Packed gravel (40 mm tire) | 0.01 | 19.3 mph | 116 W |
| Trail (2.3″ knobby) | 0.018 | 16.9 mph | 161 W |
Flat, still air, hoods. Rolling-resistance coefficients are representative; real surfaces and tires vary by ±50%, and pressure moves the number — the tire pressure calculator is where that half of the equation lives.
On tarmac, rolling resistance is a sideshow behind the air. On gravel it becomes a co-star — the same 200 W is worth several mph less — and at trail speeds it is the main event, because you're going too slowly for the air to matter much. Which is the physics behind the gravel-racing obsession with tire casings and pressure: on that surface, tires are the aero upgrade.
What does a climb do to your speed?
| Grade | Speed at 250 W | Biggest cost |
|---|---|---|
| 0% | 22.8 mph | air 83% |
| 3% | 14.9 mph | gravity 65% |
| 6% | 9.7 mph | gravity 85% |
| 9% | 7.0 mph | gravity 91% |
| 12% | 5.4 mph | gravity 93% |
The crossover is the story: somewhere around 4–5% the enemy switches from air to gravity — which is why weight matters on climbs and barely on the flat. For time up a specific climb, the climb time calculator runs this over real distances (Alpe d'Huez included).
Holding 22.8 mph on the flat: moving from the hoods to the drops saves about 26 W. Adding 2 kg (bottles, a heavier frame) costs about 1 W. On flat roads, position is worth roughly 26× more than a wheelset's worth of weight — the free upgrade is learning to ride lower comfortably. On climbs the ledger flips; the climb tools price that side.
Why is my average speed lower than the calculator says?
Because the calculator solves a steady state — constant power on a constant road with no stops — and a ride is nothing of the kind. It has coasting, corners, traffic lights, descents where you stop pedaling at 35 mph and climbs where 250 W means 8 mph, and the minutes spent on the slow parts dominate the average. Two rules the physics supports: a route's average speed is dragged toward the speed of its slowest sections, never lifted by its fastest; and the same watts give a lower average on a hilly loop than on a flat one, even though every climb comes with a descent, because you can't spend on the way down the time you lost on the way up. For a whole-ride figure use the speed · distance · time calculator; for what the watts buy on one stretch of road, this page.
How the model works — and what it assumes
This is the validated cycling power equation (Martin et al., Journal of Applied Biomechanics, 1998): power balances gravity (mass × grade), rolling resistance (Crr — 0.0045 for road tires, 0.01 gravel, 0.018 trail), and aerodynamic drag (CdA by position, air density 1.225 kg/m³ at sea level), divided by drivetrain efficiency (97.6%). In symbols: P = [m·g·v·(sin θ + Crr·cos θ) + ½·ρ·CdA·(v + w)²·v] ÷ η, where w is the headwind — and because v appears cubed, "watts for a speed" is one line of arithmetic while "speed for watts" has to be solved numerically, which the calculator does by bisection. Assumptions worth knowing: steady speed, no drafting (a wheel to follow cuts your aero cost 25–40%), sea-level air, and representative CdA values — individuals vary ±15–20%, which is why real-world speeds spread around any calculator's answer. The physics is exact; the inputs are estimates. Sanity check we run in our test suite: 6 W/kg up Alpe d'Huez computes to ~40 minutes, which is precisely what it takes the professionals.