How Fast Is 400 Watts on a Bike - Speed in mph and 40K

Watts for any speed, and the speed your watts buy

A 165 lb rider on a 20 lb road bike needs about 136 W to hold 18 mph on a flat road with hands on the hoods. Three quarters of that goes into pushing air. This cycling power calculator works out that number for your own weight, bike, position, tires, grade, wind and air, splits it into the forces that eat it, and runs the other way too: type the watts you can hold and it returns your speed, your 10-mile and 40K times, and your climbing rate on a hill.

No power meter needed. The speed from a bike computer or a GPS watch is enough. Pick a stretch where the grade and the wind stayed about the same, and the calculator estimates the watts that speed took.

A ride worked out in six steps

The example below follows the form from top to bottom. Each step says what to type and what the choice changes.

Choose the direction - "I know my speed" returns watts; "I know my watts" returns speed. Riders without a power meter start with the first. Take the average speed of one segment, not the whole ride, because stops and hills blur it.
Choose US or metric units - US units ask for mph, pounds, feet and °F. The result echoes the same units and adds kilograms where they matter, since watts per kilogram is the figure every training plan quotes.
Type both weights - your body weight, then the bike with everything on it: bottles, saddle bag, lights, shoes and helmet. A carbon road bike usually weighs 15 to 20 lb ready to ride, an aluminum gravel bike 20 to 25 lb, a loaded commuter 30 lb or more. In the example: 165 lb and 20 lb.
Describe the road - the grade in percent, minus for a descent, and the wind along the road in mph, plus for a headwind and minus for a tailwind. A wind at 60 degrees to the road counts about half its speed. Leave both empty for a flat road on a still day.
Pick a position and tires - the position sets CdA, the drag area of rider and bike: 0.40 sitting up on the tops, 0.33 on the hoods, 0.30 in the drops, 0.26 on aero bars. Tires set Crr, the rolling resistance coefficient: 0.003 for race tires on smooth asphalt, 0.004 for ordinary road tires, 0.0066 for touring tires. If you have measured values from an aero test or a roller test, choose "my own" and type them.
Add the air if it was unusual - empty boxes mean 68 °F at sea level. Cold air is denser and costs more; thin air at altitude costs less. Then read the result: the watts or the speed at the top, the split into air, tires, climbing and drivetrain below, and a table with the same ride in the other three positions.

Where 136 watts go on a flat road

The calculator uses the road-cycling model published by James Martin and colleagues, who checked it against power meter readings and found the predicted and measured watts in close agreement (R² of 0.97, a standard error of about 3 W). It adds up four forces and divides by the efficiency of the chain and gears:

Air resistance grows with the square of the speed through the air, and the power needed to beat it with the cube of your speed on a still day. The drag area is CdA plus a small extra 0.0044 m² for the spinning wheels.
Rolling resistance is Crr times total weight, so its power cost grows only in step with speed.
Wheel bearings take a few watts at any normal speed.
Gravity is total weight times the sine of the climbing angle. It is zero on a flat road and helps on a descent.

For the example ride that comes to 105 W of air resistance (77%), 28 W for tires and bearings (20%) and 3 W lost in the drivetrain, which the model treats as 97.7% efficient. Dropping from the hoods to aero bars saves 22 W at the same 18 mph; sitting up on the tops costs 22 W more.

Watts on a flat road, 165 lb rider, 20 lb bike, road tires

Position	12 mph	15 mph	18 mph	20 mph	22 mph	25 mph
Tops, upright (0.40)	57 W	99 W	158 W	210 W	272 W	388 W
Hoods (0.33)	51 W	86 W	136 W	179 W	231 W	327 W
Drops (0.30)	48 W	80 W	126 W	166 W	213 W	302 W
Aero bars (0.26)	44 W	73 W	113 W	148 W	190 W	267 W

Read across a row and the cube law shows: going from 20 to 25 mph on the hoods takes 25% more speed and 83% more power (179 W to 327 W).

Going the other way: what 100 to 300 watts are worth

With a power meter, or with the watts a training plan prescribes, switch the direction and the calculator finds the speed by searching for the point where the four forces use up exactly your watts. The table uses the same rider and bike on a flat road with no wind.

Flat-road speed for a steady power

Power	Tops	Hoods	Drops	Aero bars
100 W	15.1 mph	16.0 mph	16.4 mph	17.1 mph
150 W	17.6 mph	18.7 mph	19.2 mph	20.1 mph
200 W	19.6 mph	20.9 mph	21.5 mph	22.4 mph
250 W	21.3 mph	22.7 mph	23.3 mph	24.4 mph
300 W	22.8 mph	24.2 mph	24.9 mph	26.1 mph

Doubling the power from 150 to 300 W on the hoods adds only 5.5 mph. The same 200 W rides a 40K in 1:11:31 on the hoods and 1:06:28 on aero bars, five minutes for a change of position alone. Position is free speed.

On a climb, weight takes over from air

As the road tilts up, speed drops and the air stops mattering. Gravity becomes the main cost, so every pound counts. The result adds VAM, the vertical meters climbed per hour (from the Italian term for average climbing speed), which riders use to compare climbs of different lengths.

200 W on the hoods, 165 lb rider, 20 lb bike

Grade	Speed	Share for gravity	VAM
2%	15.1 mph	55%	485 m/h (1,590 ft/h)
4%	10.7 mph	79%	687 m/h (2,254 ft/h)
6%	7.9 mph	87%	763 m/h (2,503 ft/h)
8%	6.2 mph	91%	795 m/h (2,607 ft/h)
10%	5.1 mph	93%	811 m/h (2,660 ft/h)

On a 6% climb at 200 W, a bike 5 lb lighter lifts the speed from 7.9 to 8.1 mph, and losing 10 lb of body weight lifts it to 8.3 mph. On the flat at the same 200 W the lighter bike changes nothing you can see: the speed stays at 20.9 mph.

Wind, tires and air: the smaller settings

Wind

18 mph into a 10 mph headwind takes 288 W, more than twice the still-air 136 W. With the same wind behind you it takes only 50 W. Hold 200 W instead and the speed swings from 15.1 mph into the wind to 27.5 mph with it.

Tires

At 18 mph race tires need 129 W, ordinary road tires 136 W and touring tires 153 W. The gap of 24 W is close to what dropping from the hoods to aero bars saves.

Air

The same 18 mph costs 142 W at 40 °F, 131 W at 95 °F and 117 W at 5,280 ft, the elevation of Denver. Thin air is why hour records are attempted at altitude.

Before you trust the watts

How accurate is a power estimate without a power meter?

The physics is accurate; the inputs are the weak point. CdA is the largest unknown, because two riders on the hoods can differ by 0.05 m², which at 18 mph is a gap of about 16 W. Wind you did not notice is the second. So treat one estimate as approximate, pick calm days and steady segments, and average several of them rather than trusting one.

Why does the result say 0 W on a descent?

At 25 mph on a 5% descent gravity supplies more power than the air and tires take, so the bike accelerates without pedaling. The result says how large the surplus is (140 W for the example rider) instead of printing a negative power. On a gentler 3% descent the same speed needs a real 45 W.

Is the result the same as the number on my power meter?

It is the power at the pedals, which is what a crank or pedal power meter reports. A hub power meter sits after the chain and reads about 2% lower, the drivetrain loss shown in its own tile. A smart trainer also measures after the drivetrain.

Does the model handle accelerating, cornering or drafting?

No. It describes a steady speed on a straight road, which is why it asks for a segment rather than a whole ride. Riding in a group lowers the air resistance of the riders behind the leader considerably; to estimate that, type a smaller custom CdA.

What is a good watts per kilogram figure?

The 136 W of the example is 1.81 W/kg, an easy endurance pace for a fit rider. Sustained power over an hour is a separate question: it depends on fitness, not on the road, and is measured with a functional threshold power test.

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How fast is 400 watts on a bike on a flat road?

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