How to use the calculator
- Choose what you want to know. With Speed from my watts you enter the power you can hold and we tell you how fast you go. With Watts for a speed you enter the speed and we tell you how much power it takes.
- Enter your weight and the weight of the bike with gear. Count clothing, full bottles and bags. For the bike, figure 15-20 lb (7-9 kg) for a road bike, 20-24 lb (9-11 kg) for gravel and 26-33 lb (12-15 kg) for a mountain bike.
- Pick your position. On the flat, it changes the result more than anything else. If you’re not sure which is yours, hoods is the most common on a road bike.
- Pick the surface. A fast tire on smooth pavement rolls with almost half the resistance of a puncture-resistant tire on broken pavement.
- Add grade, wind, altitude and temperature to fine-tune. Grade is negative on descents. Wind is what hits you head-on or from behind; count a crosswind as roughly half.
The result shows your speed or watts, your watts per kilogram (W/kg), your calories per hour, where each watt goes, the power-speed curve and a table with the watts each position would need. If you ride with a power meter, save your weight in My Bike and you won’t have to enter it again in the other calculators.
How we calculate it
We use the model by Martin and colleagues (1998), the standard for estimating power on the road. They validated it against power meters on the bike itself: the model and the measured power agreed with a typical error of about 2.7 W.
The power coming out of your legs is what it takes to beat air, rolling resistance and gravity, plus what the drivetrain loses:
P = [ ½ · ρ · CdA · (v + vwind)² · v + Crr · m · g · cos θ · v + m · g · sin θ · v ] ÷ (1 − losses)
- ρ is air density. We calculate it from altitude and temperature using the standard atmosphere: 1.20 kg/m³ at sea level at 68 °F (20 °C), 1.11 at 3,281 ft (1,000 m) and 0.98 at 6,562 ft (2,000 m) at 50 °F (10 °C).
- CdA is your frontal area times your drag coefficient, in m². It runs from 0.55 sitting upright to 0.21 in a well-dialed time trial position.
- v is your speed and vwind is wind speed (positive for a headwind, negative for a tailwind), in m/s.
- Crr is the tire’s rolling resistance coefficient on that surface: 0.0035 for a fast tire on good pavement, 0.0045 on average pavement and up to 0.016 on singletrack.
- m is total mass (you, the bike and your gear), g is gravity (9.81 m/s²) and θ is the angle of the grade.
- Losses: a clean drivetrain loses 2-3%. We use 2.5% by default.
To go from watts to speed we solve the same equation backward, by successive approximation, because it has no simple direct solution. Calories per hour come from your watts at 24% efficiency: of every 100 kcal you burn, about 24 reach the pedals (we explain it in the cycling calorie calculator).
Martin’s original model also includes bearing friction and the energy needed to accelerate. Here we assume a steady speed, and bearing friction is folded into the losses.
Watts needed at 15, 18, 22 and 25 mph
A 165 lb (75 kg) rider with 18 lb (8 kg) of bike and gear, on the flat, no wind, on average pavement (Crr 0.0045), at sea level and 68 °F (20 °C).
| Position | CdA (m²) | 15 mph (24 km/h) | 18 mph (29 km/h) | 22 mph (35 km/h) | 25 mph (40 km/h) |
|---|---|---|---|---|---|
| Upright (city) | 0.55 | 128 W | 207 W | 360 W | 516 W |
| MTB or gravel, hands up | 0.48 | 115 W | 185 W | 319 W | 456 W |
| Road, hands on the tops | 0.40 | 100 W | 159 W | 272 W | 387 W |
| Road, hands on the hoods | 0.33 | 87 W | 136 W | 231 W | 326 W |
| Road, hands in the drops | 0.30 | 81 W | 127 W | 213 W | 301 W |
| Aero bars | 0.25 | 72 W | 111 W | 184 W | 257 W |
| Dialed time trial | 0.21 | 64 W | 98 W | 160 W | 223 W |
For more speeds, positions, and gravel and mountain bikes, see the chart of watts needed for each speed on flat ground.
Notice two things. First: from 18 to 25 mph the watts more than double. Second: moving your hands from the hoods to the drops saves about 26 W at 25 mph, more than any expensive set of wheels.
Where every watt goes
On the flat, air rules. At 18 mph (29 km/h) on the hoods, of 136 W, about 104 W (76%) go into the air, 29 W into rolling resistance and about 3 W are lost in the chain. At 25 mph (40 km/h), air already takes 85%.
On a climb, everything changes. At 250 W, same rider:
| Grade | Speed | Air | Grade (gravity) |
|---|---|---|---|
| 0% | 22.7 mph (36.5 km/h) | 83% | 0% |
| 2% | 17.2 mph (27.7 km/h) | 36% | 50% |
| 4% | 12.8 mph (20.6 km/h) | 15% | 74% |
| 6% | 9.7 mph (15.6 km/h) | 7% | 85% |
| 8% | 7.7 mph (12.4 km/h) | 3% | 89% |
| 10% | 6.3 mph (10.2 km/h) | 2% | 92% |
That’s why getting low pays off on the flat, while on climbs weight is what counts. If you’re into climbing, the climb time calculator runs this same calculation section by section.
Real-world examples
- Is riding in the drops worth it? At 200 W you go from 20.8 mph (33.6 km/h) on the hoods to 21.5 mph (34.6 km/h) in the drops, and 22.7 mph (36.6 km/h) on aero bars. Over an hour, that’s 0.6 and 1.9 miles more.
- Tires matter. At 200 W on the hoods, you do 21.2 mph (34.0 km/h) on a fast tire on good pavement and 20.4 mph (32.8 km/h) on a puncture-resistant tire on broken pavement. Almost 1 mph without changing anything else.
- Headwinds. Into a 10 mph (16 km/h) headwind, the same 200 W leaves you at 15.2 mph (24.4 km/h). With a 10 mph tailwind, you’re at 27.5 mph (44.3 km/h).
- Cold air slows you down. Cold air is denser: riding 22 mph (35 km/h) takes about 245 W at 32 °F (0 °C) and 224 W at 86 °F (30 °C).
- Descents. On a 5% descent without pedaling, this rider would reach about 30.5 mph (49.1 km/h) on the hoods. The calculator tells you when you don’t need to pedal.
Common mistakes
- Entering peak power instead of average power. To know how fast you’ll ride, enter the power you can hold for hours, not a sprint. If you know your FTP (the power you can sustain for about an hour), a long ride is usually at 65-75% of it. You can work it out with the FTP and power zones calculator.
- Forgetting the weight of your gear. Bottles, tools, clothing and shoes add 4.4-8.8 lb (2-4 kg). On the flat it barely matters, but on climbs it does.
- Trusting CdA to the decimal. The values for each position are typical. Two riders of different sizes in the same position can differ by 15-20%. If you’ve measured your CdA in a wind tunnel, on a velodrome or with a field-testing app, enter it under More settings.
- Comparing with your bike computer on a group ride. Drafting saves 25-40% of your effort, depending on the gap and the group. The model assumes you’re riding alone.
What it’s good for and what it isn’t
It’s good for understanding what each factor costs (position, weight, tires, wind), planning your pace and comparing scenarios. It doesn’t replace a power meter: real wind changes at every turn, so does the pavement, and your CdA changes when you move. Treat it as a solid estimate, with a margin of ±5-10% in real conditions.
Frequently asked questions
How much does a headwind slow you down on a bike?
A lot. At 200 W on the flat, a 165 lb (75 kg) rider on the hoods does 20.8 mph (33.6 km/h) with no wind, 17.9 mph (28.7 km/h) into a 5 mph (8 km/h) headwind and 15.2 mph (24.4 km/h) into a 10 mph (16 km/h) one. Holding 18 mph against a 5 mph headwind takes about 204 W instead of 136.
How many watts does an average cyclist produce?
A fit recreational rider can hold 150-250 W for one or two hours, which on a road bike on the flat is about 18.5-22.5 mph (30-36 km/h). A pro in a flat time trial holds 400-450 W for close to an hour.
Why is it so hard to go from 22 to 25 mph on a bike?
Because the power needed to beat air resistance grows with the cube of your speed. Going from 22 to 25 mph (35 to 40 km/h), about 14% faster, takes around 96 W more: roughly 41% extra power.
Are you faster on a bike at altitude?
Yes, at the same watts: at 6,560 ft (2,000 m) the air is about 22% less dense, and 200 W would take you from 20.8 to 22.5 mph (33.6 to 36.2 km/h). But your body also produces less power with less oxygen, so the real-world gain is smaller.