Why is going faster so much harder?

Aerodynamic drag rises with the cube of speed. Going from 30 to 33 km/h on the flat needs about a third more power, not ten percent.

Do aerodynamics matter on a climb?

Much less. On a steep gradient at low speed, gravity dominates and weight matters more than drag.

Health & Fitnessfitness

Cycling Power and Speed Calculator

Power required to ride at a given speed. Aerodynamic drag rises with the cube of speed, so going ten percent faster on the flat needs about a third more power.

Also called: bike power calculator, watts needed cycling.

Power required

146.1

146.1 W at 30 km/h: 113.4 W against air, 32.7 W rolling and 0 W climbing. Air is 77.63% of the total. Aerodynamic power rises with the cube of speed, so a small increase in speed costs a large increase in watts.

Aerodynamic

113.4

Rolling resistance

32.7

Against gravity

Aero share

77.63

Watts per kilogram

2.03

Power to go 5 km/h faster

218.3

On the cube law

Aerodynamic power rises with the cube of speed, so a small increase in speed costs a large increase in watts.

How this is calculated

Method and background

This is what the calculation gives for the numbers you entered. It is an estimate, not advice, and it knows nothing about your situation beyond those numbers. Rules for United States change on a published schedule; the effective date is shown on every rule-based tool.

How this is calculated

Three forces oppose a cyclist: air resistance, rolling resistance and gravity on a climb. Air resistance rises with the cube of speed, which is the dominant term above about 25 km/h and the reason a small speed increase costs so much power. Below that, and on any significant climb, gravity and rolling resistance dominate, which is why aerodynamics matters far less on a steep hill than on the flat.

aerodynamic drag rises with the cube of speed, which is why going faster gets disproportionately harder

v: Speed
C_dA: Drag area

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

30 km/h on the flat

Speed: 30 km/h
Rider mass: 72 kg
Bike mass: 8 kg
Gradient: 0 %
Drag area CdA: 0.32 m²
Rolling resistance: 0.01

Power required146.1

0.5 x 1.225 x 0.32 x 8.333^3

Open this example

a climb adds a gravity term

Speed: 15 km/h
Rider mass: 72 kg
Bike mass: 8 kg
Gradient: 8 %
Drag area CdA: 0.32 m²
Rolling resistance: 0.01

Power required291.2

boundary: aerodynamics barely matters on a steep climb

Open this example

Method and limits

What it assumes

Still air at sea level density, and no drivetrain losses.

What it deliberately does not model

Wind changes the aerodynamic term substantially and is not modelled.
Drivetrain losses of two to three percent are excluded.
CdA depends on position and clothing far more than on the bike.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Why is going faster so much harder?: Aerodynamic drag rises with the cube of speed. Going from 30 to 33 km/h on the flat needs about a third more power, not ten percent.
Do aerodynamics matter on a climb?: Much less. On a steep gradient at low speed, gravity dominates and weight matters more than drag.