Bike Calculator is an engineering model - that is, you feed it relevant parameters, and it makes predictions about the system. The system is you - the engine - supplying the power to resist the three major forces involved in cycling - gravity, rolling resistance and aerodynamic resistance.

For any set of parameters, there is a fixed relationship between power and speed. If you enter one of these two values, Bike Calculator calculates the other. Or slide the red dot back and forth to do both at the same time.

See the "Examples" page for some ideas for using Bike Calculator. A tool such as this becomes more useful with experience. It is especially important to have a sense for the accuracy of the predictions - they are not exact, and may or may not tell you what you want to know. In general, the best predictions come from exploring the effects of small changes after first settling on parameters that seem to give reasonable results.

- Power
- It is common to express this in watts, which is a unit of power, just like horsepower (1 HP is 746 watts). Power is the amount of energy supplied per second, but it also works out to be force times speed. You can enter your power numerically in the box, or move the slider (may not work in all browsers).
- Velocity
- Enter a speed numerically and the required power will be calculated. Adjusting the slider gives the appearance of changing both at the same time. You can choose kph or mph with the "Units" menu.
- Slider
- This adjusts both power and speed as you move it, and is the best way to use Bike Calculator. It relies upon some serious scripting that may not work in all browsers. It goes from zero to 500 watts, but you can enter higher power numerically.
- Units
- Metric or U.S., changing units will preserve the entered values (internally, Bike Calculator uses metric units).
- Rider weight
- Enter your weight including your clothing, helmet and shoes.
- Bicycle weight
- Enter the weight of your bike and anything attached to it. It's only important that the total weight is correct.
- Tires
- You are setting the coefficient of rolling resistance with this choice. True values are hard to come by - published measurements do not reflect real road surfaces.
- Position
- You are setting an overall aerodynamic factor here, both the shape factor Cd and frontal area. Bartops means "sitting up" on a road bike. Bar ends is the usual position for hybrid or mountain bikes. Hoods means your hands are clasping the brake hoods - the usual road bike position.
- Grade
- Grade is defined as the increase in elevation divided by the distance traveled. A descent is indicated with a negative number.
- Head wind
- Wind has a large influence on the system, but is not easily considered except for wind directly ahead or behind (enter a negative value for tail winds).
- Distance
- Distance only effects the "Other predictions" at the bottom of the calculator. If you enter a value for one of those predictions, distance will change so as to produce that prediction while maintaining all other parameters constant.
- Temperature
- This is needed to compute air density, a factor in aerodynamic resistance. Note that the possible physiological effects on power output is not considered in Bike Calculator - watts is watts, however difficult they may be to produce.
- Elevation
- Distance above sea level in either meters or feet. This is needed to compute air density, a factor in aerodynamic resistance.
- Time
- This is a simple result of the entered or predicted speed and the distance. If you enter a value, distance will be changed to match.
- Calories (food energy expenditure)
- This is figured by considering the mechanical energy produced by the rider and assuming a 25% conversion efficiency. If you enter a value, distance will be changed to match.
- Weight loss
- This is figured directly from the Calories by dividing by the energy/density of fat. If you enter a value, distance will be changed to match.