# How Does Zwift Determine My Speed?

How Does Zwift Determine My Speed?

For Wheel-on Classic Trainers:
Zwift takes the wheel speed broadcast by your speed sensor and the power curve (see example below) of the trainer you select on the Pairing Screen and converts that to virtual watts, also known as “zPower”.

Your in-game speed is determined by:
• zPower watts
• Your weight and height
• In-game bike frame and wheel set
• In-game drafting (Tri and TT bikes get no drafting boost)
• In-game road surface (Mountain bikes are fastest on dirt roads, but slowest on pavement)
• In-game virtual elevation changes.

Activating a Power Up such as the Truck (draft boost), Feather (weight reduction), or Helmet (aero boost) can further alter your in-game speed for a short duration.

This is not a real trainer curve, only to demonstrate the concept.

Example: Your wheel turns at 40 km/h then Zwift will use the curve and determine your Power as 200w. Using a set of complex equations taking your weight, height, road incline, rolling resistance and other factors in to account Zwift will calculate your virtual Speed.
So that 200w can be 45km/h on a flat road or 12km/h on a hill.

For Smart Trainers and Power Meters:
Your in-game speed is determined by:
• The power/watts broadcast by your trainer/power meter
• Your weight and height entered
• In-game bike frame and wheel set
• In-game drafting (Tri and TT bikes get no drafting boost)
• In-game road surface (Mountain bikes are fastest on dirt roads, but slowest on pavement)
• In-game virtual elevation changes

Activating a Power Up such as the Truck (draft boost), Feather (weight reduction), or Helmet (aero boost) can further alter your in-game speed for a short duration.

*****Also note that there are no stop signs, cars, or wind, and you don’t need to slow down for curves within Zwift.

Thanks to @Gerrie_Delport and @Mike_Rowe_PBR for all the help in putting this together.

Ride On!