Zwift Physics Lab | ZwiftGopher

Rider

Fills in weight, height, FTP and five-minute power from your Zwift profile.

Weight kg Height cm FTP W 5-min power W

Training level

Compare a rider you after 5 kg less, or the one who keeps dropping you

Bike

Frame Wheels Upgrade level

Aero socks

Road and company

Gradient % Surface

Riding

Compare with up to 3

Solve

Speed km/h Power W W/kg

Speed–

Power–

W/kg–

Where the watts go

What do ten more watts buy, and why does doubling your power not double your speed?

Where does a climb start to hurt, and when does weight take over from aero?

At your power, how much of it fights the air, the tyres and gravity as the road tilts?

Speed against rider weight: holding the same watts, and holding the same W/kg.

Every bike in your conditions, fastest first.

Power to hold a speed alone, on a wheel and in a bunch.

Your bike on every surface, with road, gravel and MTB tyres.

Sustainable power against time, from your FTP and five-minute power.

Time against power on a Zwift route, bike by bike.

The model. Steady-state speed from three forces: gravity, rolling resistance and air drag. It is the TT and TTT planners' model, calibrated on CTT races where every rider is on the same neutral bike, so a whole field tests it at once; it predicts those fields to within a fraction of a percent on the flat.
You. Your drag area comes from your weight and height, as Zwift's does. Your bike's drag, weight and upgrade come from Zwift's own per-item figures (Zwift Speed Lab and zwifttools), the same table the TT planner uses. Tyres decide how each surface rolls.
Company. On a wheel you still pay all of your rolling resistance and gravity and a share of your own air drag, the share the TTT calculator fits to real team time trials. That is why the draft is worth a lot on the flat and almost nothing on a climb.
Not modelled. Acceleration, cornering, power-ups, supertuck, pack dynamics beyond the shares above, and weather. Treat the numbers as the steady state they are: hold the power and you will ride close to the speed.
Sustainable power. A critical-power model: your FTP, decaying beyond an hour by training level, plus an anaerobic reserve spread over the effort, sized from your five-minute power when you give it.