These are the exact formulas the game uses to estimate watts from wheel speed on a dumb trainer. Almost nobody publishes theirs; here are ours.
Monsoon Cycles, Colorado Springs · Updated August 16, 2026
Virtual power, Zwift's zPower and every equivalent, is a lookup from rear-wheel speed to estimated watts using a resistance curve for your trainer. The estimate is only as honest as the curve, so ours are public. Wheel speed v is in meters per second.
| Curve | Formula (watts) | 15 km/h | 25 km/h | 35 km/h | 45 km/h |
|---|---|---|---|---|---|
| Magnetic, light | 3.2v + 0.18v³ | 26 | 83 | 196 | 393 |
| Magnetic, medium | 4.5v + 0.26v³ | 38 | 118 | 281 | 564 |
| Magnetic, heavy | 6.0v + 0.36v³ | 51 | 160 | 384 | 773 |
| Fluid | 2.0v + 0.063v³ + 0.30v² | 19 | 75 | 203 | 460 |
Fluid trainers rise faster with speed than magnetic ones, which is why they feel more road-like in sprints. Pick the curve that matches your trainer's character, keep your roller tension consistent, and the estimate will be consistent too. Pair it with the right wheel circumference or the speed itself will be wrong before the curve ever sees it.
USE THEM IN THE GAMEIf your magnetic trainer has a resistance lever, middle settings match the medium curve. Fluid trainers use the fluid curve. When in doubt, ride a known effort and pick the curve that feels honest.
Tire pressure, roller tension, and tire temperature all shift the real resistance. Set tension identically every ride and treat virtual power as a consistent training signal rather than a laboratory number.