The estimate formula is: VO2max = (0.01141 x Wpeak + 0.435)*1000/weight
Where Wpeak is in watts and weight is in kg. The estimate VO2max is in ml02/kg/min
Wpeak is found from a ramp test on an indoor egometer. Protocol: Start at 110 W. Increase by 35 W every 4 min, with 1 min recovery between each stage (i.e., 145, 180, 215, 250, 285 ,320, 355, 390, 425...etc). Wpeak is the power sustained in the last completed 4 min stage.
Reference: Hawley JA, Noakes TD. (1992). Peak power output predicts maximal oxygen uptake and performance time in trained cyclists. Eur J Appl Physiol Occup Physiol.; 65(1):79-83.
Thanks to Dario Fredrick on cyclingnews.com for bringing this to my attention