VDOT is a concept developed by exercise physiologist Jack Daniels and Jimmy Gilbert in their 1979 work on oxygen power and running performance. The name combines "V" (for volume) with "dot" notation (indicating rate of change) and "O2" — the same notation used in VO2 max. Numerically, VDOT values are similar to VO2 max values expressed in ml/kg/min, but they are not the same measurement.
The key distinction: VO2 max is measured in a laboratory using metabolic gas analysis under maximal exercise conditions. VDOT is derived purely from race performance using the Daniels-Gilbert equations. Two runners with identical VO2 max values can have different VDOT values if their running economy differs — a runner who converts oxygen to forward motion more efficiently will produce faster race times and therefore a higher VDOT from the same VO2 max.
VDOT's practical value is in expressing equivalent performance across distances and in deriving training paces. If you know your VDOT from one race, you can predict equivalent performances at other distances and calculate appropriate training pace zones (Easy, Threshold, Interval, Repetition) from the Daniels VDOT tables.
A VDOT of 50, for reference, corresponds approximately to a 17:50 5K, a 37:03 10K, a 1:21:46 half marathon, or a 2:54:57 marathon. These are all "equivalent" performances in the Daniels framework — a runner who achieves one of them would be expected (with appropriate training) to achieve the others.
VDOT increases over time as fitness improves. Tracking VDOT from race to race is a useful way to measure training progress that accounts for varying race distances.