Calculation is based upon a smooth model fitted to WR speeds (see Figure 3 and plots on main page) recently updated to include the top 200 performances for each distance (100% corresponds to the "average" world record, relative to the others, values higher than this are better than we expect) and can be used for any distance within the range from the drop-down list.
The age-graded performance is calculated from age-group WRs, using a simple model to provide a smooth summary of these. Again it is expressed as %AGE-group WR (speed), so that 100% is (approximately) the current age-group WR, and lower numbers are slower.
The World Association of Veteran Athletes (WAVA) produces detailed tables of age-grading for each distance and age. We use these and the WAVA standards (target times) for each event (interpolated between distances) to give a WAVA age-grading, as % of what the WAVA factors consider to be equivalent age-group performances - note that these are faster than the current age-group WRs for older runners at long distances, so that the WAVA%AGE is lower - see age-grading discussion and WAVA calculator for track and field events.
See also race pace calculator for tables of splits in races.