Enter your target total time (minutes, or mm:ss, or hh:mm:ss) and press Time to calculate your pace and mile/km splits, for any race distance. Alternatively, enter your pace and press Pace and the splits and target time will be calculated. Type distances in for those not in list and use New distance to update form.
See also race pace calculator for computing actual splits from race times, and performance calculator for comparing different distances.
Note: for spectating, I've included Start Time, to find actual time at the splits.