The forumla for a rectangular pool is:
length (in ft) x width (in ft) x average depth (in ft) x 7.48 (gallons per cubic foot) = volume in gallons.

Using BleachCalc and assuming an average depth of 5.75 feet, I get just under 28000 gallons. The steps probably only add about 300-400 gallons.

You can change Michael's BleachCalc from the metric to the imperial system by selecting "settings" from the "more calcs" menu.