ROUND (Depth x Diameter² x 5.9 = Capacity)
Most wooden hot tubs and some spas fit into the circular (or almost round) category. To figure water volume, multiply the average depth (deepest part plus shallowest part, divided by 2) times the diameter, times the diameter, times 5.9. If you have an in ground or other type spa with sloped sides, multiply result by .85.

SQUARE OR RECTANGULAR (Depth x Length x Width x 7.5 = Capacity)
Square or rectangular spas make up the vast majority. To figure water volume, multiply the average depth (deepest part plus shallowest part, divided by 2) times the length, times the width, times 7.5. If you have an in ground or other type spa with sloped sides, multiply result by .85

OVAL (Depth x Length x Width x 5.9 = Capacity)
To figure water volume of oval shaped spas, multiply the average depth (deepest part plus shallowest part, divided by 2) times the full length, times the full width, times 5.9. If you have an in ground or other type spa with sloped sides, multiply result by .85.