How to Calculate Water Storage Capacity of a Pond?

How can I calculate the capacity of a lake for storing water?

I know there is a simple equation to calculate the lake water volume using the Frustrum Method: 

V = volume of water;
H = difference in depth between two successive depth contours;
A1 = area of the lake within the outer depth contour being considered;
A2 = area of the lake within the inner contour line under consideration. 

Which other methods can you suggest for calculations for lakes with slopes? Which method will best take the slope into account and yet remain simple? 

Is there a software or an app to estimate the storage capacity of the lake?