How many rectangles are in a 5 by 4 grid?

In a grid of A x B squares, the formula to find how many unique rectangles there are (and all squares are considered to be rectangles) is:

A * (A+1) * B * ((B+1)/4)

A and B are interchangeable.

So in a 5 x 4 grid, there are

5 * (5+1) * 4 * ((4+1)/4)

Or 5 * 6 * 4 * (5/4)

Or 150 unique rectangles.

Now if we switch A and B, the equation reads:

4 * (4+1) * 5 * ((5+1)/4)

Or 4 * 5 * 5 * (6/4)

Again 150 unique rectangles.