Please use the above diagram when sizing your tank.
If you want to cover just the back you only need the Length and the height.
If you want to cover the back and one side you will need to add the width of one side to the length.
If you want to cover both the back and both sides for a 3 sided background you will need to add the length plus the width x 2 for both sides.
For example. A 40 Gallon (breeder) tank is 36"L x 18"W X 16"H
Back only is 16"H x 36"L
Back and 1 side is 16H"x54"L
Back and 2 sides is 16"H x 72"L
There are often Tall and Long versions of the same tank, so it's always best to measure if possible.
Please feel free to contact us for custom sizes.