To determine how much moulding you need:Multiply the width of the moulding by 8. For example, 1-1/4″ x 8 = 10″. And then take the frame size you intend to make and double each dimension. For example, 16″x20″ equals 16 x 2 = 32 and 20 x 2 = 40.
Add it all up. For example, 10″ + 32″ + 40″ = 82″.
Add 1/4″ to make sure you have enough moulding to build in an allowance. 82″+1/4″ = 82¼”This is how much moulding you need.
More on determining the right length of moulding