Insulation for walls, ceilings and floors comes in fibreglass rolls that are either pre-cut into 8 feet lengths, or are one continuous length. Each roll is assigned an "R" factor, grading its ability to insulate. This means that the higher the "R" factor, the thicker the insulation. Most insulation has a paper vapour retarder barrier attached to one side, that stops water/vapour soaking into the fibreglass and diminishing its effectiveness. Calculating the required insulation thickness depends on the width of the cavity space between the studs/joists, as well as the geographical location of the property.
Measure from the centre of one wall stud (or ceiling/floor joist) to the centre of its neighbour. The standard distance will be either 16 or 24 inches. Rolls of insulation come in these two widths, so it's important to know which insulation width to get.
Also measure the depth of each wall stud, or ceiling/floor joist. Wall studs can be made of 2 x 4s (roughly 3 1/2 inches wide) or 2 x 6s (roughly 5 1/2 inches wide). Floor and ceiling joists can be made from 2 x 10s (roughly 9 1/2 inches wide) or 2 x 12s (roughly 11 1/2-inches wide). It's important to know what depth the studs/joists are, as insulation made for 2 x 6 wall studs will be too thick for 2 x 4 wall studs, for example.
Call the city's building department in your area and tell them whether you are installing insulation for walls, ceilings or floors. Then tell them the depth of the studs, or joists, in your home. As the "R" value requirement will be different depending on whether you live in a colder, or hotter climate, the building department will tell you the correct "R" value for your area, also taking into account the depth of the of studs/joists in your home.