Proper ventilation of the attic space protects it against mould growth by removing moisture quickly. It's second job is equalising the temperature of the attic to the outdoor temperature. This reduces the likelihood of ice damns forming on your roof surface in the winter. Your local building code determines the ratio of ventilation space to attic area. Consult your local building authority for the exact numbers for your area. Calculate the amount of ventilation once you know the size of your attic.
Measure the attic floor's width and length with a measuring tape.
Multiply those two numbers with the calculator. The result is the area of your attic in square feet.
Divide that number by 300 or the ratio your local building code requires. The required ratio of attic ventilation is typically 1:300. That is, you need 1 square foot of ventilation for every 300 square feet of attic space.
Divide the result by two. The resulting figure is the amount of ridge and soffit venting you need to install. Divide the amount of ventilation in the attic equally between the soffit and ridge vents to have a balanced system.