Measure the dimensions of your bathroom. For rectangular rooms, measure the length and the width. For circular rooms, measure the diameter.
Calculate the square footage. For rectangular rooms, multiply the length by the width. For circular rooms, divide the diameter by two to find the radius, then square the radius and multiply by pi.
Size your fan according to the bathroom's square footage if it is less than 100 square feet. (If your bathroom is over 100 square feet, skip to Step 4.) For bathrooms between 50 and 100 square feet, use the square footage to determine the minimum necessary CFM. For example, an 80-square-foot bathroom should use a fan of at least 80 CFM. If the room is less than 50 square feet, look for a fan rated at least 50 CFM.
Consider the facilities in the bathroom when sizing your fan for bathrooms over 100 square feet. For each toilet, shower or bath add 50 CFM to the fan size calculation (for each whirlpool add 100 CFM). For example, if you have a shower, whirlpool and toilet in a 100-square-foot bathroom the fan should be at least 200 CFM.