A formula that’s recommended by Home Ventilating Institute (HVI) to calculate the speed of a ventilation fan results in speed specifications that are greater than what our experts and others believe to be necessary. HVI’s formula reads like this: If a bathroom is, say, 10 feet wide and 10 feet long and has an 8-foot ceiling, you multiply 10 x 10 x 8, which will equal 800 cubic feet. If you divide 800 cubic feet by 60 minutes (1 hour of use), this will equal 13.3. HVI would then have you multiply that number (13.3) by eight air changes per hour. You will get an estimate of 106.7 cfm to ventilate your bathroom properly