Bathroom Fan CFM Calculator

This sizes a bathroom exhaust fan three ways and then checks the duct. Enter the room dimensions for the one CFM per square foot rule and the eight air changes an hour rule side by side, or count toilets, showers and tubs for a bathroom over a hundred square feet. The duct action takes the fan rating on the box, the duct diameter and the run with its elbows and cap, and returns the static pressure against the 0.1 inches water gauge that fans are rated at, so you can see whether the fan will deliver its rating or a fraction of it. Every rate and elbow allowance is an input you can change.

Calculator Type
Room Measured In
The fan size comes back in CFM either way, because that is what fans are rated and sold in.
Leave blank for the usual rule, which is 1 CFM per square foot, or 10.76 CFM per square metre, for a bath of 100 square feet or less. Enter 0 to switch this rule off.
Eight is the figure bathroom fans are normally sized to.
ASHRAE 62.2 asks for 50 CFM on demand, or 20 CFM running continuously.
Widget
That Floor Area Is In
Only the comparison area above uses this. The fixture allowances and the answer are CFM either way.
Widget

This one action stays in inches and feet however the two above are set. Duct is stocked by the inch in the markets this rating covers, the friction equation behind the answer is defined for CFM against a diameter in inches, and what it returns is inches of water gauge, which is the pressure HVI certifies a fan at. A metre entry would have to be converted straight back, and metric duct comes in 100, 125 and 150 mm sizes that are not these sizes in another unit.

A damped wall cap or roof cap is usually counted as about 30 feet of duct.
Widget
Related Tools
Duct Size Calculator Square Footage Calculator HVAC Load Calculator Air Changes Per Hour Calculator