There are several common numerical factors in home construction. "Sixteen inch on center" refers to the distance between wall studs and floor joists. One quarter inch per foot represents the pitch of a drain pipe. One hundred CFM, cubic feet per minute, is the basis for duct design.

For heating with a gas or oil furnace, 100 CFM represents approximately 8000 BTU per hour.

For air conditioning, 100 CFM represents 6000 BTU.

The standard eight inch deep duct is designed around this 100 CFM standard. Every two inches of width equals 100 CFM. A six inch round duct also equals 100 CFM. So for every two inches of width of an eight inch trunkline equals a six inch round.

Begin with two inches of duct width for friction loss, and the formula for 1000 CFM will be 2 + (2x10)=22 for an 8 inch by 22 inch duct which will supply ten six inch rounds.

Before the ducts can be designed, the size of the furnace and/or air conditioner must be determined. The above 1000 CFM would match an 85000 BTU oil or gas furnace with 30,000 BTU of air conditioning, also referd to as 2 1/2 tons; a ton being 12000 BTU of cooling. The 8 by 22 duct would feed 10 six inch round outlets.

What goes out must come back, so the return system must be the equivalent. Returns are usually fewer in number and of larger diameter but must equal 1000 CFM total.