Range Hood CFM Calculator (Gas & Electric, Wall & Island)

How it works

gas: CFM = BTU/h ÷ 100 · electric: CFM = width (in) × 10 · island × 1.5 · equivalent duct length = run + 5 ft per elbow; derate ≈ 0.5% per ft beyond 10 ft · make-up air required above 400 CFM (US)

A range hood has to move the plume of hot air, grease and combustion products rising off the cooktop, and the plume scales with the heat: the industry rule is one cubic foot per minute for every hundred BTU of gas burner output, or ten CFM per inch of width for electric cooktops whose output is more modest. Island hoods have no wall to help contain the plume and cross-draughts scatter it, hence the 50% uplift. The fan's label CFM is measured with no duct; every foot of duct and every elbow adds resistance that lowers real airflow, so the run should be short, smooth and full-diameter. Large hoods also depressurise the house enough to pull flue gases backwards down a water heater or furnace chimney, which is why building codes demand a make-up air supply above 400 CFM.

Worked example

A 30-inch, four-burner gas cooktop totalling 40,000 BTU on a wall needs 400 CFM. Through 12 ft of duct with two elbows (22 ft equivalent) about 6% is lost, so a 450 CFM hood on a 7-inch duct delivers around 420 CFM. Being over 400 CFM rated, most US codes call for a make-up air damper; the same cooktop as an island would need 600 CFM.

Frequently asked questions