Here is an update to the poster NEHPBA originally produced for your information:
NFPA 12.4.4* Flue Cross-Sectional Area. For residental-type, natural draft solid fuel burning appliances, the flue shall meet the following conditions:
1. The cross sectional area of the fuel shall not be less than the cross-sectional area of the appliance flue collar, unless specified by the appliance manufacturer.
2. The cross-sectional area of the flue of a chimney with no walls exposed to the outside below the roofline shall not be more than three exposed to the outside below the roofline shall not be more than three times the cross sectional area of the appliance flue collar.
3. The cross-sectional area of the flue of a chimney with one or more walls exposed to the outside below the roofline shall not be more than twqo times the cross-sectional area of the appliance flue collar.
Appliance Flue Size
Flue Flue in Maximum Size Maximum Size
Square Inch Allowed Inside Size Allowed
Chimney Chimney
4" Round 12.56" 37.68" 25.12"
5" Round 19.63" 58.89" 39.26"
6" Round 28.26" 84.78" 56.52"
7" Round 38.47" 115.41" 76.94"
8" Round 50.24" 150.72" 100.84"
Chimney Size (Square Inches)
Chimney Size Area in Square Inches
6" x 6" 36"
7" x 7" 49"
8" x 8" 64"
9" x 9" 81"
10" x 10" 100"
11" x 11" 121"
12" x 12" 144"
6" x 10" 60"
7" x 11" 77"
8" x 12" 96"
9" x 13" 117"
10" x 14" 140"
11" x 15" 165"
12" x 16" 192"
Calulating Area of a Circle
The area of a circle is equal to pi times the square of the Raduis. Pi is equal to 3.14.
pi X R2 = AREA
Example: If a circle has a 16' diameter, half the diameter is 8', which is the radius. The calculation for the area of this circle is:
3.14 X 64 = 201 SQUARE FEET