Feldon's BBQ Pit/Smoker Build Calculator for All Firebox Shapes v3.7

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There are any number of ways to design and build a BBQ Smoker. Hopefully this calculator will help you with the calculations. The general rule of thumb for a horizontal smoker is that the firebox should be 1/3 the size (volume) of the cooking chamber. These are just guidelines based on years building smokers but your mileage may vary!
Cooker Chamber Type & Size
Enter your cooking chamber's dimensions below under either Cylinder, Tank, or Rectangle:
Cylinder-type Cook Chamber
"Tank" Cook Chamber
Rectangle/Square Cook Chamber
Calculations
 Diameter inches Length inches
 Tank volume gallons
 Height inches Width inches Length inches
 CookerChamber Size cubic in. RecommendedFirebox Size cubic in.
Firebox Dimensions
Enter the dimensions for your planned square or round firebox.
An undersized firebox will require frequent fueling and may not adequately heat the cook chamber. A slightly oversized firebox is preferable.
Firebox Dimensions (Square)
Firebox Dimensions (Round)
Calculations
 Height inches Width inches Length inches
 Diameter inches Length inches
 SpecifiedFirebox Size   cubic in. RecommendedFirebox Size (1/3 of cook chamber) cubic in. Size Differential   %
Chimney Size
Enter the diameter of your chimney pipe to find out how long it should be.
• A chimney that is too short may produce insufficient draft (drawing of air). A chimney that is too long may cause the air to cool before it exits, reducing effective draft and worse, dripping of exhaust materials onto food!
• Many horizontal smokers have an exhaust between 30-40 inches in length, but there is no hard and fast formula.
• If you are building a horizontal smoker with a vertical cooking cabinet, realize that the cabinet partly acts as an exhaust, thus you may greatly shorten the chimney.
Chimney Pipe Diameter
Calculations
 Pipe Diameter inches
 Recommended Chimney Volume (5% of firebox volume) cubic in. Length of Chimney   inches
Firebox Air Inlet Opening
Enter the size of the firebox air inlet (intake) from the outside.
The air intake from the Outside into your Firebox controls how much oxygen is available to your fire. If the calculator determines your size is too small, you'll need more than one intake.
Firebox Intake (Square)
Firebox Intake (Round)
Calculations
 Length inches Width inches
 Diameter inches
 RecommendedAir Inlet Area (firebox volume * .003) square in. Number of Intakes
Firebox-to-Cook Chamber Opening
• For a round or half-moon opening between your Firebox and Cook Chamber, check out the recommended Diameter you below.
• For a square, rectangle, or custom shaped opening, the Area of the opening should be equal to or slightly larger than the calculation below.
• If you intend to attach your fire box at the lowest possible point on a cylindrical cook chamber, with just enough room for a half-moon opening (which maximizes cook space by lowering the grate height), then you'll want to make your half-moon opening slightly larger and then install at least a 1 1/2 inch tall grease bridge at the bottom of the opening to prevent grease incursion into the firebox.
• If this will be a Reverse Flow smoker, then the opening at the opposite end of the cook chamber from the fire box should be at least as large as the firebox-to-cook chamber opening.
Opening Size (Round)
Opening Size (Half Moon)
Calculations
 Recommended Diameter √ ((firebox volume * .008) / (π/4) )  inches
 Recommended Diameter √ ((firebox volume * .008) / (π/2) )  * 2 inches
 Recommended Firebox-to-CookerOpening Area (firebox volume * .008) square in.