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
Cooker
Chamber Size
cubic in.
Recommended
Firebox 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
Specified
Firebox Size
 
cubic in.
Recommended
Firebox 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
Recommended
Air 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-Cooker
Opening Area
(firebox volume * .008)
square in.
Forum Link:  
Copy and paste the above code onto your favorite BBQ forum. Readers will be able to check out your BBQ Pit calculations with the numbers you've specified.
Email Link:   
Copy and paste the above code to share your BBQ Pit calculations with friends and fabricators via e-mail or other communications.
Example Pit Configurations


This calculator is based upon the BBQ pit building ratios and formulas developed by Alien BBQ in his revelatory BBQ pit building guides. One of his best articles is Lessons Learned in Pit Building / Modifying. My starting point for this calculator was the Excel worksheet created by Tom C on TheSmokeRing.com (an excellent BBQ discussion forum and website).

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