Feldon's BBQ Pit/Smoker Build Calculator for All Firebox Shapes
Metric Version v3.0

Adapted by Brent wesley of CQ Smokers, Queensland Australia brent@cqsmokers.com.au
Additional Summary Sheet, Oversized Firebox, and Firebox Dimension Estimation features by Brent Wesley
switch to USA Version
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!
Step 1 - Cooker Chamber Type & Size
Enter your cooking chamber's dimensions below under either Cylinder, Tank, or Rectangle:
An undersized firebox will require frequent fueling and may not adequately heat the cook chamber. A slightly oversized firebox is preferable. Select your oversize Percentage
Cylinder-type Cook Chamber
"Tank" Cook Chamber
Rectangle/Square Cook Chamber
Calculations
Diameter
cm
Length
cm
Tank volume
litres
Height
cm
Width
cm
Length
cm
Oversize
Percentage
Cooker
Chamber Size
litres
Recommended
Firebox Size
litres
Oversized
Firebox Size
Oversize Volume
litres
Step 2 - Firebox Dimensions - Estimator
Based on the above recomennded firebox size of 1/3 the main tank volume these are your estimated firebox dimensions.
For round fireboxes, specify a diameter to have the length calculated.
Use copy to manual for your intended firebox type.
Square Firebox
Height
cm
Width
cm
Length
cm
Volume
litres
Copy to manual

Rectangular Firebox
Height and Width are 25% smaller than Length
Height
cm
Width
cm
Length
cm
Volume
litres
Copy to manual

Round Firebox
Diameter
cm
length
cm
Volume
litres
Copy to manual
Step 3 - Firebox Dimensions - Manual
Enter the dimensions for your planned square or round firebox.
Firebox Dimensions (Square)
Firebox Dimensions (Round)
Calculations
Height
cm
Width
cm
Length
cm
Diameter
cm
Length
cm
Specified
Firebox Size
 
litres
Recommended
Firebox Size
(1/3 of cook chamber)
litres
Size Differential
 
%
Step 4 - 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 cm 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
cm
Recommended Chimney Volume
(5% of firebox volume)
litres
Length of Chimney
 
cm
Step 5 - 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
cm
Width
cm
Diameter
cm
Recommended
Air Inlet Area
(firebox volume * .003)
square cm
Number of Intakes
 
Step 6 - 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) ) 
cm
Recommended Diameter
√ ((firebox volume * .008) / (π/2) )  * 2
cm
Recommended Firebox-to-Cooker
Opening Area
(firebox volume * .008)
square cm
Step 7 - Name your smoker and Show/Print a summary sheet
Smoker Name
Smoker Type
Chamber Style
Firebox Style
Intake Type
Chamber Opening
 
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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