Combustion Air Calculator
Estimate required combustion air room volume or opening free area from total appliance BTU input using simplified indoor-air and outdoor-air methods.
This calculator is part of the HVACTools.pro free calculator collection, built for working HVAC technicians. The interactive widget loads when the page hydrates; the content below previews related tools while it loads.
Frequently asked questions
What does a combustion air calculator estimate?
A combustion air calculator estimates either the minimum room volume needed for indoor combustion air or the required free area for outdoor combustion air openings based on total appliance BTU input.
How do you calculate indoor combustion air volume?
A common simplified rule is 50 cubic feet per 1,000 BTU per hour of total appliance input. That is equivalent to dividing total BTU per hour by 20 to get required room volume in cubic feet.
How do you size outdoor combustion air openings?
A common simplified two-opening method uses 1 square inch per 4,000 BTU per hour per opening for vertical or direct openings, and 1 square inch per 2,000 BTU per hour per opening for horizontal openings.
Does this replace code or manufacturer requirements?
No. This calculator is a simplified field aid. Final combustion air sizing and compliance may depend on local code, manufacturer instructions, room conditions, venting layout, and free-area reductions from louvers or grilles.
Related calculators
Browse all HVAC calculators · Troubleshooting workflows · HVAC guides · Tool reviews