How many BTUs do I need to heat a 24x24 garage?
A 24 x 24 x 8 ft garage with average insulation and a 40°F temperature rise needs about 24,500 BTU/hr of delivered heat, or 7,200 W of electric resistance heat. The next output class is 25,000 BTU/hr.
Calculators · United States
Garage Heat Lab
Free garage heater sizing calculator using garage dimensions, insulation, and winter temperature
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A free tool that calculates the BTU/hr and watts needed to heat a garage based on its size, insulation, and local winter temperatures.
The story so far
Garage Heat Lab is a free, independent garage heater sizing calculator. It estimates how much heat a garage needs in BTU/hr and watts from its size, winter low temperature, and insulation level. The tool uses a published formula with transparent, documented sources and no product catalog. Companion tools estimate running costs and installation expenses, with guides explaining heating choices in plain English.
The calculator applies the same sizing engine across all pages and guides, so recommendations are consistent. It provides comparisons between electric, natural gas, and propane heating methods, showing estimated monthly costs for each fuel type. The site does not sell heaters, take commissions from manufacturers, or rank brands.
Typeset features
- Calculates required heat output in BTU/hr and watts from garage dimensions, ceiling height, insulation, and winter temperature
- Adjustable insulation levels from poor to excellent with whole-space planning factors
- Published formula and methodology with documented sources
- Real-time calculator updates as you type input values
- ZIP or postal code reference using NOAA weather data for local temperatures
- Embeddable calculator iframe for integration on other websites
- Companion tools for installation costs and heating method comparison
Pricing
Pricing model: Free
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Questions and answers
How do I calculate the right size garage heater for my garage?
Multiply length, width, and height in feet to get cubic feet, then multiply by the insulation factor and the temperature rise in Fahrenheit. The calculator uses factors of 0.20 for poor, 0.133 for average, 0.10 for good, and 0.08 for excellent insulation. The result estimates required BTU/hr output, not a gas heater's input rating.
Is it cheaper to heat a garage with gas or electric?
Usually gas. At 2025 U.S. average prices, a million delivered BTU costs about $18.50 with natural gas, $34.90 with propane, and $50.70 with electric resistance heat. For a default 24 x 24 garage that is about $41, $77, and $112 a month.
What is the best way to heat a garage?
At average U.S. prices, a vented natural gas unit heater has the lowest running cost where gas reaches the garage. Without gas, a 240 V electric heater is the simplest choice for small or occasional use, and a ductless mini split can cost less to run if its cold-weather output covers your design low.
Do gas garage heaters list input or output BTU?
Gas and propane units commonly show an input rating. Check the manufacturer's output rating or multiply input by the specified efficiency. In this planner, 80% efficiency means a 60,000 BTU/hr input heater delivers 48,000 BTU/hr.
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