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Shop Heater BTU Calculator - Size a Heater for Garages, Pole Barns & Workshops

Calculate the heating load for any shop or outbuilding based on dimensions, insulation, and climate zone

Free shop heater BTU sizing calculator for garages, pole barns, workshops, and agricultural outbuildings. Enter your building dimensions, insulation level, target temperature rise, and climate zone to get the required BTU output for forced-air gas, radiant tube, wood, or electric heaters. The calculator accounts for wall and ceiling heat loss, slab-on-grade edge loss, air infiltration from doors and gaps, and a safety factor for cold-start recovery. Unlike generic HVAC load calculators built for insulated homes, this tool is purpose-built for the realities of shop heating - high ceilings, minimal insulation, large overhead doors, and intermittent occupancy. It gives you an honest BTU number so you don't buy an undersized unit that can't keep up on the coldest morning or an oversized unit that short-cycles and wastes fuel. Results include recommendations for heater type based on your shop's use case, fuel cost comparison, and estimated monthly operating cost at your local energy rates.

Pro Tip: Radiant tube heaters warm objects and the floor directly instead of heating air. In a drafty pole barn with 16-foot ceilings, a forced-air unit wastes energy heating a massive column of air above your head. A radiant tube heater at 8 feet keeps the work zone comfortable with 30-40% less BTU input because it heats the slab and equipment, not the air that leaks out every time the overhead door opens.

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Shop Heater BTU Sizing Calculator
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How It Works

  1. Enter Building Dimensions

    Input length, width, and ceiling height. For pole barns with peaked roofs, use the average height from floor to the midpoint of the roof slope - this approximates the heated volume more accurately than eave height alone.

  2. Select Insulation Level

    Choose from uninsulated metal, R-11 walls/R-19 ceiling, R-13 walls/R-30 ceiling, or fully insulated with vapor barrier. Insulation quality has the single biggest impact on BTU requirements.

  3. Set Temperature Rise and Climate

    Enter the outdoor design temperature for your area (the coldest temperature you expect to heat through) and your target indoor temperature. A 40×60 shop in Minnesota needs to handle a 100°F rise; the same shop in Georgia might only need 50°F.

  4. Account for Doors and Ventilation

    Enter the number and size of overhead doors and how often they open. Each door opening dumps a massive slug of cold air into the shop. Frequent opening can double the effective heating load.

  5. Review Results and Heater Recommendations

    Get the total BTU requirement with a breakdown by loss category, plus recommendations for heater type, fuel source, and estimated monthly operating cost at your local energy rates.

Built For

  • Home shop owners sizing a gas unit heater for a detached garage
  • Farm operators choosing between radiant tube and forced-air for a pole barn
  • Woodworkers calculating BTU needs for a spray booth with ventilation requirements
  • Mechanics comparing propane vs natural gas operating costs for a service bay
  • Contractors evaluating whether an existing heater is adequate after a shop expansion

Frequently Asked Questions

A 30×40 shop (1,200 sq ft) with 12-foot ceilings needs roughly 75,000-150,000 BTU depending on insulation, climate, and door usage. Uninsulated in a cold climate with frequent door openings: closer to 150,000 BTU. Well-insulated in a mild climate with doors mostly closed: 60,000-75,000 BTU may suffice. The insulation level alone can double or halve the requirement.
Forced-air heaters are cheaper upfront, heat up fast, and work well in insulated shops with low ceilings where you keep the doors closed. Radiant tube heaters cost more to install but are superior in tall, drafty buildings like pole barns because they heat the floor and objects directly rather than the air volume. For a shop where overhead doors open frequently, radiant wins on efficiency and comfort.
Three common causes: (1) The heater was sized for the building area but not the ceiling height - a 14-foot ceiling has 40% more volume than a 10-foot ceiling. (2) Infiltration from unsealed gaps, overhead door seals, and exhaust fans was underestimated. (3) The heater's output rating is input BTU, not output BTU - a unit rated at 80,000 BTU input at 80% efficiency only delivers 64,000 BTU of heat.
Wood stoves work for small, well-insulated shops where you're present to tend the fire. They struggle in large or poorly insulated buildings because radiant output drops off sharply with distance. Insurance companies often add restrictions or surcharges for wood heat in outbuildings. For shops with flammable materials, dust, or spray finishes, a sealed-combustion gas heater is safer.
Temperature rise = desired indoor temperature minus your outdoor design temperature. Your outdoor design temperature is the 99% winter design temperature for your location (the temperature that is exceeded 99% of winter hours). In Minneapolis that is about -12°F; in Atlanta about 22°F. If you want 60°F inside, the rise is 72°F in Minneapolis and 38°F in Atlanta.
Disclaimer: This calculator provides sizing estimates for planning purposes. Actual heating requirements depend on building construction details, air sealing quality, wind exposure, and usage patterns. Gas heater installation must comply with local codes and manufacturer clearance requirements. Consult a licensed HVAC contractor for final equipment selection and installation.

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