What this tool answers
You need a first-pass heat load for a garage, hobby shop, cabin, or vehicle cabin before buying a portable diesel air heater. The calculator estimates British thermal units per hour (BTU/h), converts to kilowatts, and names the smallest common parking-heater class (2, 5, or 8 kW) whose rule-of-thumb ceiling clears that load. It is not a Manual J residential load calculation and does not replace an HVAC contractor for permanent building systems.
The formula
BTU/h ≈ cubic feet × insulation factor × temperature rise (°F). Cubic feet = length × width × height. Temperature rise (ΔT) is how many degrees Fahrenheit warmer you want the space than the outdoor design temperature. Consumer garage-heating guides such as Car Guy Garage and The Furnace Outlet publish this cubic-feet form with tabulated insulation factors. This site uses factors 0.093 (excellent), 0.113 (good), 0.133 (average), 0.153 (poor), and 0.173 (very poor), entered as insulation levels 1–5.
Kilowatts and diesel classes
One kilowatt of heat output is about 3,412 BTU/h. A 2 kW class therefore clears about 6,824 BTU/h, a 5 kW class about 17,060 BTU/h, and an 8 kW class about 27,296 BTU/h. The calculator suggests the smallest class whose ceiling is at or above your estimate. Manufacturer floor-area claims still assume insulation and sealed openings — treat them as marketing ranges, not overrides of a high calculated load.
When the answer is NO_MATCH
If the estimate exceeds the 8 kW ceiling, the tool returns NO_MATCH for a single portable diesel air heater. Typical responses: improve insulation and seal the door, heat only a partitioned work zone, or switch to a hardwired garage / shop heater sized for the full building. Forcing an undersized portable unit leads to continuous full-fire operation and cold corners.
Safety and installation notes
Diesel parking heaters need outdoor exhaust, combustion air, fuel handling, and electrical supply per the manufacturer. Exhaust must exit the occupied space. Carbon monoxide alarms are prudent. This site does not certify installations.
Sources
Sizing form and insulation-factor style tables: Car Guy Garage “Garage Heating Guide” and The Furnace Outlet garage BTU tips (consumer HVAC education, accessed 2026-09-10). Conversion 1 kW ≈ 3,412 BTU/h is the standard SI/IP heat-power conversion. VEVOR diesel heater size overview: vevor.com/diy-ideas/diesel-heater-sizes/. Product wattage ratings come from the linked VEVOR US listings dated in products.json.