So funktioniert die Berechnung
This is a Manual-J-lite heuristic, and the breakdown table shows each step. It starts at 20 BTU/hr per square foot of floor area, multiplies by three modifiers for insulation, climate zone and sun exposure, then scales by ceiling height divided by 8 feet, so a 10 foot ceiling adds 25 percent. Occupants add 600 BTU each and ticking Kitchen adds a flat 4,000 BTU. Heating mode multiplies the total by 1.10. Tonnage is BTU ÷ 12,000 and kW is BTU ÷ 3,412.
The three multipliers are where your judgement matters. Insulation runs Good 0.85, Average 1.0, Poor 1.15. Climate zone runs Hot (zone 1-2) 1.20, Warm (zone 3) 1.10, Moderate (zone 4) 1.0, Cool (zone 5-6) 0.95, Cold (zone 7-8) 0.90. Sun exposure runs Heavily shaded 0.9, Normal 1.0, Sunny (south/west facing) 1.1. The four presets simply set the area: Bedroom 150, Living room 300, Open plan 500, Whole floor 800 square feet.
The climate multipliers are written for cooling, where a hotter zone means a bigger load, and they are applied unchanged when you switch to Heating — so a cold zone still reduces the number even though heating demand rises with cold. In heating mode the only adjustment is the flat 1.10. Nothing models window area, orientation, air changes, duct losses or floor position. Use this to narrow the shortlist, then get a real Manual J from an HVAC contractor before buying.
FAQ
What size air conditioner do I need for a 500 square foot room? +
On the defaults — average insulation, moderate climate, normal sun, 8 foot ceiling — 500 sqft gives 10,000 BTU/hr, plus 600 per occupant. Two occupants brings it to 11,200 BTU/hr, or 0.93 tons, so you would shop for a 12,000 BTU unit.
Why does raising the ceiling height increase the BTU number? +
Because the load scales with volume, not floor area. The tool divides your ceiling height by the 8 foot baseline and multiplies the adjusted base by that ratio, so a 12 foot vaulted ceiling raises the requirement by 50 percent over the same footprint.
How much should I add if the room is a kitchen? +
Set Kitchen to Yes and the tool adds a flat 4,000 BTU on top of the area-driven figure, covering oven, cooktop and refrigerator heat. It is a fixed amount, not a percentage, so it matters much more in a small galley than in an open-plan space.
How do I convert BTU per hour into tons or kilowatts? +
Both appear under the main result. One ton of cooling is 12,000 BTU/hr, so the tool divides by 12,000, and kilowatts come from dividing BTU/hr by 3,412. Tons is how North American AC and heat pump capacity is usually labelled.
Can I use this to size a heat pump for heating? +
Only as a very rough first pass. Switching Mode to Heating adds a flat 10 percent, but the climate multipliers were built for cooling loads and still reduce the figure in cold zones. For heating in a cold climate, treat the result as a floor and get a proper load calculation.
