How it works
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 and sun multipliers are written for cooling, where a hotter, sunnier room means a bigger load. Switching to Heating reflects both about 1.0, so Cold (zone 7-8) becomes 1.10 instead of 0.90 and a sunny room becomes 0.90 instead of 1.10 — heating demand rises with cold and falls with solar gain. Occupants and the kitchen load are heat sources, so they are added in cooling mode and subtracted in heating mode. Heating then takes the flat 1.10 on top. 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.
Frequently asked questions
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? +
As a rough first pass. Heating mode inverts the climate and sun multipliers so a cold zone raises the load rather than lowering it, subtracts occupant and kitchen heat instead of adding it, and applies a flat 10 percent on top. That gets the direction right, but it is still a heuristic — for heating in a cold climate, treat the result as a starting point and get a proper load calculation.
Floor area is the start, and only the start
A rough heating or cooling load starts from floor area, and the multipliers — 0.85, 1.0, 1.15 — adjust for how the space actually behaves. Insulation quality, glazing area and orientation, ceiling height and air leakage each move the real figure substantially, and a well-insulated room can need barely half what a poorly insulated one of identical size does.
Ceiling height is the one most often forgotten, because the calculation is per square foot while the thing being conditioned is a volume. A room with a 12-foot ceiling is half as large again as the same footprint at 8 feet.
Why oversizing is worse than it sounds
For cooling in particular, oversizing degrades performance rather than improving it. An oversized air conditioner satisfies the thermostat quickly and shuts off, and because dehumidification happens over the length of a run cycle, short cycles leave a room that is cold and clammy. It also wears the compressor faster.
That is why a load calculation is worth doing rather than rounding up to the next unit. This tool gives a planning figure; a proper installation should be sized on a full room-by-room load calculation, which accounts for the fabric of the building rather than its footprint.
