A simple load estimate from floor area, ceiling height, insulation, sun and people. The climate factor starts at 20 BTU/h per ft² (you can type 20–25 or any override). This is not Manual J.
BTU/h = area_sqft × climate × (ceiling_ft ÷ 8) × insulation × sun + 600 × max(0, occupants − 1). Area comes from length × width, or the area field if you fill it; metre inputs are converted to square feet first. Tons = BTU/h ÷ 12,000, and kW is converted from BTU/h. This is a rule of thumb, not a Manual J heat-load calculation.
Room cooling or heating load is estimated with a rule-of-thumb formula: BTU/h = area_sqft × climate × (ceiling_ft ÷ 8) × insulation × sun + 600 × max(0, occupants − 1) — combining room size, climate zone, ceiling height, insulation quality, sun exposure, and occupancy into one estimate.
Area comes from multiplying length by width if you enter those dimensions, or directly from the area field if you fill that in instead. If you enter measurements in metres, they're converted to square feet first, since the underlying formula's coefficients are calibrated for that unit.
The result converts to tons of cooling capacity (BTU/h ÷ 12,000, the standard conversion for HVAC unit sizing) and to kilowatts. This is a rule-of-thumb estimate suitable for a general sense of what size unit you need, not a full Manual J heat-load calculation, which a professional HVAC installer would perform with far more detailed inputs for an actual installation.
It's a reasonable rule-of-thumb estimate for general planning, but not as detailed as a full Manual J heat-load calculation that a professional HVAC installer performs, which accounts for window placement, air leakage, and other detailed factors specific to your building.
A ton of cooling capacity equals 12,000 BTU/h — a standard unit used in the HVAC industry to size air conditioning equipment, derived historically from the cooling effect of melting one ton of ice per day.
Better insulation reduces the heat load, so the insulation factor in the formula scales the result down for well-insulated spaces and up for poorly insulated ones, reflecting how much heat actually needs to be managed.
Use this as a general starting estimate for budgeting or research, but for an actual purchase, a professional Manual J assessment specific to your home will give a more precise and reliable sizing recommendation.