When to use: Use this calculator during the preliminary design phase to estimate the cooling load for a residential or light commercial space. Based on the simplified ACCA Manual J method, it accounts for envelope conduction (walls, ceiling, windows), solar heat gain, occupant loads, equipment, and lighting. Use climate zone to set the outdoor design temperature differential. For permit-level design, use the full Manual J room-by-room method.
This free cooling load calculator estimates the air-conditioning capacity a building needs, in BTU per hour and in tons of cooling. It follows the same principles as an ACCA Manual J / ASHRAE load calculation, summing the heat that walls, roof, windows, sunlight, infiltration, people, lighting, and equipment add to the space. Enter the floor area, insulation level, climate zone, glazing, and internal loads, and it returns the total cooling load plus a breakdown by source. It's built for HVAC designers, contractors, mechanical engineers, and homeowners who need a quick, defensible estimate before specifying an AC unit.
The total cooling load is the sum of the sensible load (heat that raises air temperature) and the latent load (energy needed to remove moisture from the air).
Sensible heat gains come from several paths: • Conduction through the envelope — walls, roof, and windows — using Q = U × A × ΔT, where U is the assembly heat-transfer coefficient, A is the area, and ΔT is the indoor-to-outdoor temperature difference. • Solar heat gain through glass, which can dominate on sunny, west-facing facades and is far larger than conduction through the same window. • Infiltration and ventilation air that must be cooled. • Internal gains from people (about 250 BTU/hr sensible per person), lighting, and equipment (1 watt = 3.412 BTU/hr).
Latent load comes mainly from occupants (about 200 BTU/hr each), ventilation/infiltration moisture, and indoor moisture sources. Add sensible + latent to get the total, then convert to tons: tons = total BTU/hr ÷ 12,000.
Worked example: a space with 18,000 BTU/hr sensible and 4,000 BTU/hr latent has a 22,000 BTU/hr total load, which is 22,000 ÷ 12,000 ≈ 1.8 tons — so you'd select a 2-ton unit.
1. Enter the floor area and ceiling height of the conditioned space. 2. Pick the insulation level and climate zone — the zone sets the outdoor design temperature difference (ΔT). 3. Enter window area and glazing type (single, double, low-E, or triple pane) so solar and conduction gains through glass are captured. 4. Add internal loads: number of occupants, equipment wattage, and lighting power density. 5. Read the total cooling load in tons and BTU/hr, and review the load breakdown to see which sources drive the result. For permit-level work, follow up with a full room-by-room Manual J.
Sensible load controls temperature; latent load controls humidity. A unit must handle both — its sensible heat ratio (SHR) should match the building's. In humid climates, latent load matters as much as sensible, and an undersized-for-latent system leaves the space cool but clammy.
Resist the urge to oversize. An oversized AC satisfies the thermostat quickly and short-cycles, so it never runs long enough to wring moisture out of the air — giving a cold, humid, uncomfortable space, plus higher energy use and more wear. ACCA Manual J sizes the load and Manual S selects equipment to match it; good practice keeps the installed capacity close to the calculated load (typically within about 15%).
Sum the sensible heat gains (conduction through walls, roof, and windows via Q = U × A × ΔT, solar gain through glass, infiltration/ventilation, people, lighting, and equipment) and the latent gains (moisture from occupants and outdoor air). Total cooling load = sensible + latent, expressed in BTU/hr. A full ACCA Manual J does this room by room.
A very rough rule of thumb is about 20–30 BTU/hr per square foot, but it is only a sanity check. Actual load depends on climate, insulation, window area and orientation, ceiling height, and internal gains, so you should always run a Manual J calculation rather than rely on square footage alone.
Divide the total cooling load in BTU/hr by 12,000, since one ton of cooling equals 12,000 BTU/hr. For example, a 30,000 BTU/hr load needs 30,000 ÷ 12,000 = 2.5 tons. Round to a standard equipment size that matches the calculated load closely.
Sensible load is the heat that changes the air temperature — conduction, solar gain, lighting, and equipment. Latent load is the energy required to remove moisture (humidity) from the air, mostly from people and outdoor air. The total cooling load is the sum of both, and the equipment must handle each part.
An oversized unit cools the air fast and shuts off before it has run long enough to dehumidify, so it short-cycles. The result is a cold but clammy space, poor humidity control, reduced efficiency, and faster wear on the compressor. Sizing to the calculated Manual J load avoids this.
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