AC Tonnage & Cooling Load Calculator
Estimate the cooling load (BTU/hr and kW) and recommended air-conditioner tonnage for a room, from its size, occupancy, sun exposure, appliance load, and window area.
How this works
The total cooling load is the sum of the main heat sources in the room; tonnage is that load divided by 12,000 BTU/hr (one ton of refrigeration), rounded up to the next standard 0.5-ton size:
Q_envelope = floorArea_sqft × factor // 25 / 35 / 45 by exposure
Q_people = occupants × 600 BTU/hr
Q_equipment = applianceW × 3.412 BTU/hr
Q_solar = windowArea_sqft × 150 BTU/hr
Q_total = Q_envelope + Q_people + Q_equipment + Q_solar
Tonnage = Q_total / 12,000 (round up to 0.5 ton)Worked example
A 4 m × 3 m bedroom (12 m² ≈ 129 sqft), moderate sun, 2 occupants, 300 W of appliances, 2 m² of windows:
- Envelope =
129 × 35 ≈ 4,520 BTU/hr - People =
2 × 600 = 1,200 BTU/hr - Equipment =
300 × 3.412 ≈ 1,024 BTU/hr - Solar =
21.5 × 150 ≈ 3,230 BTU/hr - Total ≈ 9,970 BTU/hr ≈ 0.83 ton → recommend a 1.0 Ton AC
Sources
- Rule-of-thumb component cooling-load summation (Manual-J-lite): envelope 25/35/45 BTU/hr/sqft by exposure, 600 BTU/hr per person, 3.412 BTU/hr per watt, 150 BTU/hr/sqft solar gain; 1 ton = 12,000 BTU/hr.
FAQ
How does this calculator size an air conditioner?
It adds up the main sources of heat in a room — the building envelope (walls, roof, floor, and air leakage), the people in it, lighting and appliances, and solar heat through windows — to get the total cooling load in BTU per hour. Dividing by 12,000 BTU/hr (one ton of refrigeration) gives the tonnage, which is then rounded up to the next standard 0.5-ton split-AC size. It is a quick rule-of-thumb estimate for choosing between a 1, 1.5, or 2 ton unit.
What is a 'ton' of air conditioning?
A ton of refrigeration is a unit of cooling capacity equal to 12,000 BTU per hour, or about 3.52 kW of heat removal. It originally referred to the cooling effect of melting one ton of ice in 24 hours. Common residential splits are rated 0.75, 1.0, 1.5, and 2.0 tons. The calculator reports both the raw computed load and the nearest standard size to buy.
Why does sun exposure matter so much?
A west-facing room, a top-floor room under an exposed roof, or a room with large glazing absorbs far more heat than a shaded, ground-floor room. The calculator captures this with an envelope factor of 25 BTU/hr per sqft for low exposure, 35 for moderate, and 45 for high. Picking the right exposure level is the single biggest factor in getting a realistic estimate — when in doubt for a sunny room, choose 'high'.
How much heat does each person add?
The calculator uses 600 BTU/hr per occupant, which covers the combined sensible and latent heat a person gives off during light residential activity (sitting, sleeping, light work). Rooms used by many people at once — a living room during gatherings, for example — should be entered with the peak occupancy you want the AC to handle comfortably, not the average.
What should I enter for appliance / lighting load?
Add up the wattage of equipment that runs while the AC is on — TV, computer, refrigerator (if in the room), lights, set-top box, router. Each watt of electrical load becomes about 3.41 BTU/hr of heat. A typical bedroom is 150–400 W; a home office or kitchen-adjacent room can be 800 W or more. Leave it low if the room is mostly used for sleeping.
Why round up to the next half ton?
ACs are only sold in fixed sizes (0.75, 1.0, 1.5, 2.0 tons). Choosing the next size up from the calculated load gives the unit enough capacity on the hottest days and a margin for things the rough method does not model in detail. Avoid heavily oversizing, though — an AC that is too large short-cycles, cools quickly without removing humidity, and leaves the room clammy.
Is this accurate enough for buying an AC?
It is a sound preliminary estimate for choosing a residential split-AC size and matches the rule-of-thumb methods used by most online sizing tools. It does not replace a detailed ASHRAE / Manual J load calculation, which accounts for local design temperatures, wall and roof insulation (U-values), orientation, infiltration rate, and duct losses. For large halls, commercial spaces, or central systems, have an HVAC engineer run a full load calculation.