Estimate the cooling capacity a room needs, starting from the Department of Energy's 20 BTU per square foot baseline and adjusting for ceiling height, sun exposure, occupants and appliances.
The 20 BTU per square foot baseline assumes a standard 8-foot ceiling and average insulation. Every adjustment below is a correction to that assumption, so measure carefully before you buy.
The US Department of Energy publishes a working figure of 20 BTU per square foot for residential cooling. It is not a load calculation, it is a starting point that assumes an 8-foot ceiling and ordinary insulation. Every adjustment below is a correction to that assumption.
BTU stands for British thermal unit, and in air conditioning it measures how much heat the unit can remove in an hour. A room that needs 6,000 BTU needs a unit rated at least 6,000 BTU. Tonnage is just a bigger unit: 1 ton = 12,000 BTU per hour, which is why a 3-ton system is a 36,000 BTU system.
| Room size | Base BTU (area × 20) | Tonnage |
|---|---|---|
| 100–150 sq ft | 5,000 | — |
| 150–250 sq ft | 5,000–6,000 | — |
| 250–350 sq ft | 6,000–8,000 | — |
| 350–450 sq ft | 8,000–10,000 | — |
| 450–550 sq ft | 10,000–12,000 | 1 ton |
| 550–700 sq ft | 12,000–14,000 | 1–1.25 ton |
| 700–1,000 sq ft | 14,000–20,000 | 1.25–1.5 ton |
| 1,000–1,200 sq ft | 20,000–24,000 | 1.5–2 ton |
| 1,200–1,500 sq ft | 24,000–30,000 | 2–2.5 ton |
| 1,500–2,000 sq ft | 30,000–40,000 | 2.5–3.5 ton |
These are starting figures. A 2,000 sq ft home in a hot, humid climate with west-facing glass can need 50,000 BTU while the same floor plan in a mild climate with good insulation needs 32,000, the adjustments can move the answer by 40% or more.
| Condition | Correction | Why |
|---|---|---|
| Ceiling height other than 8 ft | × (actual height ÷ 8) | More air volume to cool for the same floor area |
| Sunny room, south or west facing | +10% | Direct solar gain through glass |
| Heavily shaded room | −10% | Little direct solar gain |
| Occupancy above two people | +600 BTU per extra person | Body heat from metabolism |
| Kitchen or heavy appliances | +4,000 BTU | Ovens, hobs, fridges and dryers all shed heat |
| Poor insulation, single-pane glass | +15% to +30% | Higher heat gain through the envelope |
| Hot and humid climate | +10% to +20% | The unit must remove moisture as well as heat |
Window and portable air conditioners come in fixed sizes, 5,000, 6,000, 8,000, 10,000, 12,000 and upward. You will almost never hit the calculated number exactly, so round up to the next standard size. An undersized unit runs flat out all afternoon and still loses ground on the hottest day of the year, the only day that actually matters.
Do not oversize it badly, either. A unit that is far too large cools the air fast and shuts off before it has removed the humidity, which leaves the room feeling cold and clammy. Portable units also need roughly 10–15% more rated capacity than a window unit to deliver the same cooling, because the unit itself sits inside the room.
The 20 BTU per square foot cooling baseline is the working figure published by the US Department of Energy and cited by ENERGY STAR room-air-conditioner guidance. The temperature-adjustment, occupancy and appliance corrections follow the standard residential rule-of-thumb method; the professional alternative is a full ACCA Manual J load calculation, which accounts for envelope, orientation and duct losses and should be used for any central system.
Twenty BTU per square foot is the baseline figure published by the US Department of Energy, and it assumes a standard 8-foot ceiling and average insulation. A 300 square foot room therefore starts at 6,000 BTU before any adjustment for sun, height, occupants or appliances.
A typical bedroom of 150–250 sq ft lands at 5,000–6,000 BTU. A standard 2-car garage of around 400–500 sq ft lands at 10,000–12,000 BTU on the base rule, and more if it faces the afternoon sun or has poor insulation.
Up, always. Units come in fixed sizes, so you buy the next size above your calculation. An undersized unit runs continuously and still cannot hold temperature on the hottest day, which is precisely when you need it.
It scales linearly: the base figure assumes 8 feet, so a 12-foot ceiling multiplies the load by 1.5. Vaulted or cathedral ceilings are the most common reason a room feels under-cooled with a unit that 'should' have been big enough.
No. It is a rule of thumb, not a load calculation. ACCA Manual J accounts for wall and roof construction, window area and orientation, duct losses, infiltration and local design temperatures. Use the rule of thumb to shop, and a Manual J for any central system or new construction.