The short answer: an air conditioner needs roughly 20 BTU per square foot of floor space, so a 12×12 room (144 sq ft) needs about 2,900 BTU and a 1,500 sq ft home about 30,000 BTU (2.5 tons) — in a temperate climate with average insulation. But the honest answer depends on your ceiling height, insulation, climate, sun, and how many people use the room. The fastest way to get it right is to run your room through the free BTU calculator — it applies every adjustment for you and even recommends the unit type. This guide explains the numbers behind it.
The quick rule of thumb
Multiply your room's square footage by about 20 BTU per square foot. That's the ENERGY STAR baseline for a room with an 8-foot ceiling, average insulation, and a moderate climate. It gets you in the ballpark — then you round up to the nearest standard AC size so you're never undersized on the hottest day.
One ton of cooling = 12,000 BTU. So a 24,000 BTU unit is a "2-ton" AC. Skip the math and size your exact room →
BTU by room size
For a single room in a temperate climate with average insulation:
| Room size | Floor area | Estimated cooling |
|---|---|---|
| 10 × 10 | 100 sq ft | ~2,000 BTU |
| 12 × 12 | 144 sq ft | ~2,900 BTU |
| 12 × 16 | 192 sq ft | ~3,800 BTU |
| 15 × 15 | 225 sq ft | ~4,500 BTU |
| 16 × 20 | 320 sq ft | ~6,400 BTU |
| 20 × 20 | 400 sq ft | ~8,000 BTU |
Small rooms round up to the smallest common window unit (5,000 BTU); larger rooms land on an 8,000–12,000 BTU unit. Each room above links to a page with its full breakdown and matching AC picks.
BTU by home square footage
Cooling a whole home or open floor plan? Here's the cooling load and the matching furnace size for heating, temperate climate:
| Home size | Cooling | Tonnage | Furnace (heating) |
|---|---|---|---|
| 500 sq ft | ~10,000 BTU | ~1 ton | ~21,000 BTU |
| 1,000 sq ft | ~20,000 BTU | ~2 tons | ~42,000 BTU |
| 1,500 sq ft | ~30,000 BTU | 2.5 tons | ~63,000 BTU |
| 2,000 sq ft | ~40,000 BTU | ~3.5 tons | ~84,000 BTU |
| 2,500 sq ft | ~50,000 BTU | ~4 tons | ~105,000 BTU |
Furnace sizing is climate-driven — a cold-climate home needs 50–60 BTU/sq-ft of heat versus ~30 in the south, so the same house needs a bigger furnace further north.
The six things that change your BTU
The rule of thumb assumes an "average" room. Adjust for reality — this is exactly what the calculator does:
- Ceiling height — taller than 8 ft scales the load up proportionally (a 10-ft ceiling adds ~25%).
- Insulation — poor/older insulation adds ~15%; a tight, well-sealed room cuts ~10%.
- Climate zone — a hot climate adds ~25%; a cool one trims it.
- Sun exposure — a very sunny room needs ~10% more; a heavily shaded one ~10% less.
- Occupants — add ~600 BTU for each person beyond two who regularly uses the room.
- Kitchen — add ~4,000 BTU for the extra heat from appliances.
These come from ENERGY STAR's window-AC sizing guidance. Stacked together they can swing the number 30–40% either way — which is why a bare square-footage estimate often buys the wrong unit.
Window, mini-split, or central?
The BTU number also tells you what kind of unit to buy:
- Up to ~12,000 BTU → a window unit or portable AC for a single room.
- 12,000–24,000 BTU → a single-zone mini-split — quiet, efficient, and it heats too.
- 24,000–36,000 BTU → a multi-zone mini-split to cool several rooms from one outdoor unit.
- Above ~36,000 BTU (3 tons) → central AC, sized with a Manual J load calculation and installed by a pro.
The calculator recommends the type for your number and lists real, neutral picks for each — we're not affiliated with any brand.
Frequently asked questions
How many BTU do I need for a 12×12 room? About 2,900 BTU in a temperate climate — round up to a 5,000 BTU unit. A very sunny or poorly insulated 12×12 room can need noticeably more.
Is it better to oversize or undersize an AC? Neither. An undersized unit runs constantly and never cools; an oversized one short-cycles — it cools fast but never runs long enough to remove humidity, leaving the room cold and clammy and wasting energy. Size it to the space.
How do I convert BTU to tons? Divide by 12,000. A 24,000 BTU unit is 2 tons; 36,000 BTU is 3 tons.
Does this work in metric? Yes — the calculator accepts square meters and shows tonnage, which matters if you shop AC by "ton" as in India and much of Asia.
The bottom line
Start from ~20 BTU/sq-ft, then adjust for ceiling, insulation, climate, sun, occupants, and kitchen — and round up to a standard size. The free BTU calculator does all of that for your exact room in a few seconds and tells you what to buy.
These are planning estimates from a simplified model. For a purchase or install, confirm with a Manual J load calculation and a licensed HVAC professional.