How to size an AC or heater
The quickest cooling estimate uses a rule of thumb of about 20 BTU per square foot of floor space. A 300 sq ft room therefore needs roughly 6,000 BTU/hr of cooling before any adjustments. BTU stands for British Thermal Unit — the amount of heat a unit can add or remove per hour.
That baseline shifts with the room. A very sunny space needs about 10% more cooling, while a heavily shaded one needs about 10% less. High ceilings add air volume, so we scale by ceiling height divided by the standard 8 feet. Each person beyond two adds about 600 BTU, and a kitchen adds roughly 4,000 BTU for the heat from appliances.
Heating is more demanding and depends heavily on climate. We multiply floor area by a per-square-foot factor that ranges from about 30 BTU/sq ft in hot regions up to 60 BTU/sq ft where winters are very cold, then scale for ceiling height. Once you have a number, divide cooling BTU by 12,000 to get tons — a 12,000 BTU/hr unit is one ton.
When to get a professional load calculation (Manual J)
Rule-of-thumb numbers are great for picking a window unit, portable AC or space heater for a single room. For whole-home systems, additions, or rooms with lots of glass, vaulted ceilings or unusual insulation, ask an HVAC professional for a Manual J load calculation. It accounts for insulation, window types, air leakage, orientation and local design temperatures so the system is matched precisely — avoiding both undersizing and the comfort and efficiency problems of oversizing.
Frequently asked questions
- How many BTUs do I need per square foot?
- For cooling, plan on roughly 20 BTU per square foot as a starting point — so a 300 sq ft room needs about 6,000 BTU/hr. This is a baseline before adjustments for sun, ceiling height and occupants.
- What size air conditioner for a 12x12 room?
- A 12×12 room is 144 sq ft, so at about 20 BTU per square foot a 5,000 BTU window or portable air conditioner is typically the right size. Nudge it up for very sunny rooms, high ceilings or extra people.
- How many BTUs are in a ton?
- One ton of cooling equals 12,000 BTU per hour. To convert, divide the cooling BTU by 12,000 — so a 24,000 BTU/hr unit is a 2-ton system.
- Is a bigger AC always better?
- No. An oversized air conditioner short-cycles — it cools the air quickly but shuts off before removing humidity, leaving the room clammy while wasting energy and wearing out the unit. Matching the unit to the actual load is more comfortable and efficient.
- How many BTU to heat vs cool a room?
- Heating typically needs more BTU than cooling and varies far more by climate — roughly 30 BTU/sq ft in hot regions up to 60 BTU/sq ft where winters are very cold. Cooling stays closer to the steady ~20 BTU/sq ft rule of thumb.
- Does ceiling height affect BTU?
- Yes. Taller rooms hold more air to condition, so they need more BTU. The standard rules of thumb assume 8-foot ceilings; scale the result up proportionally for higher ceilings.
- When should I get a professional Manual J load calculation?
- Get a Manual J load calculation for whole-home systems, additions, or rooms with lots of glass, vaulted ceilings or unusual insulation. It accounts for insulation, windows, air leakage and orientation so the system is sized precisely rather than by a rule of thumb.
AC size by room area
| Room area (sq ft) | Approx cooling BTU |
|---|---|
| 100–150 | 5,000 |
| 150–250 | 6,000 |
| 250–300 | 7,000 |
| 300–350 | 8,000 |
| 350–400 | 9,000 |
| 400–450 | 10,000 |
| 450–550 | 12,000 |
| 550–700 | 14,000 |
| 700–1,000 | 18,000 |
These are baseline values before adjustments for sun exposure, ceiling height and occupants.