What is a BTU?
A BTU (British Thermal Unit) is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. In HVAC, capacity is always stated in BTU per hour, often shortened to just "BTU." A 10,000 BTU window AC removes 10,000 BTU of heat every hour. Higher BTU = more cooling or heating capacity.
How to calculate BTUs for a room
BTU/h = chart capacity for the room's area × sun adjustment
+ 600 per regular occupant beyond 2 + 4,000 if kitchen
For cooling, this calculator follows ENERGY STAR's room air conditioner guidance: look up the room's floor area in its chart (below), then adjust:
- Sun, very sunny +10%, heavily shaded −10%.
- Occupants, +600 BTU for each person beyond 2 who regularly uses the room.
- Kitchen, +4,000 BTU.
- Ceiling height, the chart assumes 8 ft; ENERGY STAR suggests more capacity for taller rooms but gives no figure, so the calculator flags it rather than adding to it.
Climate, insulation and window count are not part of ENERGY STAR's method, so cooling results ignore them.
For heating there is no official square-foot chart. The heating tab uses a trade rule of thumb, 30 BTU/sq ft in hot climates up to 45 in very cold ones, scaled by ceiling height ÷ 8, insulation (poor +30%, good −10%, excellent −20%) and 1,000 BTU per window. It is an illustration, not an official or engineering figure.
BTU chart by room size
| Area to be cooled | Capacity (BTU/h) | Tons |
|---|---|---|
| 100 up to 150 sq ft | 5,000 | 0.42 |
| 150 up to 250 sq ft | 6,000 | 0.50 |
| 250 up to 300 sq ft | 7,000 | 0.58 |
| 300 up to 350 sq ft | 8,000 | 0.67 |
| 350 up to 400 sq ft | 9,000 | 0.75 |
| 400 up to 450 sq ft | 10,000 | 0.83 |
| 450 up to 550 sq ft | 12,000 | 1.00 |
| 550 up to 700 sq ft | 14,000 | 1.17 |
| 700 up to 1,000 sq ft | 18,000 | 1.50 |
| 1,000 up to 1,200 sq ft | 21,000 | 1.75 |
| 1,200 up to 1,400 sq ft | 23,000 | 1.92 |
| 1,400 up to 1,500 sq ft | 24,000 | 2.00 |
| 1,500 up to 2,000 sq ft | 30,000 | 2.50 |
| 2,000 up to 2,500 sq ft | 34,000 | 2.83 |
ENERGY STAR's chart for room air conditioners, before its adjustments; it assumes a 8-ft ceiling and stops at 2,500 sq ft. Tons are BTU/h ÷ 12,000.
AC tonnage chart
One ton of cooling is 12,000 BTU per hour (NIST SP 811). Floor area alone is a poor guide to a central system's tonnage; that takes a Manual J load calculation.
| Tons | BTU/hr | Cooling kW |
|---|---|---|
| 1.0 | 12,000 | 3.52 |
| 1.5 | 18,000 | 5.28 |
| 2.0 | 24,000 | 7.03 |
| 2.5 | 30,000 | 8.79 |
| 3.0 | 36,000 | 10.55 |
| 3.5 | 42,000 | 12.31 |
| 4.0 | 48,000 | 14.07 |
| 5.0 | 60,000 | 17.58 |
Window AC vs central AC vs mini-split
Window AC
- Single room only
- Easy install
- Noisy
Central AC
- Whole home
- Needs ductwork
- Most efficient for big homes
Mini-split
- One or more zones
- No ducts required
- Quiet & efficient
Oversizing vs undersizing
Too big (oversized)
- Short-cycles (on/off rapidly)
- Doesn't remove humidity, room feels clammy
- Higher purchase cost
- More wear on compressor
Too small (undersized)
- Runs constantly
- Can't hit target temperature
- Higher electric bills
- Overworked, shorter lifespan
For a room unit, take the capacity from ENERGY STAR's chart after its adjustments rather than adding a margin. For central systems, have a professional do a Manual J load calculation.
SEER and EER ratings
SEER (Seasonal Energy Efficiency Ratio) is the average cooling output divided by electricity used over a full cooling season. EER is the same ratio at peak conditions (no seasonal averaging). Higher numbers = more efficient.
- Federal minimum: since 2023, central air conditioners are rated in SEER2. A split-system AC under 45,000 BTU/h needs at least 13.4 SEER2 in the North and 14.3 in the Southeast and Southwest (10 CFR 430.32).
- Room units: compare the efficiency figure on the yellow EnergyGuide label, or look for the ENERGY STAR mark.
Because the rating is cooling per watt-hour, electricity for the same cooling scales with 1 ÷ SEER: moving from 14 to 18 cuts it by 22%, and from 14 to 16 by 13%.
Frequently asked questions
How many BTUs do I need per square foot?
There is no single per-square-foot number. ENERGY STAR publishes a chart for room units instead, and its steps are not proportional: 5,000 BTU/h covers 100 to 150 sq ft, while 1,500 to 2,000 sq ft calls for 30,000. It then adds or takes away 10% for sun or shade, 600 per regular occupant beyond 2, and 4,000 for a kitchen.
What size AC do I need for a 12x12 room?
At 144 sq ft, a 12 by 12 room sits in ENERGY STAR's smallest row, 5,000 BTU/h. Very sunny, it becomes 5,500; with four people using it regularly, 6,200; as a kitchen, 9,000. ENERGY STAR's figures assume an 8-foot ceiling, so a taller room may need more.
What is a ton of cooling?
A ton of refrigeration is 12,000 BTU per hour, or 3,517 W, in NIST's unit tables. Central systems are sold in tons, so a 3-ton unit delivers 36,000 BTU/h. Room units are labelled in BTU/h directly.
How many BTUs do I need to heat a room?
No official square-foot rule exists for heating. A common trade rule of thumb runs from 30 BTU/h per sq ft in warm climates to 45 in very cold ones; on that basis a 500 sq ft room comes to 20,000 in a cold climate and 15,000 in a hot one. Treat the heating tab as an illustration and have a heating contractor size real equipment.
What is the difference between BTU and BTU/hr?
A BTU (British thermal unit) is an amount of energy, historically the heat that warms a pound of water by one degree Fahrenheit. BTU per hour is a rate, the same energy delivered each hour. Air conditioners and heaters are rated as a rate, even when a label shortens it to 'BTU'.
How do I convert BTU to watts?
Multiply BTU/h by 0.2930711 (NIST SP 811), or divide watts by that factor to go the other way. So 12,000 BTU/h is 3,517 W, and a 1,500 W space heater gives off 5,118 BTU/h. Electricity drawn is a separate figure: at an EER of 10 (Btu/h of cooling per watt), a 12,000 BTU/h air conditioner draws 1,200 W.
Is it better to oversize or undersize an AC?
Neither. A unit that is too large cools the air quickly and shuts off before it has pulled out much moisture, so the room ends up cold and clammy, and the frequent starts wear it harder. One that is too small runs without stopping and may never reach the set temperature on the hottest days. Pick the capacity ENERGY STAR's chart gives for the room, after its adjustments.
How many BTUs for a 1,500 sq ft house?
ENERGY STAR's room-unit chart puts 1,500 sq ft at 30,000 BTU/h (2.5 tons), but that chart is for cooling a single open area with a room unit. For a whole house with central air or a heat pump, the size should come from a Manual J load calculation, which accounts for insulation, windows, air leakage and local weather.
What SEER rating should I look for?
Since 2023, federal rules (10 CFR 430.32) rate central air conditioners in SEER2. A split-system AC under 45,000 BTU/h must reach at least 13.4 SEER2 in northern states and 14.3 in the Southeast and Southwest. Above the minimum, savings follow the ratio: going from 14 to 18 cuts the electricity for the same cooling by 22%.
What size window AC do I need?
From ENERGY STAR's chart, before adjustments: 150 sq ft → 6,000, 250 sq ft → 7,000, 350 sq ft → 9,000, 450 sq ft → 12,000, 550 sq ft → 14,000, 700 sq ft → 18,000, 1,000 sq ft → 21,000. The chart stops at 2,500 sq ft.