# How many BTU does my AC need?

Computes the cooling capacity in BTU per hour for a room air conditioner from the room area, sun, people and kitchen use, by the ENERGY STAR sizing table.

- Page: https://www.acalculator.org/construction/btu-calculator
- JSON spec: https://www.acalculator.org/construction/btu-calculator.json
- Version: f7954d9d51f3

## Default answer

Example with the default inputs (Room area 200 ft², Sun Normal, People 2, Kitchen No): A 200 ft² room needs an air conditioner of about 6,000 BTU/h.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| area | Room area | The floor area of the room to cool, from 100 to 2,500 square feet. |
| sun | Sun | How much sun the room gets. |
| people | People | How many people usually use the room. |
| kitchen | Kitchen | The unit cools a kitchen. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| btu | Cooling capacity | The capacity to look for, in BTU per hour. |
| base | From the area alone | The capacity the ENERGY STAR table gives for the room area, before adjustments. |
| tons | Tons of cooling | The capacity ÷ 12,000 BTU per hour. |
| watts | Cooling in watts | The capacity in watts of heat removed (not the power the unit draws). |

## Method

capacity = table value for the area × (0.9 shaded, 1.1 very sunny) + 600 BTU/h for each person over 2 + 4,000 BTU/h for a kitchen

## Assumptions

- The table rows run from their lower area up to, not including, the next row’s; the last row includes 2,500 ft².
- The ENERGY STAR table is for room air conditioners in one room. It is not a load calculation for central air, which needs a full load calculation such as ACCA Manual J.
- The adjustments are applied in this order: sun first, as a percent of the table value, then people and kitchen as fixed amounts.

## Worked examples

1. area = 18.580608, sun = normal, people = 2, kitchen = no gives btu = 1,758.426421, tons = 0.5. Source: ENERGY STAR sizing table: 150 to 250 ft² needs 6,000 BTU/h.
2. area = 37.161216, sun = sunny, people = 4, kitchen = no gives base = 2,930.710702, btu = 3,575.467056. Source: ENERGY STAR table: 400 to 450 ft² is 10,000 BTU/h; + 10% for a very sunny room = 11,000; + 600 × 2 extra people = 12,200 (hand calculation in content.mdx).
3. area = 27.870912, sun = shaded, people = 1, kitchen = yes gives base = 2,344.568561, btu = 3,282.395986. Source: ENERGY STAR table: 300 to 350 ft² is 8,000; − 10% for heavy shade = 7,200; + 4,000 for a kitchen = 11,200 (hand calculation in content.mdx).
4. area = 232.2576, sun = normal, people = 2, kitchen = no gives btu = 9,964.416386. Source: ENERGY STAR table: 2,000 to 2,500 ft² needs 34,000 BTU/h (the last row includes 2,500).
5. area = 30, sun = normal, people = 2, kitchen = no gives btu = 2,344.568561. Source: hand calculation in content.mdx: 30 ÷ 0.09290304 = 322.9 ft², in the 300 to 350 row: 8,000 BTU/h.

## FAQ

### How many BTU do I need per square foot?

ENERGY STAR sizes room air conditioners by area bands rather than a single rate: 150 to 250 ft² needs 6,000 BTU/h, 350 to 400 ft² 9,000, 450 to 550 ft² 12,000. That is between about 14 and 50 BTU/h per square foot, falling as rooms get larger.

### What adjustments does ENERGY STAR make?

Reduce the capacity by 10% for a heavily shaded room and increase it by 10% for a very sunny one. Add 600 BTU/h for each person over two who regularly use the room, and 4,000 BTU/h if the unit is in a kitchen.

### Why not just buy a bigger air conditioner?

ENERGY STAR notes that an oversized unit cools the room before it has removed enough humidity, so the cool, moist air feels damp and clammy. Match the size to the room.

### What is a ton of air conditioning?

One ton of refrigeration is 12,000 BTU per hour (NIST SP 811). A 6,000 BTU/h unit is half a ton.

### How do I find my room’s area?

Multiply the length by the width of the floor. A room 12 ft by 16 ft is 192 ft². For an L-shaped room, split it into rectangles and add them.

### Can I use this for central air?

No. The table is for room air conditioners in one room. Sizing central air for a house takes a full load calculation that counts insulation, windows, climate and duct losses, such as ACCA Manual J.

## Sources

- ENERGY STAR, Room Air Conditioners: sizing table by room area and the adjustments for shade, sun, occupants and kitchens. https://www.energystar.gov/products/room_air_conditioners
- NIST SP 811 (2008), Appendix B: 1 BTU (International Table) = 1,055.05585262 J; 1 ton of refrigeration = 12,000 BTU/h; 1 ft = 0.3048 m exactly.