BTU Calculator
By BRIX Editorial Team · Updated
To size a room air conditioner, find the room's square footage on the ENERGY STAR chart (450–550 sq ft needs 12,000 BTU/hr), then add 10% for a very sunny room, 600 BTU per person beyond two, and 4,000 BTU for a kitchen. A sunny 20 × 18 ft room for three people needs about 10,500 BTU/hr.
- Room area
- 360 sq ft
- ENERGY STAR chart size
- 9,000 BTU/hr
- Sun or shade
- +900
- Extra occupants
- +600
- Kitchen
- +0
- In tons (÷ 12,000)
- 0.88
Look for a unit rated near 10,500 BTU/hr. This is the ENERGY STAR rule of thumb for one room with 8 ft ceilings; it is not a Manual J load calculation.
Key takeaways
- Start with the ENERGY STAR chart: square feet to BTU/hr, based on 8 ft ceilings.
- Heavily shaded: reduce 10%. Very sunny: increase 10%. Add 600 BTU for each regular occupant beyond two. Kitchen: add 4,000 BTU.
- Tons = BTU/hr ÷ 12,000. A 12,000 BTU unit is one ton.
- This is a single-room rule of thumb, not an ACCA Manual J load calculation. Central systems need Manual J and Manual S.
- Oversized units short-cycle, dehumidify poorly and wear out sooner.
How many BTUs do I need per square foot?
ENERGY STAR doesn't publish a flat BTU-per-square-foot number. Its chart runs from 5,000 BTU/hr for 100–150 sq ft to 34,000 BTU/hr for 2,000–2,500 sq ft, which works out to roughly 14–40 BTU per square foot, higher for small rooms. Look up the range your room falls in, then adjust for sun, people and cooking.
BTU/hr = chart(area) × (1 ± 0.10 for sun or shade) + 600 × (people − 2) + 4,000 if kitchen
- chart(area)
- the ENERGY STAR capacity for the room's square footage (table below)
- ± 0.10
- −10% heavily shaded, +10% very sunny
- people − 2
- regular occupants beyond two, never negative
- 4,000
- added when the unit cools a kitchen
- 1
Measure the room.
Length × width in feet. For an L-shaped room, add the two rectangles. The chart assumes an 8 ft ceiling.
- 2
Find the chart capacity.
Read the BTU/hr for the row that contains your square footage.
- 3
Adjust for sun and shade.
Subtract 10% if the room is heavily shaded; add 10% if it's very sunny.
- 4
Adjust for people and kitchens.
Add 600 BTU for each person beyond two who regularly uses the room, and 4,000 BTU if it's a kitchen.
- 5
Pick the nearest unit.
Choose a unit rated close to the result. Bigger is not better: an oversized unit cools fast, shuts off before it dries the air and leaves the room clammy.
A kitchen runs the other way: a 16 × 12 ft kitchen is 192 sq ft, which is 6,000 BTU/hr on the chart, plus 4,000 for the cooking load = 10,000 BTU/hr. The range and oven matter more than the floor area.
ENERGY STAR room air conditioner sizing chart
| Area to be cooled (sq ft) | Capacity needed (BTU/hr) | Tons |
|---|---|---|
| 100 up to 150 | 5,000 | 0.42 |
| 150 up to 250 | 6,000 | 0.5 |
| 250 up to 300 | 7,000 | 0.58 |
| 300 up to 350 | 8,000 | 0.67 |
| 350 up to 400 | 9,000 | 0.75 |
| 400 up to 450 | 10,000 | 0.83 |
| 450 up to 550 | 12,000 | 1 |
| 550 up to 700 | 14,000 | 1.17 |
| 700 up to 1,000 | 18,000 | 1.5 |
| 1,000 up to 1,200 | 21,000 | 1.75 |
| 1,200 up to 1,400 | 23,000 | 1.92 |
| 1,400 up to 1,500 | 24,000 | 2 |
| 1,500 up to 2,000 | 30,000 | 2.5 |
| 2,000 up to 2,500 | 34,000 | 2.83 |
BTU vs. tons vs. watts
A BTU (British thermal unit) is the heat needed to raise one pound of water one degree Fahrenheit; air conditioners are rated in BTU per hour of heat removed. One ton of cooling is 12,000 BTU/hr, the rate at which a ton of ice melts in a day. One watt is about 3.41 BTU/hr, so a 10,000 BTU/hr unit removes about 2,930 watts of heat (its electrical draw is much lower, because a heat pump moves heat rather than making it).
When is a BTU chart not enough?
- Whole-house and ducted systems. Use Manual J. DOE's Right-Size Heating and Cooling Equipment fact sheet warns against sizing from floor area or contractor experience, and ENERGY STAR tells homeowners to have the contractor verify size with Manual J.
- High ceilings and open plans. The chart assumes 8 ft ceilings and a closed room. Vaulted rooms and rooms open to a hallway behave differently.
- Extreme climates. The chart doesn't change with location. Phoenix and Portland are not the same job.
- Poor insulation, big windows, attics. Bonus rooms over garages and sunrooms often need far more than the chart says, which is exactly what a load calculation catches.
Common BTU sizing mistakes
- Rounding up 'to be safe.' Oversized room units short-cycle and leave humidity behind. ENERGY STAR notes oversized units tend to have shorter lives because of short cycling.
- Forgetting the sun. A west-facing room with a glass slider can need 10% more than the same room on the north side.
- Using the chart for the whole floor. A 14,000 BTU window unit won't cool three rooms through doorways. Size per room, or move to a ducted or multi-zone system.
- Ignoring ceiling height. A 10 ft ceiling adds 25% more air volume than the chart assumes.
- Skipping electrical. Units above about 15,000 BTU often need a dedicated 20 A or 240 V circuit. Check before you quote a window unit.
Frequently asked questions
How many BTUs do I need for a 12 × 12 room?
A 12 × 12 ft room is 144 sq ft, which falls in ENERGY STAR's 100 up to 150 sq ft row: 5,000 BTU/hr. Add 10% if it's very sunny, or 600 BTU for a third regular occupant.
How many BTUs for 500 square feet?
ENERGY STAR lists 12,000 BTU/hr (one ton) for 450 up to 550 sq ft. Reduce 10% if the room is heavily shaded, add 10% if very sunny, and add 4,000 BTU if it's a kitchen.
How many BTUs are in a ton of air conditioning?
One ton equals 12,000 BTU per hour. A 2-ton unit is 24,000 BTU/hr, a 3-ton is 36,000 and a 5-ton is 60,000.
Is it better to oversize an air conditioner?
No. An oversized unit satisfies the thermostat quickly and shuts off before it removes much humidity, so the room feels cold and damp. ENERGY STAR notes oversized units short-cycle, which wears the compressor. Size to the load.
Can I use this calculator for central air?
Not to pick equipment. Central systems should be sized with an ACCA Manual J load calculation and Manual S equipment selection, which account for insulation, windows, orientation, infiltration and local design temperatures. Use the room chart for window and portable units only.
How much does ceiling height change the BTU number?
The ENERGY STAR chart is based on 8 ft ceilings. Taller ceilings mean more air to cool; ENERGY STAR suggests choosing a higher-capacity unit for rooms with higher ceilings, and a proper load calculation is the right tool for very tall or vaulted rooms.
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Sources
- ENERGY STAR — Room Air Conditioners (sizing chart and adjustments) (checked October 12, 2026)
- ENERGY STAR — Using the CEE/ARI Online Database (one ton = 12,000 Btuh) (checked October 12, 2026)
- U.S. DOE — Right-Size Heating and Cooling Equipment (Technology Fact Sheet) (checked October 12, 2026)
- ENERGY STAR — Clean Heating and Cooling (Manual J, oversizing and short cycling) (checked October 12, 2026)