βοΈ Room BTU Calculator
Size your air conditioner correctly for any room.
Required Cooling Capacity
How to Use This Calculator
Measure your room length and width in feet and enter them, along with the ceiling height. Then pick the sun exposure and insulation quality that best describe the room. The calculator starts from 20 BTU per square foot and applies each adjustment to give you the capacity you actually need, plus the equivalent tonnage and the nearest standard AC size to buy.
Measure the room length and width in feet. For L-shaped rooms, split the room into two rectangles, calculate each, and add them.
Enter the ceiling height. The default is 8 feet; rooms with taller ceilings add 10 percent for the extra air volume.
Choose sun exposure (sunny rooms add 10 percent, shaded rooms subtract 10 percent) and insulation quality (poor insulation adds 15 percent, good insulation subtracts 10 percent).
Buy a unit at or just above the recommended BTU. If your number falls between standard sizes, round up, not down.
The Formula
The 20 BTU per square foot baseline comes from industry Manual J sizing guidelines simplified for residential rooms. The adjustments stack additively because each factor (extra air volume, solar gain, heat leakage) adds a roughly independent load on the system. For example, a sunny room with poor insulation gets base x 1.25, which matches field experience that such rooms run noticeably hotter.
Tonnage is simply BTU divided by 12,000. Room units are sold in BTU increments (5,000, 6,000, 8,000, 10,000, 12,000, 15,000, 18,000, 24,000), so the calculator rounds your result up to the nearest common size.
Worked Example
A 20 x 15 foot living room with 9-foot ceilings, sunny exposure, and average insulation. Base = 20 x 15 x 20 = 6,000 BTU. Adjustments: +10 percent ceiling, +10 percent sun = 6,000 x 1.20 = 7,200 BTU, or 0.6 tons. The nearest standard size is 8,000 BTU. If that same room had poor insulation instead, it would be 6,000 x 1.35 = 8,100 BTU, pushing you to a 10,000 BTU unit.
Tips
Seal air leaks and add curtains or reflective film to sunny windows before buying a bigger unit; cutting the load is cheaper than cooling it. For kitchens, add 4,000 BTU to cover cooking heat. If two or more people regularly occupy the room, add about 600 BTU per extra person beyond the first two. And remember that a correctly sized unit that runs steadily is more comfortable and cheaper to operate than an oversized unit that short cycles.
Estimates for planning only. For whole-house systems, have an HVAC contractor run a full Manual J load calculation.
Frequently Asked Questions
How many BTUs do I need per square foot?
The standard rule of thumb is 20 BTU per square foot of room area for rooms with 8-foot ceilings and average insulation. A 400 square foot room starts at 8,000 BTU. This calculator adjusts that baseline for ceiling height, sun exposure, and insulation quality, which is why a sunny room with poor insulation can need 35 percent more than the baseline.
What happens if my AC is too big or too small?
An oversized AC cools too fast and shuts off before it dehumidifies, leaving the room cold and clammy, and it wastes energy with constant short cycling. An undersized AC runs nonstop on hot days, wears out faster, and never quite reaches the set temperature. Being close to the calculated BTU matters more than most people think.
How many BTUs are in a ton of cooling?
One ton of cooling equals 12,000 BTU per hour. The term comes from the cooling power of one ton of ice melting over 24 hours. A 24,000 BTU unit is a 2-ton unit. Room ACs are usually listed in BTU (5,000 to 24,000), while central systems are rated in tons.
Does ceiling height change the BTU calculation?
Yes. The 20 BTU per square foot rule assumes 8-foot ceilings. Vaulted or taller ceilings add air volume that must be cooled, so this calculator adds 10 percent for ceilings over 8 feet. For very tall ceilings over 10 feet, consider adding another 5 percent on top.
Should I add BTUs for kitchens or sunny rooms?
Yes. Kitchens add heat from cooking, so add about 4,000 BTU for a kitchen. Sunny rooms with lots of afternoon sun need about 10 percent more capacity, while heavily shaded rooms can use 10 percent less. Poor insulation adds another 15 percent. These adjustments are exactly what the calculator applies.