By Zencalcy Editorial TeamLast reviewed About us
BTU per hour is the honest unit of cooling capacity, and room sizing starts from floor area at roughly 20 BTU per square foot. Every person adds about 600 BTU of body heat, a sunny exposure adds a wall of solar gain, and each appliance adds its own continuous load.
BTU per hour needed to cool a room.
BTU = area × 20 + 600 × (people − 2) + sun + 1,500 × appliances
Standard window and portable units are sold in 5,000 to 25,000 BTU sizes, roughly 6,000 apart. Round up to the next size rather than down, but do not jump more than one size past the calculation or the unit will short-cycle.
| Symbol | Meaning |
|---|---|
| A | Room floor area. |
| Q | Heat gain in BTU/hr. |
| n | Occupants and appliances. |
6,000 + 600 + 4,000 sun + 1,500 appliance = 12,100 BTU/hr — round to a 12,000 BTU or 14,000 BTU unit, about 1 ton.
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A 144 square foot room wants about 5,000 to 6,000 BTU on area alone, more with west-facing sun or three sleepers.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026