BTU Calculator for a Room
Estimate room AC capacity from the ENERGY STAR 100–2,500 sq ft chart, with sun, people and kitchen adjustments. See the chart band and assumptions.
This calculator applies the ENERGY STAR room air conditioner sizing chart to a rectangular room and adds its sun, occupancy and kitchen adjustments. It provides a starting capacity estimate for window or through-wall room units within the chart’s 100–2,500 square-foot range. The estimate does not verify humidity control, installation suitability or a particular product’s performance.
How to use this tool
- Choose feet or meters, then enter the room's length and width.
- Pick how much sun the room gets.
- Enter how many people regularly use the room.
- Tick the kitchen box if the unit will cool a kitchen.
- Compare the adjusted chart capacity with manufacturer guidance for the room and product.
Formula
BTU/h = chart value × sun factor + 600 × max(0, people − 2) + (4,000 if kitchen)
The chart gives area-band capacity. Sun factors are 0.9 for heavy shade, 1 for average and 1.1 for strong sun. Occupancy additions start after two people. Metric areas convert with 1 m² ≈ 10.764 ft². Shared endpoints use the higher band; 2,500 ft² is included.
Example
A 20 × 25 ft family room is 500 sq ft, inside the 450 up to 550 sq ft band, so the chart gives 12,000 BTU/h. The room is very sunny, so add 10%: 13,200. Three people use it in the evening, one more than the chart assumes, which adds 600 for a total of 13,800 BTU/h — about 4,044 W or 1.15 tons of cooling. A 14,000 BTU/h nominal product is nearby; product suitability still needs checking.
Frequently asked questions
Does ceiling height change the BTU I need?
The published chart assumes an 8-foot ceiling. Higher ceilings, unusual glass and construction can change the load; this calculator does not apply an invented height multiplier. Get a more detailed assessment when the chart assumptions do not fit.
Why not just buy a bigger unit?
Oversized units can cycle too quickly to control humidity. Compare the estimate with product performance and installation guidance rather than automatically adding a large margin.
Can I use this for portable AC or several rooms?
Portable units use different rating procedures, and doorways restrict air distribution between rooms. Do not treat this chart alone as a portable-unit or multiroom selection. Follow the manufacturer’s coverage and installation guidance.
How do BTU, watts and tons relate?
One BTU per hour is 0.293 W of cooling, and 12,000 BTU/h is one ton. That is cooling output, not electricity used: the power draw is on the unit's label, and its efficiency is its CEER — BTU of cooling per watt-hour of electricity.
Sources
Last updated: 2026-10-30