How the BTU calculation works
We start from the widely used guideline of about 20 BTU per hour for every square foot of floor space, which assumes an 8-foot ceiling, average sun and a moderate climate. That figure is then scaled by the room's actual ceiling height, because a taller room contains more air to cool.
Next we apply adjustments similar to those in the ENERGY STAR sizing guidance: subtract around 10% for a heavily shaded room, add 10% for a very sunny one, add 600 BTU for each person beyond two, and add 4,000 BTU for a kitchen. We also account for climate severity, insulation quality, large windows and heat-generating electronics.
The total is rounded up to the nearest standard AC size. Running cost is based on typical efficiency ratings — CEER for window units, SACC-based efficiency for portables and SEER2 for ductless mini-splits — multiplied by your hours of use and electricity price, with an average compressor load factor of 70%.
AC buying guide
Quick sizing chart
| Room area | Approx. m² | Capacity |
|---|
| 100–150 sq ft | 9–14 | 5,000 BTU |
| 150–250 sq ft | 14–23 | 6,000 BTU |
| 250–350 sq ft | 23–33 | 8,000 BTU |
| 350–450 sq ft | 33–42 | 10,000 BTU |
| 450–550 sq ft | 42–51 | 12,000 BTU |
| 550–700 sq ft | 51–65 | 14,000 BTU |
| 700–1,000 sq ft | 65–93 | 18,000 BTU |
| 1,000–1,400 sq ft | 93–130 | 24,000 BTU |
Window units
The most affordable way to cool one room. Modern U-shaped and inverter window units are much quieter and more efficient than older designs. Check that your window type (double-hung vs. sliding) is supported.
Portable units
Easy to move and install, but the least efficient. Choose a dual-hose model if possible and compare the SACC figure, not the ASHRAE number printed largest on the box.
Ductless mini-splits
The quietest and most efficient option, often with SEER2 ratings above 20. Most double as heat pumps for winter heating. They cost more upfront and typically need professional installation.
Efficiency labels
Higher CEER or SEER2 means lower running cost. Upgrading from a CEER 10 to a CEER 14 unit cuts electricity use by almost 30% for the same cooling.
Questions & answers
How many BTU do I need per square foot?
Roughly 20 BTU per square foot for an 8-foot ceiling in a moderate climate, adjusted for sun, people, insulation and climate.
What if my AC is too big?
It short-cycles, leaves humidity behind and wastes energy. Size as close as possible to the calculated value, or choose an inverter model that can throttle down.
Why do portables need more BTU?
Their realistic SACC capacity is often 30–50% lower than the headline ASHRAE rating because heat leaks back into the room.
Can one unit cool several rooms?
Only if air can flow freely between them. Closed doors block cooling; a multi-zone mini-split is the better choice for separate rooms.
How many BTU is one ton?
One ton of cooling equals 12,000 BTU per hour, or about 3.5 kW.
About
CoolRoom is an independent free calculator built to help people size air conditioners correctly. We are not connected to any HVAC manufacturer, installer or retailer. Our formulas follow widely published sizing guidelines and typical efficiency ratings.
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Last updated: October 2026
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