BTU Calculator

How to use the btu calculator

Measure the room you’re cooling — length and width in feet — and let the calculator find the floor area. For open layouts, measure the whole connected space the unit will actually serve, not just the room it sits in: a living room that flows into a dining area is one cooling zone, and sizing for half of it guarantees disappointment. The base capacity comes from the Energy Star square-footage table, the same chart printed on the back of most window-unit boxes.

Then tell the calculator what makes your room non-average. Ceiling height matters because the table assumes 8 ft — a 10 ft ceiling holds 25% more air, and the math scales accordingly. Sun exposure swings capacity by ±10%: pick "very sunny" for west- or south-facing rooms with big unshaded windows, "shaded" for north-facing rooms or tree cover. Add your regular occupant count (each person beyond two contributes about 600 BTU of body heat), and flip the kitchen toggle if the unit cools the kitchen — a running range and refrigerator add roughly 4,000 BTU.

Resist the instinct to size up "to be safe" — oversizing is the classic window-unit mistake, and it makes rooms feel worse, not better. An oversized unit blasts the air to temperature in minutes and shuts off before it has run long enough to pull moisture out of the air. The result is short cycling: a cold, clammy, cave-damp room and more compressor wear. A correctly sized unit runs longer, steadier cycles that dehumidify as they cool. If you land between sizes, the next size UP is right only when the room has other heat sources; otherwise trust the number.

Note that portable units need a skeptical eye: their advertised BTU often uses an older rating method that overstates real-world cooling versus a window unit of the same number (look for the SACC rating — "seasonally adjusted cooling capacity" — for the honest figure). If you’re going portable, treat this calculator’s result as the SACC you need, not the marketing number.

Once you know the size, compare efficiency between candidate units with the CEER number on the yellow EnergyGuide label (combined energy efficiency ratio — higher is better; 12+ is good for window units). Two 8,000 BTU units cool identically; the higher-CEER one just does it on fewer watts, which is the whole difference in your summer bill.

The formula

An Energy Star table lookup by floor area, scaled for the things the table assumes — 8 ft ceilings, average sun, two occupants, no kitchen:

base BTU = Energy Star table value for length × width
BTU = base × (ceiling height ÷ 8) × sun factor
      + (occupants − 2) × 600   [only above 2]
      + 4,000 if the kitchen is in the cooled space
rounded to the nearest 100 · unit = next common size ≥ BTU

Worked example with the defaults: a 15 × 12 ft bedroom is 180 sq ft, which falls in the table’s 150–250 sq ft band — 6,000 BTU. Standard 8 ft ceiling (×1), average sun (×1), two occupants (+0), no kitchen (+0) leaves it at 6,000 BTU, and the matching unit size is exactly 6,000.

A sunnier, busier example: a 20 × 20 ft family room (400 sq ft → 10,000 BTU base) with strong afternoon sun becomes 11,000 BTU — buy the 12,000 BTU unit. Above 34,000 BTU you’re past sensible window-unit territory; that’s central-air or multi-zone mini-split scale.

Frequently asked questions

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