Enter a room's area, ceiling height, sun exposure, occupants, and kitchen use to estimate the air conditioner size it needs.
How to Use the BTU Calculator
Enter the room's square footage and ceiling height, then pick a sun exposure level from None, Some, or Lots. Add the number of occupants and check the kitchen box if one applies — the recommended BTU total updates with every change.
Contractors reach for a fast shortcut long before running a full room-by-room load calculation: twenty BTU per square foot, built around a standard eight-foot ceiling.
base BTU = room area (sq ft) × 20 × (ceiling height ÷ 8)A ten-foot ceiling holds more air than an eight-foot one over the identical floor space, so the ceiling-height ratio scales that base number up when the room runs taller than standard, and down for anything shorter. Three adjustments layer on afterward. Sun exposure shifts the total by ±10% — a room with a wall of west-facing windows heats up differently than one that barely sees daylight. Every occupant past the first two adds 600 BTU, since bodies genuinely produce heat a spec sheet won't account for. A kitchen adds a flat 4,000 BTU for the stove, oven, and refrigerator running nearby.
More Isn't Automatically Better
Undersizing is the obvious failure — the unit runs nonstop on a hot afternoon and the room never quite gets there. The less obvious failure is oversizing, and it's the mistake people make when they follow the "bigger must be better" instinct. An oversized unit blasts the room down to temperature fast, shuts off, then kicks back on minutes later. That rapid cycling never runs long enough to actually pull humidity out of the air, so the room ends up cool but clammy — and the compressor wears out faster from all the extra starts and stops.
This method is exactly what its name implies: a rule of thumb, the kind an HVAC contractor uses to get in the right neighborhood fast. The real spec comes from an ACCA Manual J load calculation, which factors in insulation quality, window count and glazing type, local climate zone, and air leakage — none of which this simplified version sees. Use the number here to compare options and set a rough budget, not as the final figure you hand to an installer.