A 15 by 12 room with average sun and two people needs about 3,600 BTU/hr. For a multi-room zone or a whole floor, run each room through separately and add the totals rather than measuring the whole space as one number.
One room's total. Add totals across rooms for a multi-zone or central system.
A single window unit for one bedroom is a one-step calculation. A multi-zone ductless system or a central unit covering several rooms is a room-by-room calculation followed by addition. Run each room through the fields above with its own sun exposure and occupancy, note the BTU figure, then combine the rooms that share a system.
| Constant | Value used here |
|---|---|
| Base cooling load | 20 BTU per square foot |
| Sun exposure adjustment | 0.9x shaded, 1.0x average, 1.1x very sunny |
| Extra occupant load | 600 BTU per person beyond 2 |
| Conversion to tons | 12,000 BTU/hr = 1 ton |
A 15 by 12 bedroom with average sun and two people comes to 3,600 BTU/hr (180 sq ft x 20 x 1.0). A connecting 12 by 10 home office with heavy afternoon sun and one person comes to 2,640 BTU/hr (120 sq ft x 20 x 1.1). If both rooms share one ductless multi-zone system, the combined load is 6,240 BTU/hr, roughly half a ton. Sizing the system to that combined number, rather than to whichever single room felt like the "main" space, is what keeps both rooms comfortable instead of just one.
An oversized unit cools a room's air temperature down fast and then shuts off, which sounds efficient but actually cuts the run time short before the unit has dehumidified the air. The result is a room that reads cold on a thermostat but still feels damp. Size close to the calculated number for each room or zone rather than rounding up for a margin; a unit sized correctly and running longer cycles removes more humidity than an oversized one that short-cycles.
A kitchen open to a living room should be measured as one combined space, not two separate rooms added together, since air mixes freely between them and one system is typically cooling both at once. A closed bedroom down the hall, by contrast, should be its own line item even on the same multi-zone system, since a closed door limits how much the two spaces share conditioned air.
Enter this room's dimensions on the live page, then use this view to print the BTU total for comparing units or zones.
For a ductless multi-zone system or a central unit covering an open floor plan, run each room separately, then add the BTU totals together. For separate window units cooling separate rooms, size each unit to its own room's number instead of the combined total.
Sun exposure and occupancy are doing the work here, not square footage. A west-facing bedroom that gets afternoon sun needs more cooling capacity than an identically sized north-facing room, and a room that regularly holds more than two people needs extra capacity for their body heat.
No. An oversized unit cools the air quickly and shuts off before it has run long enough to remove much humidity, leaving a room that feels cold and clammy at the same time. Sizing close to the calculated number, not well above it, usually gives better comfort than buying bigger for a safety margin.
Measure the combined square footage of the connected space as one entry rather than running the kitchen and living room separately, since air moves freely between them and a single unit or zone is typically covering both at once.
No. This is a square-footage rule of thumb for a quick sizing estimate, not a substitute for a Manual J load calculation, which accounts for insulation, window type, ceiling height and climate zone in detail. Get a Manual J done before buying a central system for a whole house.