BTU Calculator

At this calculator's default room dimensions, switching Climate Zone between its mild and hot settings shifts Cooling Capacity Needed from about 9,000 to about 13,000 BTU/hr, a larger swing than a similarly modest change to room length or width produces -- climate zone acts as a large fixed multiplier on the whole calculation.

Total Window Area adds a fixed 100 BTU per square foot of glass directly to the cooling load calculation (200 BTU per square foot for the heating estimate), on top of the base square-footage figure -- more window area always raises both Cooling Capacity Needed and Heating Capacity Needed, since it's a straightforward additive term in both formulas.

Insulation Quality applies a fixed multiplier to the base cooling and heating loads -- poor insulation multiplies the load up by 30% for cooling (and 50% for heating), while excellent insulation cuts it down by 10% for cooling (and 15% for heating), which at default room dimensions swings the cooling estimate from about 12,000 down to about 9,000 BTU/hr between the two extremes.

Larger rooms do increase Cooling Capacity Needed, since Room Length and Room Width both raise the base square footage the whole calculation scales from, but the selectable factors (insulation, climate zone, sun exposure) can shift the estimate by a larger margin than a modest change in room size, so two similarly-sized rooms can need very different equipment.

Total Window Area is a fixed input, not something that scales automatically with room size, so its 100 BTU/sq ft (cooling) contribution gets spread across a larger base square footage as the room grows -- the per-square-foot average falls even though the total BTU requirement still rises. This is expected: it's the fixed window allowance being diluted by more floor area, not a sign the calculation is wrong.