AC BTU Calculator

BTU stands for British Thermal Unit — the amount of heat needed to raise 1 pound of water by 1°F. In AC sizing, BTU/hr measures how much heat an air conditioner can remove from a room per hour. Higher BTU = more cooling power.

An oversized AC cools the room too quickly without running long enough to remove humidity. This creates a cold but clammy feeling, causes short cycling that wears out the compressor faster, and actually uses more energy than a properly sized unit.

A common starting point is 20 BTU per square foot in moderate climates. But actual needs vary significantly with ceiling height, insulation quality, window exposure, climate zone, and heat-generating equipment. This calculator accounts for all these factors.

EER (Energy Efficiency Ratio) measures efficiency at a fixed peak condition (95°F outside). SEER (Seasonal Energy Efficiency Ratio) measures efficiency across an entire cooling season with varying temperatures. SEER is a more realistic measure of annual energy costs.

Yes, in most cases. Mini-splits offer SEER ratings of 16–26 vs 10–12 for window units. A properly sized mini-split can cut cooling costs by 30–50% compared to a window AC, often paying back the premium cost within 3–5 years depending on local electricity rates and usage hours.

Use the total connected square footage, not just one zone. Add 10–15% for south or west-facing large windows, and 600 BTU per person who regularly occupies the space. For very large open plans, a multi-zone mini-split or central air system is usually most efficient.

Size up when: the room is in a hot climate, has poor insulation or many large south/west-facing windows, is a kitchen with cooking appliances, has significant computer or server equipment, or the room is above the second floor where heat accumulates.

Portable ACs exhaust hot air through a hose, creating negative pressure that draws warm outside air in through gaps. Their EER of 8–10 vs 10–12 for window units means 20–30% higher energy costs for equivalent cooling. Use them only when a window unit isn't possible.

Yes. Our calculator uses a height factor of (ceiling height ÷ 8). A 10-foot ceiling room has 25% more air volume than the same floor area with an 8-foot ceiling, requiring proportionally more cooling capacity.

Significantly. Poor insulation increases heat gain by 15% or more. Good insulation can reduce required BTU by 10%. Upgrading insulation before buying a new AC often saves more money long-term than simply buying a larger unit.

Each watt of electrical equipment produces approximately 3.41 BTU/hr of heat. A server rack drawing 2,000 watts adds 6,820 BTU/hr — often more heat than the room itself. Use the "Server Room" room type in this calculator and enter equipment wattage for an accurate estimate.

Calculate each room individually then use the Multi-Room Planner tab to total them. For 2–4 rooms, a multi-zone mini-split is usually most efficient. For whole-home cooling with many rooms, central air with properly designed ductwork is typically the best solution.