BTU Calculator

Enter your room's dimensions, ceiling height, insulation, sun exposure, and climate to get a recommended BTU rating for cooling or heating — mapped to the standard unit sizes stores actually sell.

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How the BTU estimate works

A BTU (British thermal unit) is the energy needed to raise a pound of water 1°F; air conditioners and heaters are rated in BTU per hour of heat they can move. This calculator uses the standard sizing heuristic, and we'll show you exactly how it's built rather than pretend it's physics. Cooling starts at 25 BTU per square foot (the middle of the accepted 20-30 range), then scales by ceiling height (÷8 ft — a taller room is simply more air), insulation (poor ×1.2, average ×1.0, good ×0.85), sun exposure (shaded ×0.9, sunny ×1.1), and climate (mild ×0.85, extreme ×1.15), plus 600 BTU for each regular occupant beyond two — people are 100-watt heaters. Heating starts from climate instead: 30 BTU/sq ft mild, 45 moderate, 60 extreme, scaled by the same ceiling factor and by insulation (poor ×1.15, good ×0.85).

The formulas

Cooling: area × 25 × (ceiling ÷ 8) × insulation × sun × climate + 600 × (occupants − 2)  ·  Heating: area × baseclimate × (ceiling ÷ 8) × insulation

Area is length × width in square feet; the multipliers are the factors listed above; the heating base is 30/45/60 BTU per square foot for mild/moderate/extreme climates. Cooling results are then rounded up to the nearest standard unit size (5,000 to 24,000 BTU); heating to the nearest 500 BTU.

Worked example

Cooling: a 15 × 12 ft living room (180 sq ft), 8-ft ceiling, average insulation, very sunny, moderate climate, two occupants: 180 × 25 × 1.0 × 1.0 × 1.1 × 1.0 = 4,950 BTU — buy the 5,000 BTU unit, the smallest standard size, with nothing to spare for the sun load. Heating: the same room in a moderate climate with average insulation needs 180 × 45 = 8,100 BTU, so an 8,500 BTU heater (about 2.5 kW electric) covers it.

Why oversizing backfires — and when to skip the rule of thumb

The instinct to "go one size up, just in case" is the most common AC-buying mistake. An oversized unit blasts the room to temperature in minutes and shuts off — short-cycling — before it has run long enough to pull moisture out of the air. You get a room that's 72°F and swampy, a compressor that wears out early from constant restarts, and a higher electric bill than the smaller unit would have produced. Right-sized equipment runs long, steady cycles, which is exactly what dehumidification needs. That said, know this heuristic's blind spots: it can't see your windows (a west-facing glass wall can add thousands of BTU), your air leakage, or your ductwork. For a window unit or a space heater, the estimate here is plenty. For central air, a heat pump, new construction, or an addition, ask your contractor for a Manual J load calculation — the ACCA's room-by-room standard — and be suspicious of any installer who sizes a whole-house system by square footage alone.

Frequently asked questions

How many BTU do I need per square foot?

For cooling, the common rule of thumb is 20-30 BTU per square foot — this calculator starts at 25 and adjusts for ceiling height, insulation, sun, and climate. For heating, the range is wider: roughly 30 BTU per square foot in mild climates up to 60 in severe ones. These are sizing estimates, not physics; a Manual J calculation is the accurate version.

What size air conditioner do I need for a 12x12 room?

A 12 × 12 room is 144 square feet, which works out to roughly 3,600 BTU of cooling load under average conditions. The smallest window unit commonly sold is 5,000 BTU, so that's the right buy — and resist going bigger. A 5,000 BTU unit in a small bedroom already has capacity to spare.

Is it better to oversize an air conditioner?

No — oversizing is as bad as undersizing, just differently. An oversized AC cools the air so fast it shuts off before it has dehumidified, a pattern called short-cycling that leaves the room cold but clammy, wears out the compressor with constant starts, and wastes energy. The right-size unit runs longer, steadier cycles and actually feels more comfortable.

What is a Manual J calculation?

Manual J is the ACCA's room-by-room heating and cooling load procedure — the professional standard that accounts for window area and orientation, air leakage, duct losses, and local design temperatures. Rules of thumb like this calculator are fine for a window unit or space heater; for central systems, heat pumps, new construction, or additions, insist on a Manual J from your HVAC contractor.

How many BTU does it take to heat 1,000 square feet?

Between about 30,000 and 60,000 BTU per hour depending on climate and insulation — a well-insulated space in a mild climate sits near the bottom, a drafty one in a cold climate near the top. In a moderate climate with average insulation, plan on roughly 45,000 BTU. Note that furnaces list input and output BTU; a 60,000 BTU furnace at 80% efficiency delivers 48,000.

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