What size AC unit do I need for 1,000 square feet?

This calculator’s baseline is 1,000 × 25 BTU/hr, before ceiling, sun, insulation, and windows.

Quick answer: 1,000 × 25 = 25,000 BTU/hr on this calculator, before the other factors. DOE’s heating and cooling guide defines one ton as 12,000 Btu, so 25,000 ÷ 12,000 = 2.08 tons. That division is not a model number to order.

Step 1: the base load

1,000 sq ft × 25 BTU/hr per sq ft = 25,000 BTU/hr before ceiling, sun, insulation, and windows. With DOE’s 12,000 Btu per ton, 25,000 ÷ 12,000 = 2.08 tons.

25 BTU/hr per square foot is this calculator’s baseline.

Step 2: the adjustments

Factor	Multiplier	1,000 sq ft result
Base (moderate climate, 8 ft ceilings)	25 BTU/sq ft	25,000 BTU
Good insulation (recent, air-sealed)	−15%	21,250 BTU
Poor insulation (drafty)	+25%	31,250 BTU
Big west/south sun exposure	+15%	28,750 BTU
Heavily shaded lot	−10%	22,500 BTU
Each window (add per window)	+600 BTU	+600 each
10 ft ceilings (25 × 10 ÷ 8)	×1.25	31,250 BTU

The live calculator multiplies sun and insulation together, then adds windows, then a 10% margin, then rounds up to the nearest 500 BTU. Each row above changes only one factor, and it stops before that margin. Run the calculator for the combined result. Room-by-room heads are in the mini-split sizing guide.

The oversizing trap

An oversized unit can satisfy the thermostat before it has dehumidified, and leave the space cold and clammy. This page does not print a percent of oversize that is safe.

Ducts and airflow (central only)

Airflow is CFM = BTU/hr ÷ (ΔT × 1.08), on the duct CFM guide. This page does not assign a CFM-per-ton rule.

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A product search is not a load. Mini-splits on Amazon · line-set kits on Amazon

Honest label: this is rule-of-thumb planning math, not a Manual J. For a purchase this size, have the installer run a real load calc — and be suspicious of any who sizes by square footage alone in either direction.

Plug your own rooms, ceilings, and sun exposure into the HVAC load calculator, then check the per-square-foot reasoning in how many BTU per square foot. The ENERGY STAR room-AC chart for the next area up is what size air conditioner for 1,200 sq ft.

A portable unit is a different rating: portable air conditioner sizing. Steps that are not a larger ton are on how to improve air conditioner efficiency. When the old cabinet leaves, refrigerant recovery is how to dispose of an air conditioner.

Sources & Standards

Related standards. These pages describe the publications and do not print the rule-of-thumb factors used in the calculators: