AC BTU Calculator: What Size Air Conditioner Do You Need?

Type the square feet, pick your climate (or your state) and the calculator returns the cooling BTU and tons, the nearest standard unit size, and the heating BTU and furnace rating for the same space — with the adjustments for sun, insulation, ceiling height, people and kitchens that the rules of thumb leave out.

Choose what you are buying — a window or portable unit for one room, a ductless mini-split for a zone, a central system for the house, or a furnace or heat pump — then enter the area and the conditions. The calculator applies the standard rule of thumb for that equipment and climate, adjusts it for insulation, sun, ceiling height, occupants and a kitchen, and rounds to the sizes manufacturers sell. The heating rows always show, so you can size a heat pump that does both jobs. Treat the result as a check on a contractor’s proposal, not a replacement for a Manual J load calculation on new central equipment.

How many BTUs do I need per square foot?

Figure 20 BTU per square foot of cooling for an average house in a mixed climate, 24 in a hot-humid one and 16–18 in a cold one — so a 1,500 sq ft house needs about 30,000 BTU (2.5 tons) in the Mid-Atlantic and 36,000 BTU (3 tons) on the Gulf Coast. One room sized for a window unit uses a different scale: 500 sq ft takes a 12,000 BTU unit, 1,000 sq ft a 18,000 BTU unit. Heating runs 30–60 BTU per square foot by climate.

A BTU (British thermal unit) is the heat needed to raise one pound of water by 1 °F; an air conditioner’s rating is how many BTUs it removes per hour, and a ton is 12,000 BTU/h — the melting rate of a ton of ice in a day. Window units are sold by BTU (5,000–24,000), mini-splits by BTU (9,000–36,000 per head) and central systems by the ton (1.5–5 in half-ton steps). Manufacturers do not make a 2.7-ton unit, which is why the tool rounds up to the next standard size and tells you how far above your number it lands.

Worked examples

• 500 sq ft open-plan apartment, mixed climate, two people, average sun: Energy Star base 12,000 BTU for 450–550 sq ft → 12,000 BTU window or portable unit. With a kitchen in the room, add 4,000 → 16,000 → an 18,000 BTU unit.
• 1,000 sq ft ranch, mixed climate, average insulation: 1,000 × 20 = 20,000 BTU → 2-ton central system (24,000 BTU).
• 1,500 sq ft house in Houston (hot-humid), 9-ft ceilings, sunny: 1,500 × 24 = 36,000, × 1.125 for ceilings × 1.1 for sun = 44,600 BTU → a 4-ton system. Same house shaded with good insulation: 32,800 BTU → 3 tons. The envelope swings the answer by a full ton.

Window and portable AC sizing chart (Energy Star)

The Energy Star chart below is the one printed on the side of the box at the hardware store. It assumes two people, 8-ft ceilings and average sun; the official adjustments are +10% for a very sunny room, −10% for a heavily shaded one, +600 BTU for each person beyond two, and +4,000 BTU if the unit serves a kitchen. Measure the area the unit will actually cool — an open doorway to a hall counts, a closed bedroom door does not.

Portable units deserve a warning. A “12,000 BTU” portable is typically 6,500–8,000 BTU by the DOE test, because its single hose pulls conditioned room air out to cool the condenser and draws hot outside air in through the gaps. Size portables by the SACC number, prefer dual-hose models, and expect a window unit of the same rating to cool a room about half again as large. Mini-splits are rated honestly and run at 25–40% of a window unit’s electricity for the same cooling. Match any of these to the room with the room size calculator, which gives square feet and the BTU for every common room size.

What size central AC for a 1,500, 2,000 or 3,000 sq ft house?

Central systems are sized by the ton, in half-ton steps from 1.5 to 5, and the right number depends on the climate first and the house second. A 2,000 sq ft house is a 3.5-ton job in Florida, a 3-ton job in Virginia and a 2.5-ton job in Minnesota at average insulation; a tight 2010s house drops half a ton in any of them, and a leaky 1960s house with west-facing glass adds half a ton. The old rule of “one ton per 400 square feet” comes from pre-1980 houses in hot climates and oversizes most modern homes by a ton.

Which state is which climate zone?

The zones follow the IECC/DOE climate map, simplified to one zone per state (the tool’s state picker uses this list). States that straddle zones — California’s coast versus the Central Valley, Texas’s Panhandle versus Houston, upstate versus downstate New York — should pick the zone that matches their summer, not the state line.

• Hot-humid (Gulf, Southeast): Alabama, Arkansas, Florida, Georgia, Hawaii, Louisiana, Mississippi, Oklahoma, South Carolina, Texas.
• Hot-dry (Southwest desert): Arizona, California, Nevada, New Mexico.
• Mixed (Mid-Atlantic, Pacific NW, upper South): Delaware, District of Columbia, Kansas, Kentucky, Maryland, Missouri, New Jersey, North Carolina, Oregon, Tennessee, Virginia, Washington, West Virginia.
• Cold (Northeast, Midwest, Mountain): Colorado, Connecticut, Idaho, Illinois, Indiana, Iowa, Massachusetts, Michigan, Nebraska, New York, Ohio, Pennsylvania, Rhode Island, South Dakota, Utah, Wisconsin.
• Very cold (northern tier, Alaska): Alaska, Maine, Minnesota, Montana, New Hampshire, North Dakota, Vermont, Wyoming.

Existing equipment is the best check. If the house already has a 3-ton system that keeps up on the hottest day with the thermostat holding, 3 tons is right regardless of what any chart says; if it runs non-stop and loses ground after 3 pm, the house needs sealing and insulation before it needs a bigger unit. The HVAC sizing tool walks through the same decision with duct and equipment options, and the utility cost calculator shows what cooling costs by state.

Furnace size calculator: heating BTU per square foot

Heating is sized by heat loss, which depends on the winter design temperature far more than on square feet, so the per-square-foot factor doubles from the Gulf to the northern tier: 30 BTU per sq ft in hot climates, 40 in mixed, 50 in cold and 60 in very cold, for average insulation and 8-ft ceilings. A 2,000 sq ft house in Ohio therefore needs about 100,000 BTU/h of heat on the coldest night; the same house in Georgia needs 60,000.

Furnaces are labeled by input, not output. A 95% AFUE furnace rated 100,000 BTU input delivers 95,000 BTU of heat; an 80% furnace with the same label delivers 80,000. Divide the heat loss by the efficiency to get the input rating to buy, then round up to the sizes sold (40, 60, 80, 100, 120, 140 thousand BTU). Heat pumps are sized in tons like air conditioners, but their capacity falls as the outdoor temperature drops — a standard 3-ton heat pump makes about 3 tons at 47 °F and half that at 17 °F, which is why cold-climate models and backup heat strips exist.

Oversizing a furnace is less damaging than oversizing an air conditioner but still wastes money: a 120,000 BTU furnace in a house that loses 60,000 runs in short bursts, stresses the heat exchanger and leaves rooms far from the thermostat cold. If a contractor proposes replacing “like for like,” ask for the load calculation — most furnaces installed before 2000 were sized by rule of thumb for an uninsulated house and are one or two sizes too big for the house as it is now.

Why an oversized air conditioner is a problem

An air conditioner sized a ton too big short-cycles: it hits the thermostat setting in 8–10 minutes and shuts off before the coil has run long enough to pull moisture out of the air. The house ends up at 74 °F and 60% humidity — cold and clammy — and the owner turns the thermostat down further, which costs more and still feels wrong. Short cycling also wears out the compressor (starts are the hardest moment of its life), and an oversized system needs oversized ducts that most houses do not have, so airflow is noisy and uneven.

Undersizing by a half-ton is a far smaller problem than most people fear. The system runs long cycles on the hottest afternoons of the year and may lose a degree or two for a few hours; the rest of the summer it dehumidifies properly and uses less electricity. ACCA’s Manual S, the equipment-selection standard, lets a contractor pick a unit from 95% up to 115% of the calculated cooling load (up to 125–130% for some heat pumps in cold climates) and no more. If two quotes differ by a ton, the smaller one is usually the one that did the load calculation.

Mini-split sizing and multi-zone systems

Size a single-zone mini-split like a central system for the area it serves: 20–24 BTU per sq ft in warm climates, so a 1,500 sq ft open floor plan wants a 30,000–36,000 BTU (2.5–3-ton) single head, or more realistically two or three heads totaling that. Heads come in 6,000, 9,000, 12,000, 18,000, 24,000 and 36,000 BTU; a bedroom is a 6,000–9,000 BTU head, a living room 12,000–18,000. A head cannot cool through a closed door, so count rooms, not just square feet.

For a multi-zone system add up the heads and pick an outdoor unit rated for 80–100% of that total — manufacturers allow connecting heads up to about 130% of the outdoor unit’s capacity because the rooms never all peak at once, but running every head at full load on an undersized condenser is where complaints start. One 36,000 BTU outdoor unit with four heads (12 + 9 + 9 + 6 = 36,000) covers a typical 1,500–1,800 sq ft house in a mixed climate; hot-humid houses add a head or step the condenser up to 48,000. Inverter mini-splits also heat efficiently to about 5 °F (cold-climate models to −15 °F), so the heating rows in the calculator apply to them as well.

Manual J versus rules of thumb

Everything on this page is a rule of thumb: square feet times a factor, adjusted for the big variables. An ACCA Manual J load calculation instead models each wall, window, roof and floor with its orientation, insulation value and air leakage, the local design temperatures, the people and the appliances, and produces a sensible and a latent load for every room. It typically comes in 10–30% below a rule-of-thumb size for houses built after 1990, which is why contractors who skip it install bigger equipment than the house needs.

Ask for a Manual J whenever you replace central equipment, add a mini-split system, or finish an addition or basement; it costs $150–$400 as a stand-alone service, is free from contractors who do it routinely, and is required by code for new construction in most states. Use this calculator to sanity-check the result: if the Manual J says 2 tons and the chart says 3.5, something in the house — a wall of west glass, an uninsulated attic — is either missing from the model or is the thing to fix. For a finished basement or a new room, the room size calculator gives the area and a window-unit size; the HVAC sizing tool covers whole-house equipment choices and ductwork.