AC BTU Calculator

AC BTU Calculator: What Size AC Do You Need?

Choosing an air conditioner starts with one honest question: what size AC do I need for this space? Buy too small and the unit runs all day without ever quite catching up. Buy too big and you pay more up front for a room that feels cool but clammy. This page works the answer out from your floor area, ceiling height, insulation, climate and sun, in imperial or metric units, then explains every adjustment below so you can check the maths yourself before you spend anything.

Quick answer: BTU by room and house size

At the basic rule of thumb — 20 BTU per sq ft, 8 ft ceilings and a well-insulated room — the starting sizes look like this:

Floor area	Cooling needed	Size in tons
300 sq ft	6,000 BTU	0.5 tons
500 sq ft	10,000 BTU	0.8 tons
750 sq ft	15,000 BTU	1.3 tons
1,000 sq ft	20,000 BTU	1.7 tons
1,500 sq ft	30,000 BTU	2.5 tons
2,000 sq ft	40,000 BTU	3.3 tons
2,500 sq ft	50,000 BTU	4.2 tons

These are starting estimates — the adjustments below move real homes up or down from these, sometimes by a quarter or more. A poorly insulated room in a hot, sunny climate can need half as much again, while a shaded, well-sealed room in a mild climate may need a little less.

Important: this is a rule-of-thumb estimate, NOT an ACCA Manual J load calculation. Before installing a central system, a contractor should run a proper Manual J, which accounts for window area and orientation, duct losses, air leakage and local design temperatures.

The 20 BTU rule and its adjustments

The 20 BTU rule says a well-insulated room with standard 8 ft ceilings needs about 20 BTU of cooling for every square foot of floor. It survives because it is easy to remember and roughly right for a modern, shaded room in a moderate climate. It is a starting point, not a promise: it assumes decent insulation, ordinary windows and no unusual heat sources, and real rooms break those assumptions in predictable ways — which is why the calculator adjusts it instead of just using it.

Insulation is the biggest adjustment. An average home — older walls, a thin attic layer, a few draughts — leaks cool air fast enough to need about 25 BTU per sq ft, and a poorly insulated room needs around 32. Ceilings matter because you cool volume, not floor: scale the result by ceiling height ÷ 8, so a 10 ft ceiling adds a quarter. Climate adds up to 15% in hot, humid weather and takes 10% off in cool, mild areas, because the unit works against the gap between inside and outside. A very sunny room with large west-facing glass gains heat all afternoon, so it takes another 10%. People are heat sources too: allow 600 BTU for each person beyond the first two who regularly use the room. And a kitchen adds about 4,000 BTU, because cooking is a heat source no other room has.

Worked example

Take a 400 sq ft living room with 9 ft ceilings, average insulation, a moderate climate, normal sun and 3 occupants. Here is the calculator's working, step by step:

• Step 1 — base: 400 sq ft × 25 BTU per sq ft (average insulation) = 10,000 BTU.
• Step 2 — ceilings: 10,000 × (9 ÷ 8) = 11,250 BTU, because taller ceilings mean more air to cool.
• Step 3 — people: 3 occupants is one more than the standard two, so 11,250 + 600 = 11,850 BTU.
• Step 4 — what to buy: the nearest standard size at or above 11,850 is a 12,000 BTU (1 ton) unit.

Climate and sun stay at ×1.0 in this example, so they change nothing. Change any one input — make the room a kitchen, say — and the same steps show you exactly where the bigger number comes from.

BTU, tons and kW explained

A BTU (British thermal unit) is a small measure of heat, and an air conditioner's BTU/h rating tells you how much heat it can remove in an hour. A “ton” of cooling sounds like a weight but is really a rate: 1 ton = 12,000 BTU/h. The word comes from ice — it is the cooling you would get from melting one ton of ice over 24 hours, a definition from the days when cooled air often meant ice in a box.

Kilowatts measure the same cooling in metric terms: 1 kW ≈ 3,412 BTU/h, so a 12,000 BTU unit is about 3.5 kW of cooling — not the same figure as the electricity it draws. Australia sells and regulates air conditioners in kW, and standard split sizes there are 2.5 / 3.5 / 5 / 7 kW. The UK sells room units in both BTU and kW, so check which number a listing is quoting before you compare two models.

Why bigger is not better

An oversized unit blasts the room to the thermostat setting in a few minutes, then switches off — a pattern called short-cycling. That causes three problems. First, the air cools but its moisture does not leave with it: an air conditioner removes humidity mainly while it runs, so short runs leave the room feeling clammy even at the right temperature. Second, you paid for capacity you never use, in the purchase price and in the heavier electrical work a big unit may need. Third, constant starting and stopping wears compressors and fans faster than steady running does. A correctly sized unit runs longer, quieter cycles, dehumidifies properly and usually costs less to own. If you land between two sizes, the smaller one is often the better buy unless your room has a specific extra heat load.

This is not a Manual J

Manual J is the ACCA's standard method for working out a home's heating and cooling load room by room. Instead of one rate per square foot, it uses your local design temperatures, the area, orientation and shading of every window and door, wall and attic insulation values, duct losses and how leaky the building is. That is why no single-rate calculator — including this one — can replace it. You need a proper Manual J before a new central system is installed, when you add an extension or extra rooms, when a home never feels comfortable despite a working system, or when a contractor proposes a much bigger unit than the one being replaced. For a single window unit or a mini-split serving one room, the estimate on this page is a reasonable starting point.

Room AC vs central vs mini-split

The right type of system depends on how much of the home you need to cool and whether ducts already exist:

• Window or portable units: cheapest to buy and fine for one room, a rental or a short season. Portable units are the least efficient of the three and the noisiest, but they need no installation.
• Mini-split (ductless) systems: a good fit for zoned cooling, extensions and homes without ducts. Each indoor unit serves one area, they are efficient and quiet, and you cool only the rooms you use — at a higher upfront cost than window units.
• Central air: best for cooling a whole house evenly where ducts already exist or are planned. It costs the most to install, and it is exactly the case where a Manual J matters, because the unit and the ducts have to match the house.

Whichever type you choose, size it from the load first and shop second — the most expensive mistake is a powerful unit cooling a small room badly.

AC sizing guides

Prefer to read the reasoning behind the numbers? These two guides explain AC sizing in full:

• What Size Air Conditioner Do I Need?
• BTU Chart for Room Size

Frequently asked questions