Etoolio

AC BTU & Tonnage Calculator

Free AC size calculator. Enter a room's dimensions, climate, sun exposure and use to get the cooling capacity it needs in BTU and tons, plus the recommended standard AC unit size. Tuned for hot climates.

Units
Climate
Sun exposure
Insulation & glazing
9,500
BTU/hr needed
Recommended AC: 1 ton
12,000 BTU unit (0.8 tons calculated, round up to the next standard size)
Room area: 172 sq ft1 ton = 12,000 BTU/hr

This is a quick estimate using standard sizing rules. Very sunny walls, poor insulation, lots of glass, or heat-producing equipment can push the need higher. For an oversized or critical install, confirm with a proper Manual J load calculation from a qualified HVAC professional. An undersized unit runs constantly and struggles in peak heat; a badly oversized one short-cycles and dehumidifies poorly.

Get the AC size right before you buy

Buy too small an air conditioner and it runs flat out yet never cools the room on the hottest days. Buy too big and it blasts cold air, switches off, and leaves the room clammy because it never runs long enough to pull out humidity. This free calculator sizes the unit properly from the things that actually matter: the room, the climate, the sun, the insulation and how the room is used.

What it takes into account

  • Room size: length, width and ceiling height, in metres or feet.
  • Climate: temperate, hot, or very hot like the Gulf, which sets the base cooling per square foot.
  • Sun exposure: a shaded room needs less, a sun-facing one with lots of glass needs more.
  • Insulation and glazing, plus the number of people and whether it is a heat-heavy kitchen.

BTU and tons, explained

Cooling capacity is measured in BTU per hour, and air conditioners are sold in tons, where one ton equals 12,000 BTU per hour. So a room needing 18,000 BTU wants a 1.5 ton unit, and 24,000 BTU wants 2 tons. Because units only come in standard sizes, the calculator rounds your requirement up to the next standard size and shows both the exact BTU figure and the recommended unit.

Why hot climates change the maths

Standard rules of thumb size a room at roughly 20 BTU per square foot, but that assumes a mild climate. In very hot places the AC fights a much bigger temperature difference all day, so the same room can need 30 BTU per square foot or more. Sizing on floor area alone is why so many units in the Gulf end up underpowered. This tool adjusts for climate, sun and insulation so the estimate reflects real conditions.

A planning tool, not a substitute for an engineer

This gives a quick, sensible estimate that is ideal for planning a purchase or sanity-checking an installer's quote. For a large, unusual or critical installation, have a qualified HVAC professional run a full load calculation on your specific walls, windows and orientation. It runs entirely in your browser and is completely free.

Good to know

AC BTU & Tonnage Calculator, frequently asked questions

What size air conditioner do I need for my room?

Enter the room's length, width and ceiling height, your climate, how much sun the room gets, its insulation, how many people use it and whether it is a kitchen, and the calculator gives the cooling capacity needed in BTU per hour and tons, then recommends the nearest standard AC size. As a rough guide, cooling needs run from about 20 BTU per square foot in mild climates up to around 30 or more in very hot ones like the Gulf.

How do I convert BTU to tons?

One ton of cooling equals 12,000 BTU per hour, so you divide the BTU figure by 12,000. For example, 18,000 BTU is 1.5 tons and 24,000 BTU is 2 tons. AC units are sold in standard sizes (0.75, 1, 1.5, 2, 2.5, 3 tons and so on), so the calculator rounds your requirement up to the next standard size.

Why does climate and sun exposure change the answer so much?

The hotter the outside air and the more direct sun a room gets, the harder the AC works to keep it cool, so the same room needs more cooling capacity in Dubai than in London. Sunny west-facing rooms, lots of glass and poor insulation all raise the requirement, which is why this calculator asks about them rather than sizing on floor area alone.

Is it better to oversize or undersize an AC?

Neither extreme is good. An undersized unit runs constantly, never quite cools the room and struggles in peak heat. A badly oversized unit cools too fast then switches off (short-cycling), which wastes energy and leaves the room humid because it never runs long enough to remove moisture. The aim is a unit matched to the load, which is what this tool estimates.

Does this replace a professional load calculation?

No. It is a quick, reliable estimate using standard sizing rules, good for planning and comparing quotes. For a large, unusual or critical installation, a qualified HVAC engineer should do a full Manual J load calculation that accounts for your exact walls, windows, orientation and local design temperatures.

Does it work in both metric and imperial?

Yes. Switch between metres and feet, and enter the room dimensions in whichever you prefer. It converts internally and shows the room area in square feet along with the result.

Is it free?

Yes, free with no signup, and it runs entirely in your browser.

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