Air Conditioner Size Calculator
BTU and tons for a room, with the usual adjustments.
Work out Air Conditioner Size. BTU and tons for a room, with the usual adjustments. Free, with no account and nothing to download.
About 20 is the common starting point; poorly insulated or hot climates need more.
Two is the baseline; each extra adds 600 BTU.
Leave at zero to skip the running cost.
From your own bill, including per-kWh delivery charges.
This is a shopping estimate, not a design. A Manual J load calculation accounts for insulation, air leakage, window orientation and local design temperatures, and that is what an installed system should be sized from.
Cooling capacity
10,000 BTU/h
0.83 tons
Bigger is not better here, which is the mistake people make. An oversized unit cools fast then shuts off, leaving humidity in the air so the room feels clammy at the right temperature — and the short cycling wears the compressor. A unit slightly under-sized runs longer and dehumidifies properly.
How the Air Conditioner Size Calculator works
The starting rule is around 20 BTU per hour per square foot, adjusted for the things that change the load: a sun-facing room needs more, extra occupants add heat, and a kitchen adds a great deal. This gives the figure to shop with. It is not a Manual J load calculation, which accounts for insulation, glazing, infiltration and local design temperatures, and which is what an installed system should be sized from.
Also known as: AC BTU calculator · HVAC tonnage calculator · what size air conditioner do I need · room AC size calculator
The calculation itself
Around 20 BTU per hour per square foot is the common starting point, adjusted for the things that change the load: a sun-facing room adds about 10%, each occupant beyond two adds 600 BTU per hour, and a kitchen adds 4,000.
Those adjustments follow the widely published guidance and are all editable, because the base figure genuinely depends on ceiling height, insulation, glazing and climate.
In practice
500 square feet at 20 BTU per square foot is 10,000 BTU per hour — 0.83 tons.
Make it a sunny kitchen with four occupants and it becomes 11,000 for sun, plus 1,200 for the extra occupants, plus 4,000 for the kitchen: 16,200 BTU per hour, or 1.35 tons. The room did not change size; its load did.
At SEER 16 that draws about 1,013 W, costing around 15 cents an hour at a 15 cent tariff.
Why oversizing is the common mistake
An oversized unit cools the air quickly and shuts off. Cooling is the fast part; dehumidifying is the slow part, and it only happens while the unit runs.
So an oversized room reaches temperature with the humidity still in it, and feels clammy at a thermostat setting that should be comfortable. The instinctive response — turning the temperature down further — makes it worse.
Short cycling also wears the compressor and is less efficient than steady running, so the oversized unit costs more to buy, more to run and delivers worse comfort.
A slightly undersized unit runs longer, dehumidifies properly and generally feels better, which is the opposite of what most buyers assume.
Where the figure deceives
This is a shopping estimate, not a design. A Manual J load calculation accounts for insulation values, air infiltration, window orientation and area, and local design temperatures — and can differ substantially from a per-square-foot rule.
The rule of thumb is worst for the cases that differ most from average: a well-insulated modern room with triple glazing needs far less, and a poorly insulated room with west-facing glass needs far more.
Nor does it account for whole-house systems, where duct losses and zoning change the requirement again.
Acting on it
Use this to know roughly what to shop for, then get a proper load calculation before installing anything ducted.
Resist rounding up to the next size. If you are between sizes, the smaller usually gives better comfort.
Check the SEER and your tariff together — the running cost difference over a system's life often exceeds the price difference at purchase.
Frequently asked questions
What size air conditioner do I need?
Roughly 20 BTU per hour per square foot as a starting point, so a 500 square foot room is around 10,000 BTU — about 0.83 tons. Add for sun exposure, extra occupants and kitchens.
Is bigger better?
No, and this is the common mistake. An oversized unit cools quickly then shuts off, which leaves humidity in the air and makes a room feel clammy at the right temperature. Short cycling also wears the compressor.
How many square feet does a ton cool?
Around 600 at 20 BTU per square foot, but that varies hugely with insulation, glazing and climate. Treating it as a fixed ratio is how systems end up badly sized in both directions.
What does SEER mean for running cost?
It is cooling output in BTU per hour divided by electrical input in watts, so a higher SEER draws less power for the same cooling. Supply your SEER and tariff here to see the hourly running cost.
Do I need a proper load calculation?
For anything installed, yes. Manual J accounts for insulation, air leakage, window orientation and local design temperatures — this estimate tells you roughly what to shop for, not what to fit.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/air-conditioner-size-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Air Conditioner Size Calculator"></iframe>The only condition is that the credit line stays visible. It sits inside the frame, so you do not have to do anything to keep it.