AC Size Calculator
Work out what size air conditioner a room needs from its area, height and heat sources.
What this tool does
An AC size calculator estimates cooling capacity from room area, ceiling height and heat-load factors. As a starting rule, a room needs about 20 BTU per square foot, or roughly 65 to 80 watts per square metre. A 20 m² room at 2.4 m needs about 12,000 BTU, or one ton of cooling. Bigger is not better: an oversized AC short-cycles, never removes humidity properly, and costs more to run.
Estimates cooling capacity from room area, ceiling height and heat-load factors.
Accounts for sun exposure, insulation, occupants and appliances, which all add heat.
Shows the result in BTU, tons and kW for any AC type.
Converts between the ratings you see on boxes and the capacity your room needs.
How to use it
- Enter the room length and width, or the area directly.
- Enter the ceiling height.
- Choose sun exposure: shaded, average or sunny.
- Choose the insulation level.
- Enter the number of people and major heat-producing appliances.
- Read the required capacity in BTU, tons and kW.
Why use this calculator
Estimating cooling capacity from room area, ceiling height and heat-load factors stops the most common air conditioning mistake: buying on room area alone. Area gives a starting point, but a sunny top-floor room with four people and a fridge is not the same as a shaded ground-floor room used by one. Undersized units never catch up on a hot afternoon, and oversized ones cycle so fast that they cool the air without removing the humidity, which leaves the room clammy.
Why we built it
We built the AC size calculator to show the adjustment factors as separate multipliers, so you can see why the answer is larger than 20 BTU per square foot. The room heating calculator is the same calculation in the opposite direction, and the electricity cost calculator turns the running hours into money. For the room itself, the square footage calculator gives the area, and the water usage calculator covers the condensate drain.
On the finance side, the loan payoff calculator spreads the purchase over a term, and the discount and profit calculator checks the quote you were given.
Who this tool is for
- Anyone buying a window, portable or split AC for one room
- People comparing quotes and checking the tonnage suggested
- Landlords fitting out a rental and needing a sensible size
- Office managers cooling a single meeting room
When to use it
- Before buying, to choose the right capacity.
- When a room is always the hottest in the house.
- Comparing the running cost of different sizes.
- Checking a contractor's recommendation.
How it is calculated
Base capacity = area in square feet × 20 BTU, adjusted for ceiling height above 2.4 m, then multiplied by factors for sun exposure, insulation, occupancy and appliances. One ton of cooling equals 12,000 BTU/h, and one watt of cooling capacity is about 3.412 BTU/h, so 12,000 BTU is about 3.5 kW of cooling.
Worked example: a 20 m² room (215 ft²) at 2.4 m needs 215 × 20 = 4,300 BTU at the base. Sunny exposure adds about 10%, average insulation adds 0%, two extra people add about 8%, and a couple of appliances add about 6%, landing near 12,000 BTU - one ton, or about 3.5 kW.
BTU per hour is the capacity, not the electricity use. A 12,000 BTU AC draws about 1,000 to 1,500 watts, because it moves heat out rather than making it. That efficiency is why AC can cool a room using far less power than a heater would need to heat it.
Capacity by room size
| Room area | BTU/h | Tons | Cooling kW |
|---|---|---|---|
| 10 m² | 6,000 | 0.5 | 1.76 |
| 15 m² | 9,000 | 0.75 | 2.64 |
| 20 m² | 12,000 | 1 | 3.52 |
| 30 m² | 18,000 | 1.5 | 5.27 |
| 40 m² | 24,000 | 2 | 7.03 |
| 55 m² | 36,000 | 3 | 10.55 |
Adjustment factors
| Condition | Add |
|---|---|
| Sunny or top floor | 10% - 20% |
| Kitchens and server rooms | 4,000 - 6,000 BTU |
| Each extra person above two | 600 BTU |
| Poor insulation or single glazing | 10% - 20% |
| Ceiling above 2.4 m | Proportional to height |
| Large south-facing windows | 10% - 15% |
A worked example
These are the values the calculator opens with, so you can see exactly what it does with them. Change any value on the page and the result updates; the figures below come from the same calculation.
| Input | Value |
|---|---|
| Room size | 5 m × 4 m |
| Ceiling height | 2.4 m |
| Sun exposure | Average |
| Insulation | Average |
| Occupants | 4 |
| Major appliances | 1 |
With those inputs, the calculator returns:
- Room area: 20 m² (215 ft²)
- Base capacity: about 4,300 BTU/h
- After adjustments: about 12,000 BTU/h
- Tons: 1
- Cooling kW: 3.5
Why bigger is not better
An oversized AC reaches the set temperature quickly, then shuts off before it has removed much moisture from the air. Short cycling means the compressor starts far more often, which wears it out faster, and the room feels cool but damp. Buying the next size up 'just in case' usually costs more to buy and more to run.
| Problem | Likely cause |
|---|---|
| Room never gets cool enough | Undersized, or too much heat gain not counted. |
| Room feels damp and clammy | Oversized - cycles before dehumidifying. |
| High electricity bills | Oversized unit short cycling all day. |
| AC runs constantly and barely keeps up | Undersized, or heat sources not accounted for. |
What you enter, and what you get back
| Item | Details |
|---|---|
| Room size | Length and width, giving the area. |
| Ceiling height | Above 2.4 m increases the requirement. |
| Exposure and insulation | Set the heat-gain multipliers. |
| Occupants and appliances | Each add heat the AC must remove. |
| Result | Capacity in BTU/h, tons and cooling kW. |
Issues you may run into
| What you see | Why it happens | What to do |
|---|---|---|
| No standard size matches | Adjustments pushed the number between sizes. | Round up, never down, to the next available size. |
| Estimate much higher than the box rating | Open-plan or multi-room space. | Measure the whole cooled volume, not one room. |
| Cool but still sticky | Unit oversized for the load. | Choose the smaller size and use the fan setting. |
| Never cools on hot days | Undersized or poor insulation ignored. | Re-run with a higher heat-gain factor. |
Assumptions and limitations
- A rule-of-thumb estimate, not a professional load calculation.
- Does not model individual window U-values, wall areas or air changes.
- Ignores duct losses for central systems and heat gained through the ductwork.
- Local climate extremes may need a larger unit than the estimate suggests.
Frequently asked questions
What size AC do I need for my room?
Start at about 20 BTU per square foot, or 65 to 80 watts per square metre, then adjust for ceiling height, sun exposure, insulation, occupants and appliances. A 20 m² room at 2.4 m typically needs about 12,000 BTU, which is one ton of cooling.
How do I calculate AC tonnage?
Divide the required BTU/h by 12,000. A room needing 24,000 BTU/h needs 2 tons. One ton is 12,000 BTU/h, which is about 3.5 kW of cooling capacity. Tonnage is a capacity rating, not a weight.
How many BTU per square foot?
About 20 BTU per square foot for a standard room at 2.4 m ceiling height. Reduce to 15 BTU for a shaded, well-insulated room and go to 25 or 30 for a sunny top floor or a room with large windows. Adjust for people and appliances on top.
What is a ton of cooling?
A ton is 12,000 BTU per hour, the amount of heat removed by a ton of ice melting over 24 hours. It is about 3.5 kW of cooling capacity. Modern split systems are usually described in BTU or kW, but tonnage is still common on window units and older systems.
Is a bigger AC always better?
No. An oversized unit reaches the set temperature quickly and then short cycles, which means it never removes much humidity, feels clammy, costs more to buy and wears the compressor out faster. Round up to the next available size if you are between ratings, but do not buy two sizes larger.
How does room insulation affect AC size?
Poor insulation and single glazing can add 10% to 20% to the required capacity, because heat flows in through the fabric as fast as the AC removes it. Insulated walls, double glazing and shaded windows reduce the load, so a smaller and cheaper unit will hold the temperature.