What Size Air Conditioner Do I Need?

Last updated on April 27, 2025

Choosing the right cooling system can be confusing, and one of the biggest questions homeowners face is, “What size air conditioner do I need?” Getting the right size is crucial for comfort, efficiency, and saving on energy bills — and we’re here to guide you through it.

A 2018 survey found that more than 7.6 million (that is around 70%) Australian homes had already installed at least one form of AC to keep the indoor temperatures comfortable.

Whereas these units play such a crucial role, they are heavy consumers of energy and one of the biggest contributors to your utility bills.

A report by the Australian Government’s YourHome website claims AC can consume up to 40% of household energy use in Australia. This is quite high, and you would want to bring it as low as possible in all ways.

But how can you actually do it?

One of the biggest secrets is choosing the right size air conditioner.

In this article, we will teach you how to properly size your air conditioner so that you don’t end up consuming too much energy or lacking the comfort you need.

Why the Right AC Size Matters

When buying an AC, most homeowners think that choosing a larger unit will serve them best. The reality is that choosing too big or too small comes with various risks, as explained below:

Undersized AC:

Increased Energy Use: Smaller units must run for longer hours to control the indoor climate. This uses more energy and adds to utility bills.

Uneven Cooling: Smaller units can’t deliver uniform cooling. Some part of your home will be warmer or cooler than others, making it even more uncomfortable.

Shorter AC Life: Since small units have to work overtime, the rate at which they wear and tear is much accelerated.

Oversized AC:

Humidity Issues: A larger AC cools the room too quickly, but leaves the air damp and uncomfortable

Short Cycling: The quick cooling causes them to turn off prematurely. The AC has to keep cycling on and off too often, reducing efficiency and shortening its lifespan

Higher Costs: Larger units are pricier, and they might often need additional setup, which increases your upfront investment.

How To Determine The Right AC Size For Your Needs

Choosing the appropriate size of air conditioner begins with determining how much cooling power your home requires.

An air conditioner’s effectiveness is determined in kilowatts (kw) or British Thermal Units (BTUS), both of which indicate the amount of heat the unit can extract from a room in an hour.

The larger a room, the greater the kw or BTU rating it will require to maintain comfortable indoor temperatures.

Step-by-Step Guide to Choosing the Right AC Size

Measure the Room Area

To determine the total room area, measure the length and width and calculate:

Area = Length x Width

If it is an irregular room, partition it into smaller rectangular sections. Use the above formula to work out the area of each section, then add them all up.

Note: You can use imperial units (measurements in feet) or metric units (measurements in meters) as you like.

Work Out the Required Capacity

General Guideline:

Imperial: For every square foot of a living space, approximately 20 BTUS are needed.

Total Cooling Required = Area ( in square feet) x 20 BTU

Metrics: In the case of kilowatts (kw), convert the area into square meters and multiply by 0.15.

Total Cooling Required = Area (in square meters) × 0.15

Example Calculation:

Room Measurement: 20ft X 25ft = 500 square feet.

Cooling Required: 500 x 20 = 10,000 BTU.

For Metric

Room Measurement: 6m X 8m = 48 square meters.

Cooling Required: 48 x 0.15 = 7.2 kW.

Adjustments to Make When Sizing an Air Conditioner

After calculating the basic size, you may need to adjust the capacity based on a few other factors:

Sunlight Exposure: If the room gets a lot of direct sunlight, increase the cooling capacity by about 10%.
For rooms that are heavily shaded, you can reduce the capacity by about 10%.
Ceiling Height: The standard ceiling height is about 2.4 metres (8 feet). If your ceilings are higher, you should increase the capacity by about 10% for every additional metre.
Number of Occupants: If more than two people regularly occupy the room, add about 600 BTUS (or 0.2 kw) for each extra person.
Appliances and Equipment: Rooms with many heat-generating appliances (like kitchens) may require extra cooling capacity. Add approximately 4,000 BTUS (or 1.2 kw) for kitchen areas.
Insulation Quality: Poorly insulated rooms will need a slightly larger unit to maintain the desired temperature. Good insulation helps reduce the cooling load.

Example Calculation (with Adjustments)

Assume the same 500 sq. ft. room we used in the previous example (20ft × 25ft) has:

3m (10 ft) ceiling height
Large windows with heavy afternoon sunlight
4 occupants
Partially connected to a kitchen

Base Calculation:

500 sq. ft. × 20 = 10,000 BTU

Adjustments:

+10% for 10-ft ceiling → 10,000 × 1.10 = 11,000 BTU

+10% for heavy sunlight → 11,000 × 1.10 = 12,100 BTU

+600 BTU × 2 extra people → 12,100 + 1,200 = 13,300 BTU

+4,000 BTU for kitchen appliances → 13,300 + 4,000 = 17,300 BTU

The total BTU needed would be = 17,300 BTU

Metric Version:

Room Size: 48 square meters (6m × 8m)

Base Calculation:

48 × 0.15 = 7.2 kW

Adjustments:

+10% for 3m ceiling → 7.2 × 1.10 = 7.92 kW

+10% for heavy sunlight → 7.92 × 1.10 = 8.71 kW

+0.2 kW × 2 extra people → 8.71 + 0.4 = 9.11 kW

+1.2 kW for kitchen appliances → 9.11 + 1.2 = 10.31 kW

The total kw needed would be = 10.31 kw

Reference Charts and Sizing Tables

For a quick estimation, use the following reference charts:

Room Size	Example Areas	2–2.5 kw	Approx. Capacity (BTU)	Price Guide
Small (up to 20m²)	Bedroom, study, small kitchen	2.5–5 kw	6,800–8,500 BTU	$600–$3,300
Medium (20–40m²)	Bedroom with ensuite, small lounge	4–6 kw	8,500–17,000 BTU	$600–$4,700
Large (40–60m²)	Large bedroom, mid-sized lounge, kitchen	5–9 kw	13,600–20,500 BTU	$800–$4,700
Extra Large (60m²+)	Open-plan areas, large lounges	5–9 kW	17,000–30,700 BTU	$1,200–$4,700+

Expert Tips When Choosing the Right AC Size

Don’t estimate: Always measure the area and figure out the exact space that the AC will serve. Estimating the room area without measuring will most likely lead to an underestimation or overestimation.

Don’t forget about ceiling fans: If the room has fans installed already, you can possibly go for a slightly smaller air conditioning unit since the fans will help in cooling and circulating the air to the intended areas.

Don’t make adjustments based on other rooms: Every part of the room has a specific cooling requirement. When purchasing a unit for a single room, ensure it’s designed for that particular room rather than the entire house.

Check manufacturer guides: Most air conditioner manufacturers offer specific sizing charts and guidelines for their equipment. Always ensure you check them for correctness.

Don’t forget about the possibilities of later changes: If the use of the room is subject to change, for example, turning a study room into a nursery, factor that into your sizing choice now to avoid future upgrades.

When to Consult a Professional

DIY calculations offer a helpful starting point, but some situations call for expert advice. Professional input is recommended if:

Your home has complex layouts, such as vaulted ceilings, open-plan designs, or multiple storeys.

You’ve renovated your property or are dealing with mixed-use spaces (like residential areas combined with commercial setups).

There are concerns about ductwork or insulation, as these can significantly impact cooling performance.

In these cases, we can perform a Manual J load calculation — the industry-standard method for accurately sizing an air conditioner and assess other factors like insulation quality and system design.

Quick Tips for Choosing the Right AC Size

Measure Accurately: Ensure you don’t go wrong when taking your room’s measurements.
Consider Insulation, Sunlight, and Climate: Factor in all key elements when sizing your AC.
Use Charts and Calculators as Starting Points: These can guide you, but always verify with the detailed manual calculation.
Don’t Assume Anything: Don’t make assumptions. Take physical measurements and remember that each room is different.
Consult an Expert: If you are unsure of anything or need more help, you can always contact us, and we will be more than happy to help.

Final Verdict on What Size Air Conditioner Do I Need?

Getting the right size air conditioner is crucial for maintaining comfort, efficiency, and the long-term health of your system.

By following the simple steps outlined above, using reference tools, and consulting a professional when necessary, you can ensure that your AC is properly sized for your home.

Frequently Asked Questions (FAQS)

How do I determine if my current air conditioner is the right size for my home?

Check if the AC is cooling effectively. If it fails to do so, it could be too small. Also, check if it cycles on and off frequently. If it does, it could be too large.

Can I downsize my air conditioner if I add insulation or replace windows?

Improving insulation or upgrading to energy-efficient windows can reduce the cooling load, meaning even a smaller AC might serve you comfortably.

Does the age of my air conditioner affect its sizing and efficiency?

Older units may have lost efficiency and cooling capacity, meaning you may need a larger or newer system to meet your home’s cooling needs.

How do I know if my air conditioner is working efficiently after sizing it correctly?

A properly sized unit should maintain consistent temperatures and humidity, and it should run for longer, more efficient cycles rather than frequently switching on and off.

Can an oversized air conditioner cause issues in specific areas of my home (e.g., bedrooms or living areas)?

Yes, oversized units can cause uneven cooling, where some rooms are too cold while others remain warm, leading to discomfort.