Air Conditioner Cooling Calculator
An undersized portable air conditioner will run constantly and never quite cool a room down, while an oversized one wastes energy and money. Cooling capacity depends on the room's volume and how much extra heat it generates from sun, appliances or cooking. Enter your dimensions below to get a BTU target for shopping.
Work out the cooling capacity you need
Enter your room dimensions and pick the room type that best matches its heat load.
TVs and general electronics add some heat
For a 33.6m³ living room, look for a unit rated at approximately:
5,000 BTU
That's based on a calculated need of 4,013 BTU, rounded up to the nearest standard portable AC unit size.
Recommended products
A few well-reviewed fans and portable air conditioners.
- 9000 BTU
Currently unavailable
Pros
- Cools a room up to 28 sqm in minutes
- Window exhaust kit included in the box
- 24-hour on/off timer for scheduled cooling
Cons
- Louder than split-system air conditioning
Currently unavailable
Pros
- Cooling, dehumidifying and fan modes in one unit
- Two window venting kits included in the box
- Digital display with remote control
Cons
- Bulkier than fan-only alternatives
£433.00
Pros
- HEPA H13 filtration captures pollen and pollutants while cooling
- App-connected air quality reporting
- Bladeless design is safe around children and pets
Cons
- Significant premium over a standard pedestal fan
- Newer SKU has a thinner review history than the older TP07 model
Frequently asked questions
Why does a home office need more cooling than a bedroom?
Computers, monitors and other equipment generate a surprising amount of heat when running for hours at a time, adding to the cooling load beyond what the room size alone would suggest. A bedroom with just a lamp and phone charger has a much lower heat load.
Does a south-facing room really need that much more cooling?
Yes — direct sun through glass is one of the biggest sources of unwanted heat in a room, particularly through large or unshaded windows. Conservatories and south-facing rooms with lots of glazing can need nearly double the cooling capacity of a similarly sized, shaded room.
Is it better to size up rather than down for a portable AC unit?
Generally yes, within reason — an undersized unit runs constantly and still struggles on hot days, while a slightly oversized one cools the room faster and can cycle off periodically. Going drastically oversized can occasionally lead to a clammy feel since the unit removes heat faster than humidity.
Do portable air conditioners need a window for venting?
Most single-hose portable units need a hose vented to an open window or through a wall to expel warm air outside — factor this into where you can realistically place one before buying based on BTU rating alone.
How this calculator works
The calculator uses a standard formula based on room volume to estimate BTU requirements. It starts with your room’s length, width, and height (in metres) to calculate the internal volume in cubic meters (m³). Then, it applies a heat gain factor – currently set at 350 BTU per square meter for an average UK home. This factor accounts for typical heat sources like sunlight, appliances, and people. Finally, the calculator multiplies the room volume by this heat gain factor to arrive at the estimated BTU cooling capacity needed.
Key Considerations for UK Homes
- **Insulation:** Poorly insulated homes generate significantly more heat. Consider adding 10-20% to your calculated BTU requirement if your property has thin walls or a poorly sealed loft.
- **Window Size & Type:** Large south-facing windows dramatically increase solar gain, particularly in summer. Add an extra 15-25% for rooms with substantial window areas – double-glazed windows offer some protection but aren't fully effective against intense sunlight.
- **Furniture & Flooring:** Dark carpets and furniture absorb more heat than lighter colours. Adding a small buffer of 5-10% can help account for this increased heat generation, especially in smaller rooms.
- **Occupancy:** The calculator assumes one person is typically present in the room. Increase the BTU requirement by approximately 20-30% if multiple people regularly occupy the space.
Worked example: Cooling a Bedroom
Let's say you have a bedroom that’s 4m long, 3m wide and 2.5m high. The room volume is 4 * 3 * 2.5 = 30 m³. Applying the standard heat gain factor of 350 BTU per square meter, we get a preliminary estimate of 30 * 350 = 10,500 BTU. Considering the potential for significant solar gain through a south-facing window and assuming one occupant, we’ll add an extra 20% – increasing the final requirement to approximately 12,600 BTU. This suggests you should look for an air conditioner with a cooling capacity of around 12,000 - 13,000 BTU.
More questions answered
What does BTU actually mean?
BTU stands for British Thermal Unit. It’s a measure of heat – essentially, how much energy it takes to raise the temperature of one pound of water by one degree Fahrenheit. Air conditioners are rated in BTU because this is the standard unit used to measure their cooling capacity.
Should I round up or down when choosing an air conditioner?
It’s generally better to slightly overestimate than underestimate. An undersized unit will struggle to cool a room effectively, leading to longer run times and higher energy bills. However, don't significantly over-size the unit – this can lead to short cycling (turning on and off frequently) which is also inefficient.
What if my room has very little sunlight?
If your room receives minimal direct sunlight, you’ll need to reduce the heat gain factor. A value of 300 BTU per square meter would be more appropriate for a dark, north-facing room with limited windows.
Can I use this calculator for outdoor spaces?
No, this calculator is specifically designed for indoor rooms. Cooling outdoor areas requires a different type of unit – typically a patio heater or a portable air cooler designed for external use.
What’s the difference between BTU and wattage?
BTU measures cooling capacity while wattage measures electrical power consumption. They're related, but not directly equivalent. An air conditioner with a higher wattage will generally have a greater cooling capacity (more BTUs).
My room has lots of dark-coloured furniture – should I increase the BTU calculation?
Yes, absolutely! Dark colours absorb significantly more heat. Adding an extra 10% to your calculated BTU requirement is recommended for rooms with a lot of dark furniture or carpeting.
What if I’m using the air conditioner in a room with multiple people?
If you regularly use the room with several occupants, increase the BTU calculation by approximately 30%. This will ensure adequate cooling capacity to maintain a comfortable temperature for everyone.
Will this calculator account for heat from appliances?
The standard heat gain factor of 350 BTU per square meter accounts for typical household appliance heat. However, if you frequently use high-powered appliances like ovens or computers that generate significant heat, consider adding a further 10-15% to the calculation.
Can I adjust the heat gain factor?
Yes, you can adjust the heat gain factor based on your specific circumstances. However, be cautious when making adjustments – a significantly different value could lead to inaccurate results. Use the key considerations above as a guide.
What's the minimum BTU rating I should look for in an air conditioner?
Generally, you’ll want to aim for at least 6,000 BTU per square meter of floor space. For smaller rooms, 4,500 - 6,000 BTU might be sufficient, but larger rooms will need considerably more cooling power – often 8,000 BTU or higher.