What Size Air Conditioner Do I Need?

Choose an air conditioner from the room's cooling load, not floor area alone. Length and width provide a starting point, but ceiling height, glazing, direct sun, insulation, occupants and computers can move the requirement substantially.

Use the calculated BTU/h or cooling-kW range to shortlist products, then confirm that the exhaust arrangement works in the actual window.

Calculate the cooling capacity my room needs

What BTU per hour means

BTU/h is a rate of heat removal. Product listings often shorten it to BTU, but the capacity is normally BTU per hour. Cooling kW describes the same output in a metric unit: 1kW of cooling is approximately 3,412BTU/h.

Neither figure is the appliance's electrical input.

A portable unit advertised at 9,000BTU/h therefore does not draw 9,000BTU of electricity. Find its input watts or rated energy data separately when estimating cost.

Measure room volume

Record internal length, width and ceiling height. Two rooms with the same floor area need not have the same air volume. A tall converted room contains more air and often more external surface than a standard-height bedroom.

Open-plan spaces must be measured as the connected area the unit will actually try to cool; an open doorway does not create an invisible boundary.

Adjust for the real heat gains

• Glazing admits solar heat and usually insulates less than a solid insulated wall.
• South- and west-facing windows can receive strong afternoon or evening sun.
• Poorly insulated roofs and walls allow heat to enter faster.
• Each additional occupant adds heat to the room.
• Desktop computers, gaming PCs, monitors and televisions release most of their electrical input as heat indoors.
• Ovens, dryers and other appliances can overwhelm a small cooling plan.

The US Department of Energy advises selecting room air conditioners to suit the space and warns that significant oversizing can increase cycling and reduce moisture removal.

Its room-air-conditioner guidance is not a substitute for a UK building calculation, but the sizing principle is useful.

Practical scenarios

Small bedroom

A compact, shaded bedroom with a standard ceiling and one occupant may fall near the lower portable classes. Noise, exhaust sealing and the ability to position the unit without directing cold air at the bed may be more restrictive than capacity.

Use the room measurements rather than assuming every box room needs 7,000BTU/h.

Medium bedroom

A larger bedroom, two occupants or more glazing increases the load. If the window receives afternoon sun, account for it instead of selecting the same unit as a shaded room of equal area.

Read air conditioning for a bedroom before accepting a louder, higher-capacity model.

Home office with several computers

Add the approximate heat from PCs, monitors and other continuously powered equipment. A small office can need more cooling than a larger quiet bedroom.

Also check that the electrical circuit and socket arrangement comply with the appliance instructions; do not create an unsafe extension-lead layout to reach the window.

South-facing living room

Large glazing, direct sun, several occupants and an open doorway can push the requirement well beyond what area suggests. First reduce solar gain with suitable shading while keeping required ventilation and safety in mind.

Energy Saving Trust's home-cooling guidance recommends blocking daytime heat and ventilating when outside air becomes cooler.

Avoid serious undersizing and oversizing

An undersized machine may run continuously without reaching the target, especially during peak sun. A substantially oversized unit can cost more, take more space and cycle frequently.

Portable-unit behaviour differs from fixed systems, so do not apply a single rule blindly. Choose reasonable headroom rather than jumping several capacity classes above the estimate.

Plan the exhaust before buying

A portable air conditioner moves heat from the room into an exhaust airstream. That hot air must reach outside. Confirm the supplied kit suits a sliding, sash or casement window and that the hose can remain short and correctly positioned.

Read portable AC window kits and exhaust hoses.

Use the SetupSelect air-conditioner finder for a transparent estimate based on volume and heat-gain factors.

Then compare 7,000 vs 9,000 vs 12,000BTU, running cost, portable AC vs fan and portable AC vs air cooler.

Before you trust a capacity estimate

Write down the assumptions with the result: room dimensions, window direction, usual number of people, equipment and whether doors will stay closed. That makes the estimate repeatable and exposes the factor most likely to change it.

If a room is open to a hall or kitchen, measure the connected space or plan a physical boundary; a portable unit cannot be expected to ignore moving air.

Use the estimate as a shortlist range rather than a promise of a particular indoor temperature. Outdoor conditions, the building fabric, infiltration and installation quality all affect performance.

For a difficult loft, conservatory or large open-plan area, seek a proper cooling-load calculation.

What to compare after BTU

• Electrical input and the conditions used for the published capacity.
• Sound data for cooling mode, including the fan setting used for the figure.
• Window-kit dimensions and compatibility with the actual opening.
• Hose position, required rear clearance and condensate arrangements.
• Controls that help with the real routine, such as a timer or thermostat.
• Product dimensions, weight and the route to storage.

Capacity is only useful if the unit can be installed correctly. A suitable 9,000BTU/h model with a well-sealed, short exhaust route can be a more credible choice than a nominally larger machine attached to a poor window arrangement.

Questions people often ask

Can one unit cool several rooms? Air movement through a doorway is unpredictable, so size for the connected area rather than assuming equal cooling in separate rooms. Should I add a safety margin?

Modest headroom can cover uncertain gains, but avoid jumping several classes without a reason. Can curtains replace air conditioning? Shading can reduce the load substantially, but it does not actively remove heat already inside.