FreeHVACCalculators

AC BTU Calculator

Find the cooling capacity a single room needs, using the ENERGY STAR room air-conditioner chart and its adjustments for sun, people and kitchens.

Method reviewed October 22, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Length × width of the room. For open-plan spaces include every connected area that the unit must cool.

Sun exposure
Is the room a kitchen?

Result

Recommended cooling capacity
9,000BTU/h

Look for a unit rated at about 9,000 BTU/h. Units are sold in steps, so choose the nearest size at or slightly above this figure.

Equivalent tonnage
0.75tons
Equivalent capacity
2.64kW
Chart base capacity
9,000BTU/h
Show the working
  1. ENERGY STAR chart capacity for the floor area
    350 ft² → 9,000 BTU/h

What this calculator is for

Use it to choose a window unit, portable air conditioner or a single ductless mini-split head for one room or one open area. It answers the shopping question “how many BTUs?” — the figure printed on the box.

For a central system that cools a whole house, use the AC Tonnage Calculator instead. A whole house has walls, windows, a roof and air leakage that a floor-area chart cannot see.

How the BTU figure is worked out

ENERGY STAR publishes a chart that maps room floor area to a recommended capacity, starting at 5,000 BTU/h for 100–150 ft² and rising to 34,000 BTU/h for 2,000–2,500 ft². The chart is stepped: every size in a band gets the same answer, because room units are sold in those steps.

Four adjustments are then applied, in this order:

  • Heavily shaded room: reduce capacity by 10 %.
  • Very sunny room: increase capacity by 10 %.
  • More than two regular occupants: add 600 BTU/h per extra person.
  • Kitchen: add 4,000 BTU/h for cooking heat.

The chart assumes 8-foot ceilings. If yours are higher, we scale the result by ceiling height ÷ 8 so the extra air volume is counted. That step is ours, and it is labelled in the working so you can see it.

Reading the result

Choose the unit size nearest to the result, at or slightly above it. Resist adding a large margin: an oversized room unit cools the air quickly and shuts off before it has removed much moisture, which leaves the room cold and clammy and wastes energy on short cycles.

If the result lands far above the chart’s range, or the room is part of an open-plan floor, the space is behaving more like a small house — a load estimate from the HVAC Load Calculator will be more useful.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Room floor area
350 ft²
Ceiling height
8 ft
Sun exposure
Average
People who regularly use the room
2
Is the room a kitchen?
No

Result

Recommended cooling capacity
9,000 BTU/h
Equivalent tonnage
0.75 tons
Equivalent capacity
2.64 kW
Chart base capacity
9,000 BTU/h

Look for a unit rated at about 9,000 BTU/h. Units are sold in steps, so choose the nearest size at or slightly above this figure.

Assumptions

  • Base capacities come from the ENERGY STAR room air-conditioner sizing chart, which assumes 8 ft ceilings.
  • For ceilings above 8 ft we scale capacity in proportion to room volume. This adjustment is ours, not part of the ENERGY STAR chart.
  • Adjustments follow ENERGY STAR guidance: −10 % for heavily shaded rooms, +10 % for very sunny rooms, +600 BTU/h per occupant beyond two, +4,000 BTU/h for kitchens.

Limitations

  • Intended for room air conditioners, portable units and single-zone ductless heads — not for central systems serving a whole house.
  • Does not account for insulation quality, window area or local climate. A poorly insulated top-floor room in a hot climate can need more capacity.
  • Oversizing a room unit causes short cycling and poor dehumidification, so avoid adding a large “safety margin”.

Frequently asked questions

How many BTUs do I need per square foot?

There is no single number. The ENERGY STAR chart works out to roughly 25–50 BTU/h per square foot for the smallest rooms, falling to about 14 BTU/h per square foot for the largest band, because small rooms have more wall and window per unit of floor. Sun, occupancy and cooking change it further, which is why this calculator applies those adjustments.

Is a bigger air conditioner better?

No. An oversized unit short-cycles: it reaches the thermostat setting before it has run long enough to pull moisture out of the air. Rooms feel damp, temperature swings more, and the compressor wears faster. Choose the nearest size at or slightly above the calculated figure.

Does this work for mini-splits?

Yes, for a single-zone ductless head serving one room or one open area. For multi-zone systems, size each zone separately and then check the outdoor unit’s combined capacity against the manufacturer’s tables.

Why does a kitchen add 4,000 BTU/h?

Cooking appliances release a lot of heat while in use. ENERGY STAR’s guidance adds a fixed 4,000 BTU/h for a room unit that will cool a kitchen.

Sources

  1. Room Air Conditioners — sizing chart — ENERGY STAR (U.S. EPA / DOE). Floor-area sizing chart (5,000–34,000 BTU/h) and the shade, sun, occupancy and kitchen adjustments.

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