HVAC BTU Calculator — Rough Room Cooling Capacity

A rough cooling capacity for one room from the published ENERGY STAR sizing chart, with the adjustments printed alongside it. A starting figure to take to a contractor, not a load calculation.

Showing US units

Rough cooling capacity EXAMPLE8,000 BTU/hr

An answer marked EXAMPLE is worked out from the page’s sample values. Type your own over the values marked SAMPLE to make it yours.

sq ftSAMPLE

The area of the room’s floor, measured out to the inside faces of its walls. One room, served by one room or window air conditioner. The published chart covers 100 square feet up to, but not including, 2,500 — 9.3 to 232 m² — and this calculator will not go outside it.

SAMPLE

The publisher's phrase is "regularly occupy" — the people who are usually in it, not the most it has ever held. Beyond two, each one adds 600 BTU per hour.

A kitchen adds 4,000 BTU per hour. It is the only appliance-heat adjustment the publisher prints.

No spare-material allowance on this calculator

There is no spare allowance on this calculator, and adding one would be the mistake it is trying to prevent. An oversized air conditioner cools the room before it has taken the humidity out, so it costs more to buy, uses more to run, and leaves the room damp. Source

Rough cooling capacity EXAMPLE8,000 BTU/hr Source

A 340 sq ft room with two people in it, on the sun and kitchen answers above.

This is a starting figure from a published chart, not a Manual J load calculation. The chart has no input for climate, insulation, windows or which floor the room is on, and it assumes an 8-foot ceiling.

The next step with a figure like this one is a Manual J load calculation, done for the actual building by an HVAC contractor. That is the step this page cannot do.

From the chart, for 300 up to 350 sq ft Source8,000 BTU/hr

  • The capacity stays in BTU per hour whichever units the room is measured in. The chart is published in BTU per hour and that is what the machines are labeled with, so restating it in watts would hand over a number no product carries.
  • The published capacities are worked out for an 8-foot ceiling. There is no ceiling-height question on this calculator because the publisher gives no correction for one — so a room with a different ceiling is a room this figure does not describe. Source
  • Bigger is not safer here. An oversized unit cools the room before it has taken the humidity out, so it costs more, uses more, and leaves the room damp. Source

The printed sheet carries the figure, the room it describes and where it came from.

HVAC BTU Calculator — Rough Room Cooling Capacity

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EXAMPLE — not a measured job. Every quantity below is worked out from this page’s sample measurements. Enter the real measurements and print it again before anyone orders from it.

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Rough cooling capacity: 8,000 BTU/hr [1]

This is a starting figure from a published chart, not a Manual J load calculation. The chart has no input for climate, insulation, windows or which floor the room is on, and it assumes an 8-foot ceiling.

The next step with a figure like this one is a Manual J load calculation, done for the actual building by an HVAC contractor. That is the step this page cannot do.

This figure is for one room cooled by one room air conditioner, worked out from the room air conditioner chart ENERGY STAR publishes and the adjustments printed with it; it is not a size for a central system. ENERGY STAR: “An oversized air conditioner costs more to purchase, wastes energy and does not provide better cooling.”

Worked out from

Floor area of the roomThe area of the room’s floor, measured out to the inside faces of its walls.340 sq ft (sample)
Sun on the roomNeither — ordinary
People who regularly occupy the room2 (sample)
Is this a kitchen?No

Estimates only — this is a starting figure, not a specification. Check it with an HVAC contractor performing a Manual J load calculation before you buy anything.

Where these figures come from

A number in brackets after a figure on this sheet is its entry in this list.

  1. Room air conditioner cooling capacity chart — room area to BTU per hour (Room air conditioners — cooling capacity chart, "Area To Be Cooled (square feet)" against "Capacity Needed (BTUs per hour)"); The chart assumes an 8-foot ceiling (Room air conditioners — the paragraph directly above the cooling capacity chart) — ENERGY STAR (U.S. Environmental Protection Agency), Government publication. The program's product page carries no commencement date; current as of the retrieved date. Retrieved October 11, 2026.
  2. Heavily shaded room — reduce capacity by 10% (Room air conditioners — "Make any adjustments for the following circumstances"); What an oversized unit does — costs more, wastes energy, and leaves the room damp (Room air conditioners — why size matters) — ENERGY STAR (U.S. Environmental Protection Agency), Government publication. The program's product page carries no commencement date; current as of the retrieved date. Retrieved August 28, 2026.
  3. The recognized method for sizing equipment is a Manual J load calculation (Manual J — Residential Load Calculation, description) — Air Conditioning Contractors of America, Trade association publication. The association's page carries no commencement date; current as of the retrieved date. Retrieved August 28, 2026.
  4. Oversizing reduces efficiency, wears the equipment, and raises summer humidity (HVAC sizing research — the problem being studied) — U.S. Department of Energy, Building Technologies Office, Government publication. The article carries no commencement date; current as of the retrieved date. Retrieved August 28, 2026.

This list, with a link to each document: concretetally.com/hvac-btu-calculator/#sources-heading

Every table ConcreteTally reads, with its dates: concretetally.com/sources

This is a rough figure from a published chart, not a Manual J load calculation.

Where these figures come from

Every table ConcreteTally reads, with its dates: all sources.