Sizing and calculators
AC BTU calculator: Method and Sizing Limits
Check a single room against the ENERGY STAR sizing chart.
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Enter the required data. Unknown assumptions do not become guessed values.
Method v1.0. Checked October 14, 2026. Sources and limits.
Paper method and limits
Use the visible ENERGY STAR room chart only within its stated area and ceiling limits. Adjust the base by minus 10% for heavy shade or plus 10% for strong sun. Then add 600 BTU/h for each person above two and 4,000 for a kitchen. Do not extrapolate.
| Room area (ft²) | Base cooling (BTU/h) |
|---|---|
| 100 to 150 (upper endpoint excluded) | 5,000 |
| 150 to 250 (upper endpoint excluded) | 6,000 |
| 250 to 300 (upper endpoint excluded) | 7,000 |
| 300 to 350 (upper endpoint excluded) | 8,000 |
| 350 to 400 (upper endpoint excluded) | 9,000 |
| 400 to 450 (upper endpoint excluded) | 10,000 |
| 450 to 550 (upper endpoint excluded) | 12,000 |
| 550 to 700 (upper endpoint excluded) | 14,000 |
| 700 to 1000 (upper endpoint excluded) | 18,000 |
| 1000 to 1200 (upper endpoint excluded) | 21,000 |
| 1200 to 1400 (upper endpoint excluded) | 23,000 |
| 1400 to 1500 (upper endpoint excluded) | 24,000 |
| 1500 to 2000 (upper endpoint excluded) | 30,000 |
| 2000 to 2500 inclusive | 34,000 |
ENERGY STAR room chart, checked October 14, 2026. Heavy shade: subtract 10%; very sunny: add 10%. Add 600 BTU/h for each person above two; add 4,000 BTU/h for a kitchen. These are room-chart adjustments, not whole-house load rules.
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This ac btu calculator uses the official ENERGY STAR room air conditioner chart. Enter one room's dimensions or area. Then state sun exposure, usual occupants and kitchen use. The result shows the chart base and each adjustment. It does not turn a room-chart figure into a whole-house central cooling load.
The chart covers rooms from 100 to 2,500 ft² with an 8-foot ceiling. Heavy shade reduces the base by 10%. Very sunny exposure increases it by 10%. Each person above two adds 600 BTU/h, and a kitchen adds 4,000 BTU/h. The tool stops for other ceilings, open-plan/multi-room use or outside-chart area rather than inventing an extrapolated capacity.
AC BTU calculator: measure the room
Use length and width in feet for a rectangular room. Multiply them to obtain area. If the room is irregular, enter a measured area directly rather than treating the whole space as one guessed rectangle.
The ENERGY STAR room AC page provides the source chart and adjustments. We checked that public record on October 14, 2026. This is a chart starting point. It does not promise how a unit will cool your room.
| Input | Unit or state | Limit |
|---|---|---|
| Length/width | Feet | Positive dimensions |
| Entered area | ft² | Use instead of dimensions, not in addition |
| Ceiling | Feet | Simple chart assumes 8 ft |
| Usual occupants | Whole people | Extra adjustment applies above two |
| Sun | Typical, heavy shade or very sunny | Source-defined choice |
| Kitchen | Yes/no | Source adjustment |
| Unusual/open plan | Stated coverage flag | No simple extrapolation |
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Room use, sun and occupancy adjustments
Apply the sun percent to the chart base. Then add the extra-person and kitchen amounts. Do not apply the percent to the kitchen allowance as well. The tool prints that sequence so you can reproduce it on paper.
Usual occupants are people regularly in the room. The chart adds only the number above two. A two-person room has no extra-person increment under this rule.
Kitchen use has its own fixed increment. It is not permission to ignore unusual heat sources or an open-plan layout. If the room falls outside the source's simple terms, use a fuller room load record.
AC BTU calculator: result and boundary convention
The source chart lists adjacent bands. This tool uses the lower endpoint inclusive and upper endpoint exclusive. The final 2,500 ft² endpoint is included. An exact shared boundary chooses the next band.
| Declared room example | Base and adjustment | Result |
|---|---|---|
| 10 ft by 15 ft, typical sun, two people | 150 ft² enters the next chart band | 6,000 BTU/h |
| Same room, very sunny | Base times 1.1 | 6,600 BTU/h |
| Same sunny room, three people | Add one 600 BTU/h increment | 7,200 BTU/h |
| Same scenario, kitchen | Add 4,000 BTU/h | 11,200 BTU/h |
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These are independently worked examples of the published rules. They are not measured room loads or product recommendations. The output is not rounded to an assumed available stock size.
Read the source sizing table
The table above is rendered in HTML, so it remains available without JavaScript. Keep its units and band boundary with the result. The result also states the area and chosen adjustments.
ENERGY STAR's page contains a higher-ceiling sentence referring to CADR, an air-cleaner metric. We do not use that sentence as an AC sizing rule. Nonstandard ceilings are outside this tool's simple chart coverage.
For a paper worksheet, choose the base band, apply the sun adjustment. Then add the stated occupancy and kitchen increments. Record the ceiling and coverage flags. A correct sum still needs the room to lie inside the chart's scope.
When room sizing does not apply
This tool does not serve whole-house central AC. Use the central sizing calculator for a known whole-house load and unit-size changes. The sizing guide explains the task boundaries.
Open-plan space, multiple rooms and unusual ceilings do not get a made-up multiplier here. The tool reports that the room is outside source coverage. Keep the details for a room load check rather than assuming the chart scales indefinitely.
Unit fit after the result
A chart capacity does not establish window, wall or power fit. Keep the selected unit's actual maker room-use and install records. This page provides no power connection or mounting instructions.
Check the unit's stated cooling performance and install terms. A rated capacity near the worksheet result is one comparison field, not a full choice. Keep the chart math and selected full model as separate records.
FAQ
Is the chart a whole-house cooling design?
No. It is a room-air-conditioner starting chart with stated area and ceiling coverage. A central system needs its own whole-house load and unit-choice records. Use the linked central tool for a unit-size change from a declared load. Do not add room-chart capacities and present the sum as a checked ducted-system design for the home.
What happens at an exact area boundary?
The tool explicitly selects the next band at a shared endpoint. It treats lower bounds as inclusive and upper bounds as exclusive, except the final endpoint is included. That convention resolves adjacent source wording. The result prints the area and base so you can see which chart step was used before the adjustments are applied.
Why does the tool stop for a higher ceiling?
The simple source chart assumes an 8-foot ceiling. A nonstandard height changes the room boundary, and this tool does not invent a height multiplier. The source page also contains a mismatched CADR reference that is not used here. Keep the actual room details for a fuller load record rather than extrapolating the chart silently.
Does the output tell me which AC to buy?
No. It shows a chart starting point and its published adjustments. The full unit must still suit the room, the opening and the maker's requirements. No fixed rounding to a stock model is applied. Keep the math and selected product record separate, and do not treat the worksheet as power or mounting instructions for installation.
Verdict: use the AC BTU calculator within the chart's coverage
The ac btu calculator keeps base and adjustments and limits visible. Print the math with the room details. Return to the sizing guide for whole-house tasks, or the central AC cost guide for a separate install budget.