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BTU Cooling / AC Sizing Calculator

Get a rough estimate of the air conditioner capacity (BTU) needed for a room, adjusted for ceiling height and sun exposure.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-25 · v1.0.0

Market
Imperial · sales tax
13.12 ft11.48 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Recommended cooling capacity

3013.89 BTU/hr

Check your inputs

This is a rough planning estimate, not a Manual J load calculation — actual sizing should be verified by an HVAC professional, especially for whole-house systems.

Room area
150.69 sq ft
Baseline (20 BTU/sq ft)
3013.89 BTU/hr

With the figures above, the recommended cooling capacity comes to 3014 BTU/hr. Behind that figure, baseline (20 btu/sq ft) is the biggest single quantity at 3014 BTU/hr; start there if the total looks wrong. The method behind this is well established, though site conditions and material batches will move it somewhat. This is calculated for United States and cites IRC 2024. Building in another market? Change the selector above so the units and code reference follow.

Add the equipment this sizes

This result is a specification — 3,013.895 BTU/hr — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Rule-of-thumb baseline: approximately 20 BTU per square foot for cooling, within a commonly cited 18-30 BTU/sq ft range depending on insulation, ceiling height, and climate
  • A precise sizing should use a full Manual J load calculation performed by an HVAC professional — this tool gives a rough planning estimate only

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Frequently asked questions

How accurate is this?
It's a rough planning estimate using a widely-cited rule of thumb, not a substitute for a Manual J load calculation, which accounts for insulation quality, window count and orientation, local climate, and occupancy.
What does BTU mean here?
British Thermal Units per hour — a measure of cooling (or heating) capacity. Air conditioner capacity is also often listed in 'tons,' where 1 ton = 12,000 BTU/hr.
Is bigger always better for an air conditioner?
No — an oversized AC cools the room quickly but shuts off before properly removing humidity, leaving the room feeling clammy. Right-sizing matters more than maximizing capacity.