HVAC
AC Size Calculator
Estimate cooling capacity for a space as a cross-check on a load calculation, not as a replacement for one.
Result
This is a preliminary sizing estimate using industry rules of thumb. Actual equipment selection requires a room-by-room load calculation (ACCA Manual J) and duct design (Manual D) by a qualified professional, oversized equipment short-cycles, dehumidifies poorly and fails early.
Get a Free Business AuditWhat moves the number
- Sensible and latent load, since humidity removal is a large part of the job in humid climates.
- Window area and shading, which drive peak afternoon load.
- Duct location, because ducts in an unconditioned attic lose capacity before the air arrives.
Where this one goes wrong
Sizing up to be safe. An oversized air conditioner cools quickly, shuts off before it has removed moisture, and leaves the house cold and damp. In cooling, bigger is not a safety margin.
Before you quote
- Whether the existing ductwork can deliver the airflow the tonnage requires.
- Whether the electrical service supports the unit.
- The manufacturer's expanded performance data for the actual conditions, not the nameplate.
Questions
About this calculator
Should I round up to the next tonnage?
Not by default. Rounding up costs dehumidification and run time, which is why undersized-feeling but correctly sized systems often deliver better comfort. Where the load sits between sizes, look at whether a variable capacity unit solves it.
Does this replace a Manual J?
No. It gives a planning figure and a sanity check. Equipment selection should come from a load calculation and the manufacturer's performance data for the design conditions.
Why does duct location matter to sizing?
Ducts running through an unconditioned attic pick up heat and leak, so the capacity that reaches the room is less than the capacity at the equipment. Sizing that ignores it under-delivers.
Put this calculator on your site
Free to embed. Paste this where you want it to appear. It stays up to date automatically because it loads from BDEVY.
Please keep the attribution line. It is the only thing we ask in return.
How to use this calculator
- Conditioned floor area (sq ft) (sq ft). Enter your own figure; the page starts at 1,800 sq ft.
- Climate zone (cooling BTU per sq ft). Enter your own figure; the page starts at Warm south.
- Insulation and windows. Enter your own figure; the page starts at Average.
- Average ceiling height (ft) (ft). Enter your own figure; the page starts at 9 ft.
The result updates as you type, so you can try a figure, see what it does to system size, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Conditioned floor area (sq ft) | 1,800 sq ft |
| Climate zone (cooling BTU per sq ft) | Warm south |
| Insulation and windows | Average |
| Average ceiling height (ft) | 9 ft |
What comes out
| Result | Value |
|---|---|
| System size main result | 3.4 |
| Capacity | 40,590 |
| Nearest standard size | 3.5 ton |
| Airflow needed | 1,400 |
About 3.4 tons (40,590 BTU/h). The nearest standard system is 3.5 ton, needing roughly 1,400 CFM of airflow.
How sensitive the answer is
Holding everything else steady and changing only conditioned floor area (sq ft), this is what happens to system size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Conditioned floor area (sq ft) (sq ft) | System size |
|---|---|
| 900 sq ft | 1.7 |
| 1,350 sq ft | 2.5 |
| 1,800 sq ft example above | 3.4 |
| 2,700 sq ft | 5.1 |
| 3,600 sq ft | 6.8 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Conditioned floor area (sq ft) (sq ft)
- Measured in sq ft, starting at 1,800 sq ft.
- Climate zone (cooling BTU per sq ft)
- Choose from Cool north, Moderate / mid, Warm south, Hot south, Hot / humid south, Desert / extreme heat.
- Insulation and windows
- Choose from Older / poorly insulated, Average, Well insulated / new.
- Average ceiling height (ft) (ft)
- Measured in ft, between 0 and 30, starting at 9 ft.
What the results tell you
| Result | What it is |
|---|---|
| System size | The headline result. A decimal figure. |
| Capacity | A number. |
| Nearest standard size | A category or planning selection, rather than a measured quantity. |
| Airflow needed | A number. |
Before you rely on this number
Sizing, capacity and load figures here are planning estimates. A real installation depends on the manufacturer's data for the specific equipment, the duct system it is going into, and a proper load calculation for the building. Use this to sanity-check a number or price a job, not to select equipment.
Keep going
Related calculators
HVAC BTU Calculator
Get a planning figure for the BTU a space needs, as a sanity check against a proper load calculation rather than a substitute for one.
Open calculatorFurnace Size Calculator
Estimate heating input and output for a space, with the distinction between the two made explicit.
Open calculatorCFM Calculator
Work out the airflow a space or a system needs, and check it against what the equipment and ducts can actually deliver.
Open calculatorEstimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.