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AC size · by square footage
AC size by square footage and climate zone
The tables below give the cooling size for 7 floor areas in 14 US climate zones. Each number comes from a Manual J load run on a standard house, once as a 1985 build with ducts in the attic and once as a tight 2015 build. Between 1,000 and 3,000 sq ft the results run from 0.5 tons to 6.5 tons.
- Pick the column closest to the conditioned floor area of the house.
- Pick the row for the IECC climate zone of the job, or a city in the same zone.
- Use the first table for an older house with attic ducts and the second for a tight newer house.
- Enter the ZIP and the house details in the calculator for a load that fits the job.
Example result for the default inputs
ZIP 89101 · Clark County, NV · IECC zone 3B · elevation 1,877 ft · design 100°F DB / 67°F WB cooling, 25°F heating
Cooling load
29,099BTU/h
≈ 2.5 ton (nearest 0.5) · SHR 0.97
Heating load
32,746BTU/h
- Furnace input bin 60,000 BTU/h at 80% AFUE
- Heat pump 48,000 BTU/h at 47°F (38,400 at design temp)
Load breakdown by component
| Component | Cooling | Heating |
|---|---|---|
| Transmission (walls, ceiling, floor, glass, doors) | 9,733 | 21,296 |
| Solar through glass | 12,325 | - |
| Infiltration | 1,089 | 5,992 |
| Internal gains (people, appliances) | 2,920 | - |
| Ducts in unconditioned attic | 3,032 | 5,458 |
- These design temperatures are the worst case for the IECC climate zone. BuildSolver estimates ZIP-level values can differ by 2-8°F.
- These are preliminary design conditions. For a permit load, take the design temperatures from ASHRAE Handbook - Fundamentals 2021, Chapter 14.
For estimation purposes only. Consult a licensed engineer for final designs.
Room-by-room Manual J and branded PDF reports · Pro plan
How the tonnage in the tables is calculated
Every cell is a separate run of the BuildSolver load engine, the same code behind the calculator form and the assistant. The engine applies the whole-house Speed Sheet simplification of ACCA Manual J 8th Edition at the design temperatures of each climate zone. Tonnage is rounded to the nearest half ton.
- Cooling load
Q_cool = transmission + solar + infiltration + internal + ductSummed at the 1% cooling design dry-bulb and its mean coincident wet-bulb for the IECC climate zone, with the house held at 75°F and 50% RH.- Tonnage
tons = Q_cool / 12,000, rounded to the nearest 0.5Rounded to the nearest half ton, so the size can land slightly under the load. The calculator adds a warning when the rounded size lands above 107% of the load, inside the 115% single-speed limit in ACCA Manual S.- Square feet per ton
sq ft per ton = floor area / (Q_cool / 12,000)Computed from the unrounded load. The 400 to 600 sq ft per ton rule of thumb tries to guess this number without a load.- BTU per square foot
BTU/h per sq ft = Q_cool / floor areaUse it to compare houses side by side. It comes out of the load calculation, so it cannot go back in as a sizing input.- Envelope defaults
U = 1 / R from year built and climate zoneInsulation and window type come from the year-built default for the zone. Glass is 15% of floor area, split evenly by orientation, with 8 ft ceilings and a square footprint.- Heating load
Q_heat = transmission + slab edge + infiltration + ductSummed at the 99% heating design dry-bulb with the house at 70°F. The furnace input bin is the smallest of 40,000, 60,000, 80,000, 100,000 and 120,000 BTU/h that covers the load divided by the year-built AFUE default.
AC tonnage by floor area and climate zone
Built 1985, average air sealing, ducts in a vented attic. Single story up to 1,800 sq ft and two stories above, on a slab. Click a floor area for the page with loads in BTU/h, heating numbers and notes for that size.
| Zone and city | 1,000 | 1,200 | 1,500 | 1,800 | 2,000 | 2,500 | 3,000 |
|---|---|---|---|---|---|---|---|
| 1A Miami | 2 | 2 | 2.5 | 3 | 3.5 | 4 | 5 |
| 2A Houston | 2 | 2.5 | 3 | 3.5 | 3.5 | 4.5 | 5.5 |
| 2B Phoenix | 2.5 | 3 | 3.5 | 4 | 4.5 | 5.5 | 6.5 |
| 3A Atlanta | 1.5 | 1.5 | 2 | 2.5 | 2.5 | 3 | 3.5 |
| 3B Las Vegas | 1.5 | 1.5 | 2 | 2.5 | 2.5 | 3 | 4 |
| 3C San Francisco | 1 | 1 | 1.5 | 1.5 | 2 | 2 | 2.5 |
| 4A St. Louis | 1 | 1.5 | 1.5 | 2 | 2.5 | 2.5 | 3 |
| 4B Albuquerque | 1 | 1.5 | 1.5 | 2 | 2.5 | 2.5 | 3 |
| 4C Seattle | 1 | 1 | 1 | 1.5 | 1.5 | 1.5 | 2 |
| 5A Chicago | 1 | 1.5 | 1.5 | 2 | 2 | 2.5 | 3 |
| 5B Denver | 1 | 1.5 | 1.5 | 2 | 2 | 2.5 | 3 |
| 6A Minneapolis | 1 | 1.5 | 1.5 | 2 | 2 | 2.5 | 3 |
| 6B Billings | 1 | 1.5 | 1.5 | 2 | 2 | 2.5 | 3 |
| 7 Duluth | 1 | 1 | 1.5 | 1.5 | 1.5 | 2 | 2.5 |
Built 2015, tight envelope, ducts inside conditioned space. Same floor areas and climates.
| Zone and city | 1,000 | 1,200 | 1,500 | 1,800 | 2,000 | 2,500 | 3,000 |
|---|---|---|---|---|---|---|---|
| 1A Miami | 1 | 1 | 1.5 | 1.5 | 1.5 | 2 | 2.5 |
| 2A Houston | 1 | 1 | 1.5 | 1.5 | 2 | 2 | 2.5 |
| 2B Phoenix | 1.5 | 1.5 | 1.5 | 2 | 2 | 2.5 | 3 |
| 3A Atlanta | 1 | 1 | 1.5 | 1.5 | 1.5 | 2 | 2.5 |
| 3B Las Vegas | 1 | 1 | 1.5 | 1.5 | 2 | 2 | 2.5 |
| 3C San Francisco | 0.5 | 1 | 1 | 1 | 1 | 1.5 | 1.5 |
| 4A St. Louis | 1 | 1 | 1 | 1.5 | 1.5 | 2 | 2 |
| 4B Albuquerque | 1 | 1 | 1 | 1.5 | 1.5 | 2 | 2 |
| 4C Seattle | 0.5 | 0.5 | 1 | 1 | 1 | 1 | 1.5 |
| 5A Chicago | 1 | 1 | 1 | 1.5 | 1.5 | 1.5 | 2 |
| 5B Denver | 1 | 1 | 1 | 1.5 | 1.5 | 1.5 | 2 |
| 6A Minneapolis | 0.5 | 1 | 1 | 1 | 1.5 | 1.5 | 2 |
| 6B Billings | 0.5 | 1 | 1 | 1 | 1.5 | 1.5 | 2 |
| 7 Duluth | 0.5 | 0.5 | 1 | 1 | 1 | 1.5 | 1.5 |
Standards and data sources
- ACCA Manual J 8th Edition - residential load procedure. The tables use its whole-house Speed Sheet simplification, which gives a single load for the whole house.
- ACCA Manual S - equipment selection against the Manual J load, including the 115% limit on cooling capacity for a single-speed unit. Selection is a separate step and part of the Pro plan.
- IECC 2021 climate zones (PNNL county map) - ZIP to county to climate zone. The zone sets the design temperatures, solar factor and envelope defaults for each row.
- NREL TMY3 weather data - outdoor design conditions as percentiles of typical-year hourly data, applied as the worst case of each climate zone. City values are published in ASHRAE Handbook Fundamentals, Chapter 14.
- ASHRAE Handbook Fundamentals - the 1.08 sensible and 4,840 latent infiltration constants with a standard-atmosphere density correction by ZIP elevation.
What the tables leave out
- The tables model a standard house for each floor area. The house on your job will differ in insulation, glass, orientation and air leakage, and the 1985 and 2015 columns show how far those inputs move the size.
- Each row uses the worst-case design temperature for its IECC climate zone. BuildSolver estimates ZIP-level values can differ by 2-8°F, so check the city's own 1% value in ASHRAE Handbook - Fundamentals 2021, Chapter 14, before a final load.
- The house is one zone with 8 ft ceilings, a square footprint on a slab and glass at 15% of floor area. Vaulted rooms, window walls and bonus rooms over a garage are not modeled.
- Whole-house load only. It does not size registers, split the load between floors or pick a model. Room-by-room Manual J and Manual S equipment selection are in the Pro plan.
- Preliminary sales-phase estimate. Not ACCA-approved software output and not accepted for permit submission.
Common sizing mistakes
- Reading a size off square footage. The same floor area needs very different equipment in different climates and envelopes. Find the row for your zone before you quote.
- Matching the unit coming out. The old nameplate may have been picked by rule of thumb, and matching it carries over whatever oversizing the last installer chose.
- Two rounds of safety margin. Manual J already sizes at the 1% cooling design temperature, so the load carries margin. Add 10% to a 2-ton load, round up to 2.5 tons, then buy the next size up, and the house gets 3 tons.
- A bigger condenser for one hot room. One room that never cools usually points to a duct, return or glass problem in that room. A bigger unit short-cycles the rest of the house and leaves that room where it was.
AC size for each floor area
- What size AC for a 1,000 sq ft house 0.5 tons to 2.5 tons in the table, against 1.67 to 2.5 tons by rule of thumb.
- What size AC for a 1,200 sq ft house 0.5 tons to 3 tons in the table, against 2 to 3 tons by rule of thumb.
- What size AC for a 1,500 sq ft house 1 ton to 3.5 tons in the table, against 2.5 to 3.75 tons by rule of thumb.
- What size AC for a 1,800 sq ft house 1 ton to 4 tons in the table, against 3 to 4.5 tons by rule of thumb.
- What size AC for a 2,000 sq ft house 1 ton to 4.5 tons in the table, against 3.33 to 5 tons by rule of thumb.
- What size AC for a 2,500 sq ft house 1 ton to 5.5 tons in the table, against 4.17 to 6.25 tons by rule of thumb.
- What size AC for a 3,000 sq ft house 1.5 tons to 6.5 tons in the table, against 5 to 7.5 tons by rule of thumb.
Questions about AC size by square footage
How many square feet does one ton of AC cool?
Across every house and climate in these tables, one ton of load covers anywhere from 414 to 2,197 sq ft. Only 20 of the 196 results fall inside the 400 to 600 sq ft per ton rule.
What size AC for a 2,000 sq ft house?
As a 1985 build with attic ducts it needs 4.5 tons in Phoenix and 1.5 tons in Seattle. The 2,000 sq ft page has the full table for 14 climate zones.
Why do the tables show two houses?
Year built, air sealing and duct location change the load as much as the climate does. Showing a 1985 house with attic ducts next to a tight 2015 house gives the realistic range for a replacement quote.
Are these numbers good enough to order equipment?
They are preliminary whole-house estimates for a standard house. Run the load for the actual house before ordering, and check the unit against manufacturer data. Room-by-room Manual J and Manual S equipment selection are part of the Pro plan.
Does the calculator on this page cost anything?
No. The whole-house calculator is free, with up to 10 calculations a day shared across the load calculators on BuildSolver pages. The tables are precomputed and do not use that limit.
Used on the same job
Size the actual house in chat
Start with the ZIP. Describe the house you are quoting and the assistant runs the same load tools, asks for the windows, insulation and duct location, and explains the result in terms you can show the homeowner.
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