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AC size · 1,800 sq ft
What size AC for a 1,800 sq ft house
A 1,800 sq ft house needs anywhere from 1 ton to 4 tons of cooling in the 14 climates below. The low end is a tight 2015 house in San Francisco, CA and the high end is a 1985 house with attic ducts in Phoenix, AZ. The 400 to 600 sq ft per ton rule gives 3 to 4.5 tons for all of them. Check your zone first.
Run your own house in the calculator. It starts with a 1,800 sq ft single-story house in Las Vegas, NV, built in 1985 with ducts in the attic.
- Type the 5-digit ZIP of the job. The calculator looks up the climate zone and its design temperatures.
- Enter the conditioned floor area, stories and year built. The form starts at 1,800 sq ft.
- Pick air tightness, foundation and window type, and check the box if the ducts run through an unconditioned attic.
- Read the cooling load and tonnage, then find the row for your climate zone in the table below to see how the envelope changes the size.
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 table is calculated
Each of the 28 cells in the table is its own run of the BuildSolver load engine on a 1,800 sq ft house, using the whole-house Speed Sheet simplification of ACCA Manual J 8th Edition. The AC size hub has the full method and every floor area side by side.
- House geometry
footprint = 1,800 / 1 = 1,800 sq ft; side = 42 ftThe engine models a square single-story footprint with 8 ft ceilings, which gives about 1,358 sq ft of gross wall and 1,800 sq ft of ceiling under the attic.- Tonnage
tons = Q_cool / 12,000, rounded to the nearest 0.5Loads are rounded to the nearest half ton, which puts this floor area between 1 ton and 4 tons.- Square feet per ton
sq ft per ton = 1,800 / (Q_cool / 12,000)Computed from the unrounded load. Across this table one ton covers 440 to 1,984 sq ft.
AC size for a 1,800 sq ft house in 14 climate zones
Each row is the same 1,800 sq ft single-story house on a slab with 4 occupants and glass at 15% of floor area. Only the climate zone and the envelope change. The first house column is a 1985 build with average air sealing and ducts in a vented attic. The second is a 2015 build with a tight envelope and ducts inside conditioned space. Tons are rounded to the nearest half ton. Each cell also shows the load in BTU/h and how many square feet one ton covers.
| Zone | Reference city | Cooling design | 1985 house, attic ducts | 2015 house, ducts inside |
|---|---|---|---|---|
| 1A | Miami, FL | 92°F | 3 tons37,763 BTU/h · 572 sq ft/ton | 1.5 tons18,762 BTU/h · 1,151 sq ft/ton |
| 2A | Houston, TX | 96°F | 3.5 tons40,315 BTU/h · 536 sq ft/ton | 1.5 tons20,051 BTU/h · 1,077 sq ft/ton |
| 2B | Phoenix, AZ | 110°F | 4 tons49,037 BTU/h · 440 sq ft/ton | 2 tons24,450 BTU/h · 883 sq ft/ton |
| 3A | Atlanta, GA | 95°F | 2.5 tons27,203 BTU/h · 794 sq ft/ton | 1.5 tons17,902 BTU/h · 1,207 sq ft/ton |
| 3B | Las Vegas, NV | 100°F | 2.5 tons29,099 BTU/h · 742 sq ft/ton | 1.5 tons19,314 BTU/h · 1,118 sq ft/ton |
| 3C | San Francisco, CA | 84°F | 1.5 tons19,192 BTU/h · 1,125 sq ft/ton | 1 ton12,670 BTU/h · 1,705 sq ft/ton |
| 4A | St. Louis, MO | 92°F | 2 tons24,683 BTU/h · 875 sq ft/ton | 1.5 tons16,236 BTU/h · 1,330 sq ft/ton |
| 4B | Albuquerque, NM | 95°F | 2 tons24,771 BTU/h · 872 sq ft/ton | 1.5 tons16,491 BTU/h · 1,310 sq ft/ton |
| 4C | Seattle, WA | 84°F | 1.5 tons16,129 BTU/h · 1,339 sq ft/ton | 1 ton10,885 BTU/h · 1,984 sq ft/ton |
| 5A | Chicago, IL | 91°F | 2 tons24,000 BTU/h · 900 sq ft/ton | 1.5 tons15,509 BTU/h · 1,393 sq ft/ton |
| 5B | Denver, CO | 93°F | 2 tons23,547 BTU/h · 917 sq ft/ton | 1.5 tons15,362 BTU/h · 1,406 sq ft/ton |
| 6A | Minneapolis, MN | 90°F | 2 tons21,900 BTU/h · 986 sq ft/ton | 1 ton14,419 BTU/h · 1,498 sq ft/ton |
| 6B | Billings, MT | 92°F | 2 tons21,735 BTU/h · 994 sq ft/ton | 1 ton14,445 BTU/h · 1,495 sq ft/ton |
| 7 | Duluth, MN | 86°F | 1.5 tons18,019 BTU/h · 1,199 sq ft/ton | 1 ton12,004 BTU/h · 1,799 sq ft/ton |
Only 3 of the 28 engine results land inside the rule-of-thumb range of 3 to 4.5 tons, the 1985 house in zones 1A, 2A and 2B.
In Houston, TX the 1985 house with attic ducts needs 3.5 tons and the tight 2015 house with its ducts inside needs 1.5 tons, 2 tons apart on the same floor area.
The highlighted Las Vegas row is the calculator's starting point. There the 1985 house needs 29,099 BTU/h, or 2.5 tons, and a heating load of 32,746 BTU/h at 25°F. Built tight in 2015 with the ducts inside, the same house needs 1.5 tons. A 500 sq ft per ton quote would put 3.5 tons on both. Size each house from its own row, then check the existing ducts and registers before you write the quote.
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.
What the table leaves out
- Every row is one standard 1,800 sq ft house. On a real job, insulation, glass, orientation and air leakage move the load, and in this table the 1985 and 2015 builds in Houston, TX already land 2 tons apart.
- Each row uses the worst-case design temperature of its IECC climate zone, from 84°F in the San Francisco zone to 110°F in the Phoenix zone. BuildSolver estimates ZIP-level values can differ by 2-8°F, and ASHRAE Handbook - Fundamentals 2021, Chapter 14, lists the city values.
- Single story on a slab. A crawlspace or basement changes the floor loss, and a vaulted great room adds wall and volume the page does not model.
- Whole-house load only. Room-by-room Manual J and Manual S equipment selection are part of the Pro plan.
Common sizing mistakes on a 1,800 sq ft house
- Quoting 4.5 tons because the house is 1,800 sq ft. That is the top of the 400 sq ft per ton rule. In the St. Louis, MO row the engine puts the 1985 house at 2 tons and the 2015 house at 1.5 tons.
- Reusing the tonnage from a job in another market. The same 1985 house with attic ducts needs 1.5 tons in San Francisco, CA and 4 tons in Phoenix, AZ.
- Sizing off the nameplate of the unit coming out. It may have been picked by rule of thumb, and matching it repeats whatever oversizing the last installer chose.
- Letting the heating load set the AC. In Duluth, MN the 1985 house needs 63,808 BTU/h of heat and 1.5 tons of cooling. Pick the furnace and the AC from their own loads.
What to check on a 1,800 sq ft house
An 1,800 sq ft house is a larger single story or a smaller two-story, usually three or four bedrooms with an open kitchen and family room. If it is a tract home from the 2000s, look for a vaulted great room and a wall of glass on the back, and walk the house before you trust the quick form.
- Vaulted ceilings and tall windows are not on the quick form. The page assumes 8 ft ceilings and glass at 15% of floor area. A great room with a two-story window wall needs the room-by-room load in the Pro plan or your own Manual J software.
- In humid zones look at two-stage equipment at this size. A single-stage unit sized for the design day runs short cycles for most of the season and pulls less moisture on mild days.
- Check the orientation of the back wall. West glass on an 1,800 sq ft house in a desert zone can add more load than the ducts.
Questions about AC size for 1,800 sq ft
What size AC do I need for a 1,800 sq ft house?
Climate and construction decide it. In the table on this page the answer runs from 1 ton in San Francisco, CA to 4 tons in Phoenix, AZ. In Chicago, IL the same house comes out at 2 tons as a 1985 build and 1.5 tons as a 2015 build. Enter your ZIP and the house details in the calculator for your own number.
How many BTU does a 1,800 sq ft house need for cooling?
The cooling loads in the table run from 10,885 to 49,037 BTU/h. Per square foot that is 6 to 27.2 BTU/h, so no single BTU per square foot number fits a house this size. Run the actual house in the calculator above for its number.
Is a 4.5-ton AC too big for a 1,800 sq ft house?
ACCA Manual S limits a single-speed cooling unit to 115% of the Manual J load. By that limit a 4.5-ton single-speed unit is oversized for the 1985 house in 13 of the 14 climates and for the 2015 house in 14 of them.
What size furnace goes with a 1,800 sq ft house?
The heating loads in the same runs go from 11,280 to 63,808 BTU/h, the highest for the 1985 house in Duluth, MN. With the engine's AFUE default for the year built, those loads call for furnace inputs from the 40,000 to the 80,000 BTU/h bin. The furnace size calculator shows the heating result for a single ZIP.
Why does a 1,800 sq ft house need a different size in Phoenix and Seattle?
Design temperature and solar load. The table uses 110°F for the Phoenix zone and 84°F for the Seattle zone, so the same 1985 house needs 4 tons in Phoenix and 1.5 tons in Seattle.
Can I quote a 1,800 sq ft job from this table?
Use it for the first conversation with the homeowner. In Las Vegas, NV the same 1,800 sq ft comes out at 2.5 tons or 1.5 tons depending on how the house was built, so run the load for the actual house before you order equipment. Room-by-room Manual J and Manual S equipment selection are in the Pro plan. If the job needs a permit, ask the building department whether it requires a load report from ACCA-approved software.
Used on the same job
Size the actual house in chat
Start with the ZIP. Then describe the house you are quoting, including the windows, insulation and where the ducts run, and the assistant runs the same load tools, asks for what is missing and explains the result in terms you can show the homeowner.
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