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AC size · 800 sq ft
What size AC for an 800 sq ft house
An 800 sq ft house needs anywhere from 0.5 tons to 2 tons of cooling in the 14 climates below. The low end is a tight 2015 house in Seattle, WA and the high end is a 1985 house with attic ducts in Phoenix, AZ. At the low end 0.5 tons is a load, not a unit you can order. The 400 to 600 sq ft per ton rule gives 1.33 to 2 tons for all of them. Check your zone first.
Run your own house in the calculator. It starts with an 800 sq ft single-story house in Houston, TX, 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 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 77002 · Harris County, TX · IECC zone 2A · elevation 43 ft · design 96°F DB / 78°F WB cooling, 25°F heating
Cooling load
19,663BTU/h
≈ 1.5 ton (nearest 0.5) · SHR 0.95
Heating load
23,060BTU/h
- Furnace input bin 40,000 BTU/h at 80% AFUE
- Heat pump 30,000 BTU/h at 47°F (24,000 at design temp)
Load breakdown by component
| Component | Cooling | Heating |
|---|---|---|
| Transmission (walls, ceiling, floor, glass, doors) | 6,503 | 16,370 |
| Solar through glass | 7,998 | - |
| Infiltration | 1,111 | 2,847 |
| Internal gains (people, appliances) | 2,060 | - |
| Ducts in unconditioned attic | 1,991 | 3,843 |
- 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 an 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 = 800 / 1 = 800 sq ft; side = 28 ftThe engine models a square single-story footprint with 8 ft ceilings, which gives about 905 sq ft of gross wall and 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 0.5 tons and 2 tons.- Square feet per ton
sq ft per ton = 800 / (Q_cool / 12,000)Computed from the unrounded load. Across this table one ton covers 398 to 1,632 sq ft.
AC size for an 800 sq ft house in 14 climate zones
Each row is the same 800 sq ft single-story house on a slab with 2 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 | 1.5 tons18,358 BTU/h · 523 sq ft/ton | 1 ton9,672 BTU/h · 993 sq ft/ton |
| 2A | Houston, TX | 96°F | 1.5 tons19,663 BTU/h · 488 sq ft/ton | 1 ton10,380 BTU/h · 925 sq ft/ton |
| 2B | Phoenix, AZ | 110°F | 2 tons24,136 BTU/h · 398 sq ft/ton | 1 ton12,805 BTU/h · 750 sq ft/ton |
| 3A | Atlanta, GA | 95°F | 1 ton13,735 BTU/h · 699 sq ft/ton | 1 ton9,348 BTU/h · 1,027 sq ft/ton |
| 3B | Las Vegas, NV | 100°F | 1 ton14,777 BTU/h · 650 sq ft/ton | 1 ton10,133 BTU/h · 947 sq ft/ton |
| 3C | San Francisco, CA | 84°F | 1 ton9,737 BTU/h · 986 sq ft/ton | 0.5 tons6,677 BTU/h · 1,438 sq ft/ton |
| 4A | St. Louis, MO | 92°F | 1 ton12,418 BTU/h · 773 sq ft/ton | 0.5 tons8,513 BTU/h · 1,128 sq ft/ton |
| 4B | Albuquerque, NM | 95°F | 1 ton12,562 BTU/h · 764 sq ft/ton | 0.5 tons8,721 BTU/h · 1,101 sq ft/ton |
| 4C | Seattle, WA | 84°F | 0.5 tons8,334 BTU/h · 1,152 sq ft/ton | 0.5 tons5,883 BTU/h · 1,632 sq ft/ton |
| 5A | Chicago, IL | 91°F | 1 ton12,079 BTU/h · 795 sq ft/ton | 0.5 tons8,044 BTU/h · 1,193 sq ft/ton |
| 5B | Denver, CO | 93°F | 1 ton11,948 BTU/h · 803 sq ft/ton | 0.5 tons8,028 BTU/h · 1,196 sq ft/ton |
| 6A | Minneapolis, MN | 90°F | 1 ton10,978 BTU/h · 874 sq ft/ton | 0.5 tons7,503 BTU/h · 1,279 sq ft/ton |
| 6B | Billings, MT | 92°F | 1 ton10,958 BTU/h · 876 sq ft/ton | 0.5 tons7,560 BTU/h · 1,270 sq ft/ton |
| 7 | Duluth, MN | 86°F | 1 ton9,148 BTU/h · 1,049 sq ft/ton | 0.5 tons6,341 BTU/h · 1,514 sq ft/ton |
25 of the 28 sizes above are under 1.5 tons. Those numbers are loads, not equipment sizes, because the ducted lineup this calculator sizes against starts at 18,000 BTU/h.
Only 3 of the 28 engine results land inside the rule-of-thumb range of 1.33 to 2 tons, the 1985 house in zones 1A, 2A and 2B.
Phoenix, AZ splits the widest. The 1985 house with attic ducts needs 2 tons and the tight 2015 house with its ducts inside needs 1 ton, a full ton apart on the same floor area.
In 11 of the 14 climates, zones 3A, 3B, 3C, 4A, 4B, 4C, 5A, 5B, 6A, 6B and 7, even the older house rounds to 1 ton or less, on cooling loads from 8,334 to 14,777 BTU/h. The smallest ducted system BuildSolver sizes against is 18,000 BTU/h, 1.5 tons, and these loads sit under it. Size a mini split to the load, ducted or ductless, or let one system cover this space and the rooms next to it, and put that in the quote before the homeowner asks why the new unit is smaller than the one coming out.
The highlighted Houston row is the calculator's starting point. There the 1985 house needs 19,663 BTU/h, or 1.5 tons, and a heating load of 23,060 BTU/h at 25°F. Built tight in 2015 with the ducts inside, the same house needs 1 ton. 1 ton is a load, not a size the ducted lineup carries. That lineup starts at 1.5 tons, 18,000 BTU/h, so the tight build is a mini split job. A 500 sq ft per ton quote would put 1.5 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 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 Phoenix, AZ already land 1 ton 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 an 800 sq ft house
- Quoting 2 tons because the house is 800 sq ft. That is the top of the 400 sq ft per ton rule. The St. Louis, MO row puts the 1985 house at 1 ton and the 2015 house at 0.5 tons. Neither size is one the ducted lineup carries, so that row is a mini split job at either vintage.
- Reusing the tonnage from a job in another market. Carry the Phoenix, AZ tonnage to a Seattle, WA job and you have sold 1.5 tons more equipment than the house asks for.
- 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. Duluth, MN asks the 1985 house for 35,349 BTU/h of heat and 1 ton of cooling. Pick the furnace and the AC from their own loads.
What to check on an 800 sq ft house
At 800 sq ft you are usually looking at a one-bedroom cottage, a converted garage or a detached ADU behind a bigger house. Most of the runs on this page land under the smallest ducted unit, so the equipment list is what limits the job. Ask first whether the space gets its own system, a zone off the main house or a single ductless head, and whether the owner expects it to hold temperature while the main house sits empty.
- Count how many of the four walls actually face outdoors. A converted garage or an attached in-law unit usually has one wall against the main house, and the table models all four as exposed, so the real load on an attached space comes in under every row here.
- The engine gives every house the same baseline appliance gain and the same gain per person, so on 800 sq ft those gains carry a far larger share of the total than they do on a big house. Count the standing freezer in the corner and the home office gear into the load.
- In the humid zones a house this small spends nearly the whole season far below the design load, and a single-stage condenser satisfies the thermostat long before it pulls much moisture out of the air. Ask whether the space feels clammy on mild days, then price a variable-capacity head or a standalone dehumidifier.
Questions about AC size for 800 sq ft
What size AC do I need for an 800 sq ft house?
Climate and construction decide it. In the table on this page the answer runs from 0.5 tons in Seattle, WA to 2 tons in Phoenix, AZ. Chicago, IL puts the same house at 1 ton as a 1985 build and 0.5 tons as a 2015 build. On this page any size under 1.5 tons is a load, not a unit you can order. The ducted lineup starts at 18,000 BTU/h, so those houses take a mini split. Enter your ZIP and the house details in the calculator for your own number.
How many BTU does an 800 sq ft house need for cooling?
The cooling loads in the table run from 5,883 to 24,136 BTU/h. Per square foot that is 7.4 to 30.2 BTU/h, so no single BTU per square foot number fits a house this size. 25 of the 28 runs on this page round to a size under the 18,000 BTU/h floor of the ducted lineup, where the job goes to a mini split. Run the actual house in the calculator above for its number.
Is a 2-ton AC too big for an 800 sq ft house?
ACCA Manual S limits a single-speed cooling unit to 115% of the Manual J load. By that limit a 2-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. Going down from 2 tons stops at 18,000 BTU/h, 1.5 tons, where the ducted lineup this calculator sizes against ends. Loads under that number belong to a mini split.
What size furnace goes with an 800 sq ft house?
The heating loads in the same runs go from 6,580 to 35,349 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 60,000 BTU/h bin. At the warm end 4 runs come in light enough that the engine asks for electric resistance heat rather than a furnace. The furnace size calculator shows the heating result for a single ZIP.
Why does an 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 2 tons in Phoenix and 0.5 tons in Seattle. The residential ducted lineup this page sizes against starts at 1.5 tons, so the Seattle number lands below it. Mini splits are made in smaller sizes, ducted and ductless, so that job goes to one of those.
Can I quote an 800 sq ft job from this table?
Use it for the first conversation with the homeowner. Houston, TX puts the same 800 sq ft at 1.5 tons or 1 ton depending on how the house was built, so run the load for the actual house before you order equipment. The smaller of those two sits under the 18,000 BTU/h bottom of the lineup, so that build is a mini split job. 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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