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AC size · 1,200 sq ft
What size AC for a 1,200 sq ft house
A 1,200 sq ft house needs anywhere from 0.5 tons to 3 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 2 to 3 tons for all of them. Check your zone first.
Run your own house in the calculator. It starts with a 1,200 sq ft single-story house in Phoenix, AZ, 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 1,200 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 85003 · Maricopa County, AZ · IECC zone 2B · elevation 1,085 ft · design 110°F DB / 71°F WB cooling, 32°F heating
Cooling load
34,280BTU/h
≈ 3 ton (nearest 0.5) · SHR 0.98
Heating load
26,314BTU/h
- Furnace input bin 40,000 BTU/h at 80% AFUE
- Heat pump 36,000 BTU/h at 47°F (31,320 at design temp)
Load breakdown by component
| Component | Cooling | Heating |
|---|---|---|
| Transmission (walls, ceiling, floor, glass, doors) | 15,138 | 18,456 |
| Solar through glass | 11,997 | - |
| Infiltration | 1,047 | 3,472 |
| Internal gains (people, appliances) | 2,490 | - |
| Ducts in unconditioned attic | 3,609 | 4,386 |
- These design temperatures are an approximate estimate for the IECC climate zone, not location-specific. For permit work use the design conditions for your exact location.
- 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.
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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 1,200 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 = 1,200 / 1 = 1,200 sq ft; side = 35 ftThe engine models a square single-story footprint with 8 ft ceilings, which gives about 1,109 sq ft of gross wall and 1,200 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 3 tons.- Square feet per ton
sq ft per ton = 1,200 / (Q_cool / 12,000)Computed from the unrounded load. Across this table one ton covers 420 to 1,803 sq ft.
AC size for a 1,200 sq ft house in 14 climate zones
Each row is the same 1,200 sq ft single-story house on a slab with 3 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 ZIP | Cooling design, zone estimate | 1985 house, attic ducts | 2015 house, ducts inside |
|---|---|---|---|---|
| 1A | Miami, FL 33101 | 92°F | 2 tons26,256 BTU/h · 548 sq ft/ton | 1 ton13,428 BTU/h · 1,072 sq ft/ton |
| 2A | Houston, TX 77002 | 96°F | 2.5 tons28,070 BTU/h · 513 sq ft/ton | 1 ton14,376 BTU/h · 1,002 sq ft/ton |
| 2B | Phoenix, AZ 85003 | 110°F | 3 tons34,280 BTU/h · 420 sq ft/ton | 1.5 tons17,618 BTU/h · 817 sq ft/ton |
| 3A | Atlanta, GA 30303 | 95°F | 1.5 tons19,262 BTU/h · 748 sq ft/ton | 1 ton12,892 BTU/h · 1,117 sq ft/ton |
| 3B | Las Vegas, NV 89101 | 100°F | 1.5 tons20,657 BTU/h · 697 sq ft/ton | 1 ton13,936 BTU/h · 1,033 sq ft/ton |
| 3C | San Francisco, CA 94103 | 84°F | 1 ton13,632 BTU/h · 1,056 sq ft/ton | 1 ton9,176 BTU/h · 1,569 sq ft/ton |
| 4A | St. Louis, MO 63101 | 92°F | 1.5 tons17,450 BTU/h · 825 sq ft/ton | 1 ton11,719 BTU/h · 1,229 sq ft/ton |
| 4B | Albuquerque, NM 87102 | 95°F | 1.5 tons17,578 BTU/h · 819 sq ft/ton | 1 ton11,951 BTU/h · 1,205 sq ft/ton |
| 4C | Seattle, WA 98101 | 84°F | 1 ton11,562 BTU/h · 1,245 sq ft/ton | 0.5 tons7,986 BTU/h · 1,803 sq ft/ton |
| 5A | Chicago, IL 60614 | 91°F | 1.5 tons16,971 BTU/h · 849 sq ft/ton | 1 ton11,136 BTU/h · 1,293 sq ft/ton |
| 5B | Denver, CO 80202 | 93°F | 1.5 tons16,716 BTU/h · 861 sq ft/ton | 1 ton11,069 BTU/h · 1,301 sq ft/ton |
| 6A | Minneapolis, MN 55401 | 90°F | 1.5 tons15,459 BTU/h · 931 sq ft/ton | 1 ton10,372 BTU/h · 1,388 sq ft/ton |
| 6B | Billings, MT 59101 | 92°F | 1.5 tons15,383 BTU/h · 936 sq ft/ton | 1 ton10,418 BTU/h · 1,382 sq ft/ton |
| 7 | Duluth, MN 55802 | 86°F | 1 ton12,802 BTU/h · 1,125 sq ft/ton | 0.5 tons8,704 BTU/h · 1,654 sq ft/ton |
16 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 2 to 3 tons, the 1985 house in zones 1A, 2A and 2B.
Houston, TX splits the widest. The 1985 house with attic ducts needs 2.5 tons and the tight 2015 house with its ducts inside needs 1 ton, 1.5 tons apart on the same floor area.
In 3 of the 14 climates, zones 3C, 4C and 7, even the older house rounds to 1 ton or less, on cooling loads from 11,562 to 13,632 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 Phoenix row is the calculator's starting point. There the 1985 house needs 34,280 BTU/h, or 3 tons, and a heating load of 26,314 BTU/h at 32°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 2.5 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 1,200 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 1.5 tons apart.
- Each row uses an approximate design temperature for its IECC climate zone, from 84°F in the San Francisco zone to 110°F in the Phoenix zone. These are not location-specific, 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 1,200 sq ft house
- Quoting 3 tons because the house is 1,200 sq ft. That is the top of the 400 sq ft per ton rule. The St. Louis, MO row puts the 1985 house at 1.5 tons and the 2015 house at 1 ton. The 2015 number is under the smallest size the ducted lineup carries, so that one is a mini split job.
- Reusing the tonnage from a job in another market. Carry the Phoenix, AZ tonnage to a San Francisco, CA job and you have sold 2 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 47,186 BTU/h of heat and 1 ton of cooling. Pick the furnace and the AC from their own loads.
What to check on a 1,200 sq ft house
On most jobs a 1,200 sq ft house is a single-story ranch on a slab or crawlspace, three bedrooms and one bath. Expect one return in the hallway and ducts in the attic. Write down the tonnage on the existing condenser and ask for the old load report. If there is none, assume the size came from square footage.
- A single central return starves closed bedrooms. Check for jumper ducts or transfer grilles before you blame the equipment size for hot back rooms.
- Measure total external static pressure on the existing system. Attic ducts sized for a 3-ton unit can put a smaller replacement at the wrong airflow, so check the blower table before you drop tonnage.
- In hot-dry zones the attic ducts and the glass carry most of the extra load. Price sealing and burying the ducts against the step up to the next equipment size before you quote.
Questions about AC size for 1,200 sq ft
What size AC do I need for a 1,200 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 3 tons in Phoenix, AZ. Chicago, IL puts the same house at 1.5 tons as a 1985 build and 1 ton 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 a 1,200 sq ft house need for cooling?
The cooling loads in the table run from 7,986 to 34,280 BTU/h. Per square foot that is 6.7 to 28.6 BTU/h, so no single BTU per square foot number fits a house this size. 16 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 3-ton AC too big for a 1,200 sq ft house?
ACCA Manual S limits a single-speed cooling unit to 115% of the Manual J load. By that limit a 3-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 3 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 a 1,200 sq ft house?
The heating loads in the same runs go from 8,564 to 47,186 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 one run comes 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 a 1,200 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 1,200 sq ft house needs 3 tons in Phoenix and 1 ton 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 a 1,200 sq ft job from this table?
Use it for the first conversation with the homeowner. Phoenix, AZ puts the same 1,200 sq ft at 3 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
- Manual J calculator The whole-house load for the actual house from its ZIP code.
- HVAC design temperatures by ZIP The outdoor design conditions behind every row of the table.
- HVAC CFM calculator The airflow the ducts have to carry at the tonnage you picked.
- BTU to tons converter The same load in tons, kW or MBH for a data sheet in other units.
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.
Open the chat