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AC size · 3,500 sq ft
What size AC for a 3,500 sq ft house
A 3,500 sq ft house needs anywhere from 1.5 tons to 7 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 high end 7 tons is a load, not a unit you can order. The 400 to 600 sq ft per ton rule gives 5.83 to 8.75 tons for all of them, and its top end is past 5 tons, the largest size in the residential ducted lineup this calculator sizes against. Check your zone first.
Run your own house in the calculator. It starts with a 3,500 sq ft two-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 3,500 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
84,727BTU/h
≈ 7 ton (nearest 0.5) · SHR 0.99
Heating load
58,367BTU/h
- Furnace input bin 80,000 BTU/h at 80% AFUE
- No single residential heat pump carries this heating load, so no heat pump size is shown
Load breakdown by component
| Component | Cooling | Heating |
|---|---|---|
| Transmission (walls, ceiling, floor, glass, doors) | 34,362 | 38,512 |
| Solar through glass | 34,991 | - |
| Infiltration | 3,053 | 10,127 |
| Internal gains (people, appliances) | 3,350 | - |
| Ducts in unconditioned attic | 8,971 | 9,728 |
- 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.
- A heat pump sized to carry this heating load alone would need 67,089 BTU/h of rated capacity at 47°F, more than one 5-ton residential unit delivers. Plan on furnace heat or a heat pump with furnace backup, and have an engineer check any all-electric design.
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 3,500 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 = 3,500 / 2 = 1,750 sq ft; side = 42 ftThe engine models a square two-story footprint with 8 ft ceilings, which gives about 2,677 sq ft of gross wall and 1,750 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 1.5 tons and 7 tons.- Square feet per ton
sq ft per ton = 3,500 / (Q_cool / 12,000)Computed from the unrounded load. Across this table one ton covers 496 to 2,365 sq ft.
AC size for a 3,500 sq ft house in 14 climate zones
Each row is the same 3,500 sq ft two-story house on a slab with 5 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 | 5.5 tons66,962 BTU/h · 627 sq ft/ton | 2.5 tons31,431 BTU/h · 1,336 sq ft/ton |
| 2A | Houston, TX 77002 | 96°F | 6 tons71,001 BTU/h · 592 sq ft/ton | 3 tons33,298 BTU/h · 1,261 sq ft/ton |
| 2B | Phoenix, AZ 85003 | 110°F | 7 tons84,727 BTU/h · 496 sq ft/ton | 3.5 tons39,609 BTU/h · 1,060 sq ft/ton |
| 3A | Atlanta, GA 30303 | 95°F | 4 tons47,106 BTU/h · 892 sq ft/ton | 2.5 tons29,842 BTU/h · 1,407 sq ft/ton |
| 3B | Las Vegas, NV 89101 | 100°F | 4 tons49,980 BTU/h · 840 sq ft/ton | 2.5 tons31,927 BTU/h · 1,316 sq ft/ton |
| 3C | San Francisco, CA 94103 | 84°F | 3 tons33,317 BTU/h · 1,261 sq ft/ton | 2 tons21,121 BTU/h · 1,989 sq ft/ton |
| 4A | St. Louis, MO 63101 | 92°F | 3.5 tons42,672 BTU/h · 984 sq ft/ton | 2.5 tons27,201 BTU/h · 1,544 sq ft/ton |
| 4B | Albuquerque, NM 87102 | 95°F | 3.5 tons42,352 BTU/h · 992 sq ft/ton | 2.5 tons27,337 BTU/h · 1,536 sq ft/ton |
| 4C | Seattle, WA 98101 | 84°F | 2.5 tons27,349 BTU/h · 1,536 sq ft/ton | 1.5 tons17,757 BTU/h · 2,365 sq ft/ton |
| 5A | Chicago, IL 60614 | 91°F | 3.5 tons41,761 BTU/h · 1,006 sq ft/ton | 2 tons25,876 BTU/h · 1,623 sq ft/ton |
| 5B | Denver, CO 80202 | 93°F | 3.5 tons40,585 BTU/h · 1,035 sq ft/ton | 2 tons25,345 BTU/h · 1,657 sq ft/ton |
| 6A | Minneapolis, MN 55401 | 90°F | 3 tons37,907 BTU/h · 1,108 sq ft/ton | 2 tons23,950 BTU/h · 1,754 sq ft/ton |
| 6B | Billings, MT 59101 | 92°F | 3 tons37,301 BTU/h · 1,126 sq ft/ton | 2 tons23,767 BTU/h · 1,767 sq ft/ton |
| 7 | Duluth, MN 55802 | 86°F | 2.5 tons30,930 BTU/h · 1,358 sq ft/ton | 1.5 tons19,723 BTU/h · 2,129 sq ft/ton |
3 of the 28 sizes above are past 5 tons. Those numbers are loads, not equipment sizes, because the ducted lineup this calculator sizes against stops at 60,000 BTU/h.
Only 2 of the 28 engine results land inside the rule-of-thumb range of 5.83 to 8.75 tons, the 1985 house in zones 2A and 2B.
Phoenix, AZ splits the widest. The 1985 house with attic ducts needs 7 tons and the tight 2015 house with its ducts inside needs 3.5 tons, 3.5 tons apart on the same floor area.
Miami, FL (66,962 BTU/h), Houston, TX (71,001 BTU/h) and Phoenix, AZ (84,727 BTU/h) put the older house's cooling load at 65,000 BTU/h or more. DOE's residential central air conditioner rules cover units rated under 65,000 BTU/h, so a unit rated above that line is certified as commercial equipment. Plan on two residential systems for the older house in each of those zones.
The highlighted Phoenix row is the calculator's starting point. There the 1985 house needs 84,727 BTU/h, or 7 tons, and a heating load of 58,367 BTU/h at 32°F. Built tight in 2015 with the ducts inside, the same house needs 3.5 tons. 7 tons is a load, and residential ducted equipment stops at 5 tons, so the 1985 build takes two systems. A 500 sq ft per ton quote would come to 7 tons for both, past the largest residential split system. 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 3,500 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 3.5 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.
- The engine treats both stories as one zone. The page cannot split the load between floors, and a bonus room over a garage is not modeled.
- 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 3,500 sq ft house
- Quoting 5 tons because the house is 3,500 sq ft. The 400 sq ft per ton rule gives 8.75 tons here, more than residential ducted equipment carries, so a quote lands on 5 tons, the largest size in it. The rule hands the same answer to a 1985 build in Phoenix and a 2015 build in Seattle, because floor area is all it reads. The St. Louis, MO row puts the 1985 house at 3.5 tons and the 2015 house at 2.5 tons.
- Reusing the tonnage from a job in another market. Carry the Phoenix, AZ tonnage to a Seattle, WA job and you have sold 4.5 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 99,008 BTU/h of heat and 2.5 tons of cooling. Pick the furnace and the AC from their own loads.
What to check on a 3,500 sq ft house
A 3,500 sq ft house is where residential sizing runs out of room. In the hot zones the 1985 column on this page passes what one residential split system can deliver, and the job becomes two systems, a zoned variable-capacity system or light commercial equipment. Expect four or five bedrooms over two floors, a room over the garage, and at least one wall of glass nobody has measured.
- When the load calls for two systems, split the house by floor and by use before you pick tonnage. Two units sized at half the whole-house number will both be wrong, because one of them gets the west glass and the attic ducts and the other gets the bedrooms.
- The coldest row on this page outruns the residential furnace series. That job takes two furnaces on separate zones or a single input above the series, and the combustion air and the venting get sized for whichever one you pick.
- Glass is what moves the load most on a house this size. The page splits window area evenly across the four orientations at 15% of floor area, and a rear wall of west-facing windows, a two-story great room or a sunroom off the kitchen can put more load on one side of the house than the whole upstairs carries.
Questions about AC size for 3,500 sq ft
What size AC do I need for a 3,500 sq ft house?
Climate and construction decide it. In the table on this page the answer runs from 1.5 tons in Seattle, WA to 7 tons in Phoenix, AZ. Chicago, IL puts the same house at 3.5 tons as a 1985 build and 2 tons as a 2015 build. Any size above 5 tons is a load, and residential ducted equipment stops at 5 tons, so that house takes two systems. Enter your ZIP and the house details in the calculator for your own number.
How many BTU does a 3,500 sq ft house need for cooling?
The cooling loads in the table run from 17,757 to 84,727 BTU/h. Per square foot that is 5.1 to 24.2 BTU/h, so no single BTU per square foot number fits a house this size. The top of that range is past the 5-ton limit on residential ducted equipment, so that house takes two systems. Run the actual house in the calculator above for its number.
Is a 5-ton AC too big for a 3,500 sq ft house?
The 400 sq ft per ton rule gives 8.75 tons here, past the 5-ton top of that lineup, so the largest unit a rule-of-thumb quote can land on is 5 tons. ACCA Manual S limits a single-speed cooling unit to 115% of the Manual J load. By that limit a 5-ton single-speed unit is oversized for the 1985 house in 11 of the 14 climates and for the 2015 house in 14 of them.
What size furnace goes with a 3,500 sq ft house?
The heating loads in the same runs go from 16,637 to 99,008 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 120,000 BTU/h bin. One run at the cold end of the list needs more input than the 120,000 BTU/h top of that series, so no bin on this page covers it. Size that house from its own heating load and take the input off your supplier's full furnace table. The furnace size calculator shows the heating result for a single ZIP.
Why does a 3,500 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 3,500 sq ft house needs 7 tons in Phoenix and 2.5 tons in Seattle. The Phoenix number is past the 5-ton top of residential ducted equipment, so that job is two systems.
Can I quote a 3,500 sq ft job from this table?
Use it for the first conversation with the homeowner. Phoenix, AZ puts the same 3,500 sq ft at 7 tons or 3.5 tons depending on how the house was built, so run the load for the actual house before you order equipment. The larger of those two is past the 5-ton top of the lineup, so that build takes two systems. 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.
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