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Calculator · Load per square foot
BTU per square foot calculator
A BTU per square foot number is only as good as the load behind it. Start with the load. This page runs a whole-house Manual J load for the house you describe and gives you the per square foot figure as a result, and the tables below run the same engine on a standard house in 14 climate zones so you can see how far a fixed 20 to 30 BTU factor lands from a real load in each one.
- Type the 5-digit ZIP of the job so the calculator can pick the climate zone and design temperatures.
- Enter the conditioned floor area, stories, year built, air tightness, foundation and windows.
- Press Calculate load to get the whole-house cooling and heating loads in BTU/h.
- Divide each load by the conditioned floor area for BTU per square foot, then compare with the tables below.
Example result for the default inputs
ZIP 87102 · Bernalillo County, NM · IECC zone 4B · elevation 4,982 ft · design 95°F DB / 66°F WB cooling, 14°F heating
Cooling load
27,213BTU/h
≈ 2.5 ton (nearest 0.5) · SHR 0.97
Heating load
39,261BTU/h
- Furnace input bin 60,000 BTU/h at 80% AFUE
- Heat pump 60,000 BTU/h at 47°F (40,650 at design temp)
- Flagged for a dual-fuel system
Load breakdown by component
| Component | Cooling | Heating |
|---|---|---|
| Transmission (walls, ceiling, floor, glass, doors) | 8,347 | 25,332 |
| Solar through glass | 12,253 | - |
| Infiltration | 863 | 7,386 |
| Internal gains (people, appliances) | 2,920 | - |
| Ducts in unconditioned attic | 2,830 | 6,543 |
- 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.
- Recommended 2.5 tons rounds to 110% of computed load. Per ACCA Manual S, oversize >115% may impair dehumidification. Consider a 2-stage or variable-capacity unit.
For estimation purposes only. Consult a licensed engineer for final designs.
Room-by-room Manual J and branded PDF reports · Pro plan
How BTU per square foot is calculated
BTU per square foot is an output of a load calculation. The calculator runs the whole-house Speed Sheet simplification of ACCA Manual J 8th Edition for the house you describe, then divides the cooling and heating loads by the conditioned floor area. The tables run the same engine on a standard house in 14 climate zones.
- Cooling per sq ft
BTU/h per sq ft = Q_cool / floor areaQ_cool is the sum of transmission, solar, infiltration, internal and duct gains at the 1% cooling design condition of the climate zone.- Heating per sq ft
BTU/h per sq ft = Q_heat / floor areaQ_heat is the sum of transmission, slab edge, infiltration and duct losses at the 99% heating design dry-bulb, with no credit for solar or internal gains.- Tons from the load
tons = Q_cool / 12,000The calculator divides the cooling load by 12,000 and rounds to the nearest half ton.- The rule of thumb
Q_guess = floor area x 20 to 30 BTU/h per sq ftA 20 to 30 BTU per sq ft factor guesses the load before you know anything about the house. The tables show how far the engine lands from that range.
BTU per square foot for a 2,000 sq ft house in 14 climate zones
The calculator starts with a 2,000 sq ft two-story house in Albuquerque, NM, built in 1985 with ducts in the attic. Its cooling load of 27,213 BTU/h works out to 13.6 BTU/h per sq ft, and the heating load of 39,261 BTU/h to 19.6 BTU/h per sq ft.
Now move that house. The 1985 columns assume average air sealing and ducts in a vented attic. The 2015 columns assume a tight envelope and ducts inside conditioned space. Both houses run in all 14 reference climates.
| Zone and city | 1985 cooling | 1985 heating | 2015 cooling | 2015 heating |
|---|---|---|---|---|
| 1A Miami | 20.6 | 11.3 | 10.2 | 5.6 |
| 2A Houston | 22 | 22.1 | 11 | 11 |
| 2B Phoenix | 26.7 | 18.5 | 13.4 | 9.2 |
| 3A Atlanta | 15 | 19.6 | 9.9 | 12.4 |
| 3B Las Vegas | 16.1 | 16.8 | 10.7 | 10.7 |
| 3C San Francisco | 10.5 | 13.3 | 6.9 | 8.4 |
| 4A St. Louis | 13.6 | 22.3 | 9 | 14.4 |
| 4B Albuquerque | 13.6 | 19.6 | 9.2 | 12.9 |
| 4C Seattle | 8.8 | 16.1 | 6 | 10.4 |
| 5A Chicago | 13.2 | 28.1 | 8.5 | 16.9 |
| 5B Denver | 13 | 24.5 | 8.4 | 14.9 |
| 6A Minneapolis | 12 | 28.8 | 7.9 | 18.2 |
| 6B Billings | 11.9 | 26.2 | 7.9 | 16.6 |
| 7 Duluth | 9.8 | 31.9 | 6.5 | 20.1 |
Floor area moves the number as well. Here is the 1985 house in the Atlanta, GA zone from 1,000 to 3,000 sq ft, run as a one-story up to 1,800 sq ft and a two-story from 2,000 sq ft up.
| Floor area | Cooling load | Cooling per sq ft | Heating per sq ft |
|---|---|---|---|
| 1,000 sq ft | 16,281 BTU/h | 16.3 | 25 |
| 1,200 sq ft | 19,262 BTU/h | 16.1 | 23.7 |
| 1,500 sq ft | 23,017 BTU/h | 15.3 | 22.2 |
| 1,800 sq ft | 27,203 BTU/h | 15.1 | 21.2 |
| 2,000 sq ft | 30,086 BTU/h | 15 | 19.6 |
| 2,500 sq ft | 36,174 BTU/h | 14.5 | 18.5 |
| 3,000 sq ft | 42,672 BTU/h | 14.2 | 17.7 |
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.
Limitations of per square foot numbers
- Per square foot figures come from a whole-house load. They cannot tell a hot bonus room from a cool basement.
- Design temperatures are the worst case of the ZIP's IECC climate zone. BuildSolver estimates ZIP-level values can differ by 2-8°F, and ASHRAE Handbook - Fundamentals, Chapter 14, lists the city values.
- Insulation and window type default from year built and climate zone unless you pick the window type. Upgrades since construction lower the load.
- Ceilings are 8 ft. A house with 10 ft ceilings or vaulted rooms has more volume and wall per square foot than the calculator assumes.
- Preliminary sales-phase estimate. Not ACCA-approved software output and not accepted for permit submission.
Common BTU per square foot mistakes
- Using one factor for every house. In the 2,000 sq ft table the leaky 1985 house in Houston needs more than three times the cooling per square foot of the tight 2015 house in Seattle, so a factor that fits one badly oversizes the other. Run the load first and read the factor off it.
- Same factor for cooling and heating. Heating per square foot swings far more with climate than cooling does. In the cold zones of the table the heating figure is well above the cooling figure, so check both columns before you reuse a factor.
- Scaling a small house factor to a big house. A larger house has less wall and roof per square foot of floor, so the load per square foot drops as the house grows.
- Counting unconditioned space. Pull the garage, unfinished basement and attic out of the square footage before you apply any per square foot shortcut.
Questions about BTU per square foot
How many BTU per square foot do I need for cooling?
For a 2,000 sq ft house the engine gives 6 to 26.7 BTU/h per sq ft across 14 climate zones and two house ages. Only 3 of the 28 results fall inside the 20 to 30 BTU per sq ft rule of thumb.
How many BTU per square foot do I need for heating?
The heating load of the same house runs from 5.6 to 31.9 BTU/h per sq ft. The spread is wider than for cooling because the winter design temperature ranges from 47°F in the Miami zone to -25°F in the Duluth zone.
Why does BTU per square foot drop as the house gets bigger?
Wall and roof area grow slower than floor area, and a two-story house puts half its floor under the other half. In the Atlanta, GA zone a 1985 house needs 16.3 BTU/h per sq ft at 1,000 sq ft and 14.2 at 3,000 sq ft.
Is 25 BTU per square foot a good estimate?
For cooling, the 2,000 sq ft table comes within 3 BTU of 25 in 2 of 28 results. In 26 of them the engine is under 22, so 25 would oversize the equipment. Check it against a load for the actual house.
Can I size equipment from BTU per square foot?
No. Size from the load of the actual house. The calculator above gives the whole-house load for free, and room-by-room Manual J with Manual S equipment selection are part of the Pro plan.
How many calculations can I run?
BuildSolver load calculators share a limit of 10 calculations per day and 50 per month per visitor. Precomputed tables like these do not count.
Used on the same job
Run the actual house in chat
Bring the house you are quoting. Describe the windows, the insulation and where the ducts run, and the assistant runs the same load tools and turns the result into a tonnage and a furnace input bin.
Open the chat