SizingKitHVAC · square-foot estimate
BTU calculator — the square-foot rule and its error band
Free and with no signup: enter a floor area and a climate zone and this returns the heating and cooling BTU/h a square-foot rule of thumb gives, as a range rather than a single number. The heating span comes from the five-band trade table, 30 to 60 BTU/h per square foot; the cooling span from the one-ton-per-400-to-700-square-feet table, 30 down to 17. No standard publishes either of them, and this page says so beside every figure instead of in a footnote.
- 100% free
- No signup
- 8 IECC climate zones
- 30–60 BTU/h per ft² heating
- 17–30 cooling
- Answer is a band, not a point
The house, as a square-foot rule sees it
Six answers, four of which the rule cannot really use. That is the point of the width of the answer it gives back.
Heated and cooled space only. Garages, unconditioned basements and vented attics are not in it.
Assigned by county in IECC Table R301.1, not computed from your weather.
You pick this one: the cooling table carries no climate zones, and this site will not invent a mapping it does not have.
ACCA Manual J, 8th edition: unless the owner states otherwise, the occupancy of a residence is taken as the number of bedrooms plus one.
Default 4, worth 1,720 BTU/h of cooling. Type over it if the house is fuller or emptier than its bedroom count suggests.
The estimate
Heating, furnace output
81,000 BTU/h – 90,000 BTU/h
23.7 kW to 26.4 kW · 45–50 BTU/h per ft²
Cooling, total capacity
37,720 BTU/h
3.14 tons · 20 BTU/h per ft² plus 1,720 BTU/h of people
The heating figures behind that answer span 11% from one end to the other, and the table’s five bands together run 30 to 60 BTU/h per ft² — a factor of 2 across climates. The width, rather than any point inside it, is what a square-foot rule actually tells you.
Heating band: 45–50 BTU/h per ft², the cool band. The whole published band, end to end. This is the house the figures were written for, so this row uses them exactly as the table states them rather than reading a position inside them. A trade rule of thumb with no standard behind it. These are the bands that circulate in US HVAC sales material; they describe furnace output BTU/h per square foot of conditioned floor area for an average existing house. The IECC zone column is an approximate mapping added here, not part of the original.
Cooling row: Temperate — the customary default at 600 ft² per ton. Stays on the row you picked. The four rows already span 17 to 30 BTU/h per ft², so the row itself is the answer's precision. Occupants add 230 BTU/h sensible and 200 latent each and are counted in cooling only — ACCA Manual J, 8th edition: the sensible heat gain assigned to one occupant of a residence.
Two things this page is blind to. It is per square foot of floor, so a room with a 12 ft ceiling and a room with an 8 ft ceiling read identically although one holds half again as much air to heat. And the heating figure is furnace output: the input rating stamped on the appliance is larger by its efficiency, which is what the furnace size calculator turns it into.
A square-foot rule of thumb is not a load calculation. It knows nothing about your insulation, your windows, which way the house faces, how leaky it is, how many people live in it or what your local design temperature is — and those are the things that decide the answer. Two houses of the same floor area in the same city routinely differ by a factor of two. Use this to sanity-check a quote or to guess at a budget; do not buy equipment on it. What sizes equipment is an ACCA Manual J load calculation, room by room, and a permit that asks for a load calculation is not asking for this.
Climate zones and the heating band each one lands in
The zone column is a definition you can check. The BTU/h column is not — it is the trade figure, printed here so that the difference between the two is visible on one sheet.
| Zone | Name | Degree-day criterion | BTU/h per ft² | 1,800 ft² house |
|---|---|---|---|---|
| 1 | Very hot | CDD 50 °F > 9000 | 30–35 | 54,000–63,000 |
| 2 | Hot | 6300 < CDD 50 °F ≤ 9000 | 30–40 | 54,000–72,000 |
| 3 | Warm | 4500 < CDD 50 °F ≤ 6300, or HDD 65 °F ≤ 3600 (3C, marine) | 35–45 | 63,000–81,000 |
| 4 | Mixed | CDD 50 °F ≤ 4500 and HDD 65 °F ≤ 5400; 3600 < HDD ≤ 5400 for 4C marine | 40–45 | 72,000–81,000 |
| 5 | Cool | 5400 < HDD 65 °F ≤ 7200 | 45–50 | 81,000–90,000 |
| 6 | Cold | 7200 < HDD 65 °F ≤ 9000 | 50–60 | 90,000–108,000 |
| 7 | Very cold | 9000 < HDD 65 °F ≤ 12,600 | 50–60 | 90,000–108,000 |
| 8 | Subarctic / Arctic | HDD 65 °F > 12,600 | 50–60 | 90,000–108,000 |
Zone definitions: International Energy Conservation Code, Table R301.3 / C301.3 (International Climate Zone Definitions), converted to IP degree days; the same zones as ASHRAE Standard 169. Zone 8 is 7000 HDD 18.3 °C, which is 12,600 HDD 65 °F.
How to read a square-foot BTU estimate without being misled by it
Three steps, the third of which is the one that matters.
Give it the conditioned floor area and your county's climate zone
Measure heated and cooled space only — a garage, a vented attic and an unconditioned basement are outside the envelope and are not in the figure. The climate zone is assigned to your county by IECC Table R301.1; it is not something you compute from local weather.
Place the house in the band rather than at a point in it
The five envelope choices move you from the bottom of the published band to the top of it. The outermost two do not extrapolate past the table: they return the band edge and tell you the table has stopped, because a tight new build and an uninsulated solid-wall house are both off the ends of a figure written for an average existing house.
Read the width, then decide whether the width is acceptable
The page prints how far apart the two ends of the heating answer are as a percentage. If a 30% spread is fine for a budget conversation, stop here. If you are choosing equipment, the spread is the reason to go and do a room-by-room load calculation instead.
Technical specifications
| Heating figures | Five bands, 30 to 60 BTU/h per ft² of conditioned floor area, describing furnace output for an average existing house. No code, no ACCA manual and no ASHRAE table publishes them. |
|---|---|
| Cooling figures | Four rows, 400 / 500 / 600 / 700 ft² per ton, printed as 30 / 24 / 20 / 17.1 BTU/h per ft². The BTU column is computed as 12,000 ÷ ft² per ton rather than stated separately, so the two cannot disagree. |
| Climate zones | All 8 IECC / DOE zones with the heating and cooling degree-day criteria that define them, from IECC Table R301.3 — the same zones as ASHRAE Standard 169. |
| Zones claimed twice | Zones 2 and 3 each appear in two bands of the heating table. The page returns the union of both — 30–40 for zone 2, 35–45 for zone 3 — rather than picking one. |
| Occupant gain | 230 BTU/h sensible and 200 BTU/h latent per person, ACCA Manual J 8th edition, at bedrooms plus one unless you enter a count. Added to cooling only, never to heating. |
| Floor area accepted | 20 to 200,000 ft² (1.9 to 18,580 m²), entered in either unit, with the answer always shown in BTU/h and kW together. |
| What the method cannot see | Ceiling height, window area by orientation, air leakage, duct location, and your actual outdoor design temperature. Four of those change the answer by more than the climate zone does. |
| Where the arithmetic runs | In this browser tab, on every input change, with nothing sent anywhere — so it works on a phone in a basement with no signal. |
Frequently asked questions
Is 20 BTU per square foot the right number for cooling?
It is the middle of a range that runs from 17 to 30, not a constant. The 20 comes from one row of a four-row table — 600 ft² per ton, and 12,000 ÷ 600 is exactly 20 — and the rows either side of it are 24 and 17.1. A house that lands on the 400 ft²-per-ton row needs half again as much machine per square foot as one on the 600 row, and nothing about floor area tells you which row you are on.
Why does this return a range when every other BTU calculator returns a number?
Because the underlying table is a range and printing its midpoint hides that. The five heating bands are each written as a span — 45 to 50 BTU/h per ft² for a cool climate, for instance — and collapsing that to 47.5 implies a precision the figure has never had. Two houses of the same floor area in the same city routinely differ by a factor of two, which is wider than this page's entire output.
My climate zone appears in two rows of the heating table. Which applies?
Both, so the page returns the union of the two. The zone column in the heating table is an approximate mapping added when the table was compiled and is not part of the original figures, so zones 2 and 3 are each claimed by two bands. Taking the wider span is the only reading that does not silently make a choice on your behalf.
Does ceiling height change the result?
Not here, and that is a defect of the method rather than a feature of your house. The rule is written per square foot of floor, so a great room with a 16 ft vaulted ceiling and a bedroom with an 8 ft one read the same per square foot despite holding twice the air. Any method that starts from volume or from air changes will see it.
Can I buy a furnace or an air conditioner from this number?
No, and a permit office asking for a load calculation will not accept it. Equipment selection runs the other way round: a room-by-room load at your own design conditions, then a selection from the manufacturer's expanded performance data at those same conditions. An air conditioner chosen a ton too large satisfies the thermostat before it has pulled any water out of the air, which is how a house ends up cold and clammy at 74 °F.
Why do people and windows only move the cooling figure?
A heating load takes no credit for internal gains and a cooling load takes them all. That asymmetry is deliberate: heat has to be available at four in the morning in January when nobody is awake, no lights are on and the sun has been down for fourteen hours, so a load calculation that leaned on body heat would undersize the furnace for exactly the conditions it exists for. Cooling peaks in the afternoon with the house full and the sun on the glass, so those gains are real and are counted.
The contractor quoted twice this. Is one of us wrong?
Possibly neither — but ask which method produced the quote. Replacement equipment is very often sized to whatever was there before, and the thing that was there before was frequently oversized too, so the practice reproduces itself across a house's whole service life. If the quote came with a printed room-by-room load, compare that against this instead; if it came from the label on the old furnace, this page is the more considered of the two guesses.
Where BTU per square foot came from, and what it forgets
A BTU is a quantity of energy and a BTU/h is a rate, and every piece of equipment in this subject is sized in the second. When a nameplate says 36,000 BTU it means 36,000 BTU/h; the hour is dropped in conversation and never in the arithmetic. That rate is also what a ton means — 12,000 BTU/h, the rate that melts a short ton of ice in a day, a survival from the ice trade that the AC tonnage calculator and the BTU to tons calculator take apart properly. The square-foot figures on this page are that rate divided by floor area, and they exist because a salesperson standing in a driveway needs an answer before the visit is over.
What they forget is everything that decides the answer. Solar gain through glazing is the largest single cooling term in most houses and depends on window area, orientation, shading and the glass’s own solar heat gain coefficient — none of which is a function of floor area. Air leakage can be a third of a heating load in an old house and almost nothing in a new one, at identical square footage. Ducts run through a vented attic can lose a fifth of what the furnace makes before it reaches a register. And the outdoor design temperature, which sets the temperature difference the whole calculation multiplies by, varies by more than 60 °F across the eight climate zones while the square-foot figure varies by a factor of two. Insulation is the one term you can partially recover here, by choosing where in the band the house sits — and the insulation R-value calculator shows why the number on the batt is not the number the wall performs at.
The honest use of this page is a sanity check. If a quote comes back at four times what the band says, something is wrong with the quote or with what you told the contractor about the house. If it comes back inside the band, the band has told you nothing you did not already know and the next step is a real calculation: fabric and ventilation heat loss if the question is heating, a room-by-room sensible and latent load if it is cooling, and what a certified Manual J adds to both if a permit is involved.
Where the floor area you typed goes
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The bedroom count and the floor area are the only two things this page could identify a property by, and neither leaves the tab — there is no request to a server on this page at all, so nothing about your house is available to be stored.