Heated and cooled space only.
You cool volume, not floor.
IECC zone. Matters more than floor area.
The single biggest variable.
Advanced options
Variable speed tolerates more error.
0 to use the zone default.
0 to skip running cost.
This BTU calculator sizes heating and cooling from your climate zone and building envelope rather than floor area alone — and shows why an oversized system is the more expensive mistake, not the safe one.
The short answer
A 2,000 ft² home in a mixed climate with average insulation needs roughly 44,600 BTU of cooling — about 3.7 tons — and 80,000 BTU of heating. Those two numbers are not related, and both depend far more on your envelope than your floor area.
The “500 square feet per ton” rule is a 1970s figure that ACCA Manual J explicitly deprecates. Measured modern homes average closer to 856 ft² per ton.
On this page
How to use this BTU calculator
Three modes:
- Whole home — conditioned floor area, ceiling height and occupants
- Single room — for a window unit, mini split, or checking zone balance
- Check a quote — enter what a contractor proposed and see how far off it is
Then set your climate zone and envelope quality. Those two inputs move the answer more than anything else, and they’re exactly what square-footage rules ignore.
The header toggle switches between BTU and kW, since most of the world sizes in kilowatts.
Why bigger is worse
The instinct is that a larger unit is the safe choice — worst case, it’s a bit over. That instinct is wrong, and it’s the single most useful thing on this page.
An air conditioner does two jobs: it lowers temperature (sensible cooling) and removes moisture (latent cooling). The second only happens while the coil is actually running. An oversized unit satisfies the thermostat quickly and shuts off — before it has dehumidified.
| Unit size vs load | Runtime on a design day | Result |
|---|---|---|
| 95% | 100% | Runs continuously, fully dehumidifies |
| 100% | 100% | The target |
| 115% | 87% | Top of the Manual S window |
| 130% | 77% | Cycling begins, humidity slips |
| 160% | 63% | Short cycling, clammy |
| 200% | 50% | Cold and wet, high wear, high bills |
You pay more for the equipment. You get a house that reads 72°F and feels clammy, because the humidity never came out. The compressor — the most expensive component — takes start-stop wear instead of steady running. And your bills go up, because the first minute of every cycle is the least efficient minute of operation.
Undersizing has one failure mode: the system runs flat out and can’t quite keep up on the two or three worst afternoons of the year. That’s a real problem, but it’s a smaller one, and it doesn’t cost you comfort for the other 362 days.
ACCA Manual S sets the correct window at 95–115% of the calculated load for single-stage equipment. Two-stage tolerates up to about 125%, and variable-speed up to 130%, because they can modulate down rather than cycling.
The rule of thumb is obsolete
“400 to 600 square feet per ton”, or equivalently “20 to 25 BTU per square foot”, is still quoted constantly. It comes from housing built between roughly 1960 and 1985: R-11 walls, single-pane aluminium-framed windows, and envelopes that leaked badly.
ACCA Manual J section 1 explicitly deprecates it. Applied to a home built to 2015 IECC or later — R-20 walls, low-E double glazing, blower-door tested — it produces a load well above reality.
| Method | ft² per ton | 2,000 ft² home |
|---|---|---|
| Rule of thumb, aggressive | 400 | 5.0 tons |
| Rule of thumb, typical | 500 | 4.0 tons |
| Rule of thumb, conservative | 600 | 3.3 tons |
| Measured, real Manual J jobs | 856 | 2.3 tons |
Analysis of actual Manual J calculations across a batch of real homes found an average of 856 ft² per ton. Against the typical rule that’s 1.7× oversized — a 4-ton unit where 2.3 tons was needed.
Contractors round up because nobody wants a callback on the hottest day of the year. It’s an understandable incentive that produces a worse system.
Climate zone
| IECC zone | Examples | Cooling BTU/ft² | Heating BTU/ft² |
|---|---|---|---|
| 1 | Miami, Houston | 30 | 25 |
| 2 | Phoenix, Orlando | 27 | 28 |
| 3 | Atlanta, Los Angeles | 24 | 32 |
| 4 | Kansas City, New York | 20 | 40 |
| 5 | Chicago, Denver | 18 | 50 |
| 6 | Minneapolis | 16 | 55 |
| 7 | Duluth | 14 | 60 |
The same 2,500 ft² house needs around 5.4 tons in Houston and 3.5 tons in Chicago — a 1.5× difference for identical construction. Any calculator that doesn’t ask where you are is guessing.
Envelope beats floor area
| Factor | Multiplier |
|---|---|
| Poor envelope — pre-1980, single pane | ×1.30 |
| Average — 1980s to 2000s | ×1.00 |
| Good — 2015 IECC or better | ×0.80 |
| Excellent — passive house | ×0.55 |
| 10 ft ceilings vs 8 ft | ×1.25 |
| Heavy sun, west facing | ×1.10 |
| Heavily shaded | ×0.90 |
| Kitchen | +4,000 BTU |
| Each occupant over two | +600 BTU |
Insulation and glazing alone swing the answer by a factor of 2.4 — from ×1.30 to ×0.55 — on an identical footprint. That single variable outweighs everything a square-footage rule can see.
Ceiling height matters because you condition volume, not floor. A room with 10 ft ceilings holds 25% more air than the same footprint at 8 ft, and needs about 25% more capacity. Floor-area rules miss this entirely, which is why open-plan homes with vaulted ceilings are so often undersized by them.
Heating and cooling diverge
These are separate calculations driven by different physics — cooling by solar gain and humidity, heating by temperature difference alone. They do not track each other:
| 2,000 ft², average envelope | Cooling | Heating | Ratio |
|---|---|---|---|
| Miami | 60,000 BTU | 50,000 BTU | 0.83 |
| Atlanta | 48,000 BTU | 64,000 BTU | 1.33 |
| New York | 40,000 BTU | 80,000 BTU | 2.00 |
| Chicago | 36,000 BTU | 100,000 BTU | 2.78 |
| Duluth | 28,000 BTU | 120,000 BTU | 4.29 |
Chicago needs nearly three times more heating than cooling. Miami needs more cooling than heating. A single unit doing both — a heat pump — can’t be optimal for both, which is what the balance point conversation is about: the outdoor temperature below which the heat pump can no longer meet the heating load and supplemental heat takes over.
Size a heat pump to the cooling load in a cold climate and you’ll lean heavily on expensive backup heat. Size it to the heating load and you’ve oversized the cooling and reintroduced the humidity problem. It’s a genuine trade-off worth discussing with your contractor rather than leaving to a default.
Tons, BTU and kW
| Tons | BTU/h | kW | Zone 4, average envelope |
|---|---|---|---|
| 1.5 | 18,000 | 5.28 | ~900 ft² |
| 2 | 24,000 | 7.03 | ~1,200 ft² |
| 2.5 | 30,000 | 8.79 | ~1,500 ft² |
| 3 | 36,000 | 10.55 | ~1,800 ft² |
| 3.5 | 42,000 | 12.31 | ~2,100 ft² |
| 4 | 48,000 | 14.07 | ~2,400 ft² |
| 5 | 60,000 | 17.58 | ~3,000 ft² |
The area column is the zone 4 baseline of 20 BTU/ft² at an average envelope — not a universal rule. In zone 1 the same tonnage covers about a third less area; in zone 6, a quarter more.
A “ton” is the cooling produced by melting a ton of ice over 24 hours — an unlikely-sounding unit that has stuck since the days of ice-based refrigeration. Residential equipment comes in half-ton steps, so the answer always gets rounded to an available size.
Round toward the calculated load, not away from it. If your load is 3.6 tons, 3.5 usually beats 4.
Sizing a single room
For a window unit or a mini split head, use the room mode. The same envelope and climate factors apply, but drop the kitchen allowance unless the room actually contains one.
Two things to watch. A room that’s part of an open plan isn’t thermally separate, so sizing it in isolation overstates. And a room with an exterior door in constant use, or a large west-facing window, carries load well beyond its floor area.
Checking a quote
Use the quote-check mode, then ask one question: “Can I see the Manual J?”
A contractor who has done one will produce it. One who sized from square footage won’t, and that answer tells you what you need to know. Many jurisdictions now require a Manual J for permits, and some manufacturers require one for warranty compliance.
If the quoted size is more than about 25% above the calculated load, that’s worth a conversation before signing — not because this estimate is authoritative, but because the gap suggests nobody calculated anything.
HVAC sizing mistakes to avoid
- Rounding up “to be safe”. Oversizing has no upside.
- Using 400–600 ft² per ton. Manual J deprecates it; measured homes average 856.
- Sizing on floor area alone. Envelope swings it 2.4×.
- Ignoring ceiling height. You condition volume, not floor.
- Assuming heating tracks cooling. Chicago needs 2.8× more heating.
- Sizing a heat pump to cooling in a cold climate. Check the balance point.
- Accepting a size without a Manual J. Ask to see it.
- Confusing SEER2 with capacity. Efficiency doesn’t change BTU needed.
Frequently asked questions
How many BTU do I need per square foot?
It depends far more on climate and envelope than floor area. Cooling runs roughly 30 BTU/ft² in Miami down to 14 in Duluth, then multiplied by 0.55 to 1.30 for envelope quality. The old “20 BTU per square foot” figure is a 1970s number that ACCA Manual J explicitly deprecates.
What size air conditioner do I need for 2,000 square feet?
Somewhere between about 2.3 and 5 tons, depending on where you are and how the house is built. A mixed climate with average insulation lands near 3.7 tons; the same footprint in Miami needs 5 and in Minneapolis 2.7. Anyone who answers from square footage alone is guessing.
Is it better to oversize or undersize an AC?
Undersize, if you must choose. An oversized unit satisfies the thermostat before it removes humidity, leaving a cold, clammy house, and short cycling wears the compressor while raising bills. Undersizing means struggling on the two or three hottest afternoons a year. ACCA Manual S puts the correct range at 95–115% of load.
What is short cycling?
An oversized system reaching setpoint fast, shutting off, then restarting soon after. Dehumidification only happens while the coil runs, so short cycles leave moisture in the house. It also concentrates wear on the compressor and wastes energy, since the start of each cycle is the least efficient moment of operation.
How many square feet per ton of air conditioning?
The old rule says 400 to 600, but analysis of real Manual J calculations found modern homes averaging around 856 ft² per ton. On a 2,000 ft² home that’s the difference between 4 tons and 2.3 — roughly 1.7× oversized if you use the rule.
What is a Manual J calculation?
The ACCA residential load calculation standard. It measures every window by orientation and shading, each wall assembly, duct location and leakage, and a blower-door infiltration rate against local design temperatures. It’s the industry standard, increasingly required for permits, and this calculator is a sanity check against it rather than a substitute.
How many BTU is a ton?
12,000 BTU per hour, or 3.52 kW. The unit comes from the cooling produced by melting one ton of ice over 24 hours, dating from ice-based refrigeration. Residential equipment is sold in half-ton steps from 1.5 to 5 tons.
Does ceiling height affect BTU requirements?
Yes, substantially — you condition volume, not floor area. A room with 10 ft ceilings holds 25% more air than the same footprint at 8 ft and needs roughly 25% more capacity. This is why floor-area rules regularly undersize open-plan homes with vaulted ceilings.
Why are my heating and cooling loads so different?
Because they’re driven by different things — cooling by solar gain and humidity, heating by temperature difference. A 2,000 ft² home in Chicago needs about 36,000 BTU of cooling and 100,000 of heating, nearly three times as much. Miami is the reverse.
Does a higher SEER2 rating change what size I need?
No. SEER2 measures efficiency — how much electricity it takes to deliver the cooling — not capacity. A 3-ton unit delivers 36,000 BTU whether it’s SEER2 14 or 20. Efficiency changes your running cost, not your required size.
Can I use this BTU calculator in kilowatts?
Yes — the header toggle switches all capacity figures to kW, which is how most of the world sizes equipment. One ton is 3.52 kW, and 1 kW equals 3,412 BTU per hour. Floor area is entered in square feet; multiply square metres by 10.76.
What size mini split do I need for one room?
Use the single-room mode with your room dimensions, climate and envelope. Drop the kitchen allowance unless the room has one. Watch for rooms that aren’t thermally separate — sizing part of an open plan in isolation overstates — and for large west-facing glazing, which carries load well beyond floor area.
Standards and sources
- ACCA Manual J — residential load calculation standard, and the source of the deprecation of square-footage rules of thumb
- ACCA Manual S — equipment selection, giving the 95–115% sizing window for single-stage cooling equipment
- IECC climate zone designations used for the per-square-foot baseline figures
- The 856 ft² per ton figure comes from published analysis of a batch of real Manual J calculations, against a rule-of-thumb range of 400–600
- Per-square-foot loads and envelope multipliers here are planning estimates. A Manual J typically lands within a few percent; this method does not claim that accuracy
- Design temperatures, duct losses, infiltration rates and window orientation all materially affect real loads and are not modelled individually here
Related calculators
How this BTU calculator works out its numbers
Cooling load starts from a per-square-foot figure set by IECC climate zone, then multiplies by envelope quality, sun exposure and a ceiling-height factor referenced to 8 ft. Fixed additions follow for a kitchen and for occupants beyond two, since those are absolute heat sources rather than proportional ones. Heating uses the same envelope and volume factors against a separate per-square-foot figure, because the two loads are driven by different physics.
The Manual S window is applied to the cooling result — 95–115% for single-stage, widening for two-stage and variable-speed equipment, which can modulate rather than cycle. The nearest available half-ton size is then reported, with a note if it falls above the window.
Quote-check mode compares a proposed size against the calculated load and reports the runtime fraction on a design day, which is what actually determines whether the system will dehumidify.
This is an estimate, not a Manual J. A real load calculation accounts for each window by orientation and shading, individual wall and roof assemblies, duct location and leakage, and a measured infiltration rate against local design temperatures — none of which a per-square-foot method can capture. Use this to sanity-check a quote and to understand which variables matter, then ask your contractor for the Manual J that supports their number. If they can’t produce one, that’s the finding.