BTU Calculator
How much heating and cooling capacity a room needs, from its floor area and seven things that change the answer — with every adjustment shown as its own line, so you can see which of them is driving the number. In BTU per hour, in tons, and in kilowatts.
The room you are sizing for
Length × width of the space the unit has to condition. An open plan room and the hallway it opens onto are one space, not two.
The rule of thumb assumes 8 ft. Equipment conditions volume, not floor, so a 12 ft ceiling is half again the load.
The biggest swing in the heating figure — a very cold zone asks twice the heating rate of a hot one.
Poor adds 25%. Good takes 10% off — better than average helps less than worse than average hurts.
Glass is the weakest part of any wall, in both directions. A patio door or single glazing adds 15%.
Applied to cooling only, at ENERGY STAR’s ±10%. Winter sun is not something a furnace can be sized against.
Normally at once, not the household total. The first two are already inside the base rule; each one after adds 600 BTU/hr.
A flat 4,000 BTU/hr on cooling. Ovens and hobs put out real heat, and the fridge dumps everything it removes back into the room.
That is 0.50 tons of cooling, or 1.76 kW. Heating the same room takes about 12,000 BTU/hr (3.52 kW). Both are rule-of-thumb estimates, not a Manual J load calculation.
- Cooling load
- 6,000 BTU/hr
- Heating load
- 12,000 BTU/hr
- Cooling in tons
- 0.50 tons
- Cooling in kilowatts
- 1.76 kW
- Heating in kilowatts
- 3.52 kW
| Factor | Your answer | Cooling BTU/hr | Heating BTU/hr |
|---|---|---|---|
| Base rule — floor area | 300 sq ft | 6,000 | 12,000 |
| Ceiling height | 8 ft | 0 | 0 |
| Climate zone | Mixed (zones 3–4) | 0 | 0 |
| Insulation | Average | 0 | 0 |
| Window area | Typical | 0 | 0 |
| Sun exposure | Average | 0 | — |
| Occupants | 2 people | 0 | — |
| Kitchen appliances | No | 0 | — |
| Total | — | 6,000 | 12,000 |
- Bigger is not safer. An oversized air conditioner cools the air to setpoint and switches off before it has removed the humidity, leaving the room cold and clammy — and the short-cycling is what wears the compressor out.
- Percentages compound in the order shown, so each line is a share of the running load rather than of the base. Reordering them would move the lines around and leave the totals unchanged.
- Sun, occupants, and kitchen heat are added to cooling only. Heating equipment is sized for a winter night with the room empty and the oven cold, so none of them can be counted on.
- Kilowatts here are heat moved, not electricity drawn. An air conditioner or heat pump shifts several times more heat than the power it consumes; only electric resistance heating draws its full rating from the wall.
A floor area, a base rate, and seven adjustments
Both answers start the same way: multiply the floor area by a rate in BTU per hour per square foot. Cooling uses 20, which is the figure ENERGY STAR publishes for sizing a room air conditioner. Heating uses a rate that depends on where you live — 30 in the hottest zones, 60 in the coldest — and the calculator measures that against 40, the mixed-climate middle, so the climate line in the table shows the difference your location makes rather than hiding it inside the base.
Cooling = A × 20 × (H ÷ 8) × fclimate × finsul × fglass × fsun+ 600 × max(0, P − 2) + K • Heating = A × Rzone × (H ÷ 8) × finsul × fglass- A, H
- Floor area in square feet and ceiling height in feet; ÷ 8 because the base rates assume an 8 ft ceiling
- Rzone
- Heating rate: 30, 40, 50 or 60 BTU/hr per square foot for IECC zones 1–2, 3–4, 5–6 and 7–8
- fclimate
- Cooling only, and small: +10% hot, 0% mixed, −5% cold, −10% very cold
- finsul, fglass
- Insulation (+25% poor, −10% good) and glazing (+15% large or single, −10% few and small), both applied to heating and cooling alike
- fsun, P, K
- Cooling only: ±10% for shade or full sun, 600 BTU/hr for each occupant past two, and 4,000 BTU/hr if the room is a kitchen
The percentage factors compound rather than being added up, in the order the table lists them. Multiplication commutes, so the totals do not depend on that order — but each individual line does, because it is a percentage of the running load rather than of the base. That is the honest arrangement: a 25% insulation penalty on a room with a 12 ft ceiling really is worth more BTU than the same penalty on an 8 ft one.
The last three terms are cooling-only, and that is a deliberate choice rather than an oversight. Bodies, ovens, and afternoon sun are real heat, and they really do take work off a furnace — but heating equipment is sized for the design condition, which is a winter night with the room empty, the oven cold, and the sun down. Crediting a dinner party against the furnace sizes it for the dinner party.
An oversized air conditioner is not a safety margin
Almost every other page that will give you a BTU number leaves this out, and it is the most useful thing on this one. The intuition is that if 6,000 BTU/hr is enough, 10,000 is enough with room to spare. It is not, and the reason is mechanical rather than a matter of taste.
An air conditioner does two jobs at once. It lowers the air temperature, which happens quickly, and it removes water vapour, which happens only while the evaporator coil is running cold and wet — and that takes a run of some length. An oversized unit hits the thermostat setpoint before it has been running long enough to do the second job, so it shuts off with the room cold and the humidity essentially where it started. Cold, damp air feels clammy, and it feels worse at 72 °F and 65% relative humidity than dry air does at 75 °F. People respond by dropping the thermostat further, which makes the cycling worse.
The equipment pays for it too. Nearly all of the mechanical and electrical stress in a compressor lives in the starts, not in the running, and a unit that satisfies the thermostat in four minutes is starting several times an hour all afternoon. Short-cycling is a wear mechanism, and it is one that arrives dressed as extra capacity.
Undersizing fails far more gracefully. A unit a little too small runs long cycles, which is exactly the condition in which it dehumidifies best, and it falls behind only on the handful of worst afternoons in a year. Given the two errors, aim at the number and take the nearest available size — and when the choice is between the size below and the size above, the size below is usually the better room to be in.
A 400 sq ft sunroom in a cold climate, line by line
Four hundred square feet, a 10 ft ceiling, IECC zone 5, poor insulation in an older wall, a wall of south-facing glass, full afternoon sun, and four people in it. Cooling starts at 400 × 20 = 8,000 BTU/hr and heating at 400 × 40 = 16,000 BTU/hr, both against the reference 8 ft ceiling.
The ceiling comes first: 10 ÷ 8 = 1.25, adding 2,000 to cooling and 4,000 to heating, so 10,000 and 20,000. The climate zone is next, and it splits sharply — cold takes 5% off cooling (−500, so 9,500) and adds 25% to heating, because zone 5 is sized at 50 rather than 40 BTU/hr per square foot (+5,000, so 25,000). Poor insulation then adds 25% to both: +2,375 and +6,250, giving 11,875 and 31,250. The glass adds 15% to both: +1,781 and +4,688, giving 13,656 and 35,938.
The last three lines are cooling only. Full sun adds 10% (+1,366, so 15,022), and the two occupants past the first two add 600 each (+1,200). It is not a kitchen, so nothing there. The room needs 16,222 BTU/hr of cooling — 1.35 tons, or 4.75 kW of heat moved — and 35,938 BTU/hr of heating, or 10.53 kW.
Read the table rather than the total. Of the 8,222 BTU/hr of cooling added to the base, the ceiling and the insulation account for more than half between them. On the heating side the four lines run +4,000 for the ceiling, +5,000 for the zone, +6,250 for the insulation and +4,688 for the glass — the insulation penalty is the largest of them, and largely because it is applied after the zone has already inflated the running total. Reorder the factors and the lines move around; the total does not. Either way the two biggest levers you can actually pull are insulating the walls and doing something about the glass. Neither is a dropdown, and both are cheaper than the next size of equipment.
A rule of thumb, and where Manual J takes over
The industry standard for sizing residential equipment is ACCA Manual J, an ANSI-recognised procedure that calculates a load from the building rather than from its floor plan. It works through the U-value of every wall, window, door, floor, and ceiling assembly; the orientation, shading, and solar heat gain coefficient of each glazed opening; air infiltration; duct gains and losses through unconditioned attics and crawl spaces; internal gains from lighting, appliances, and people; and the local 1% cooling and 99% heating design temperatures for your specific location. It reports the sensible and latent loads separately, which is what actually determines whether a given unit will dehumidify the space.
Two rooms with identical floor areas in the same town can differ by a factor of two once all of that is counted, and no page that asks eight questions can find that difference. What this calculator is good for is knowing roughly what to expect: whether a quote is in the right neighbourhood, whether the window unit on the shelf is plausibly the right one, and which of your building’s weaknesses is doing the most damage. Before anyone installs equipment, ask for the Manual J.
Measuring the room, and running the unit
- Square footage calculator — for the number this page starts from. It adds several rooms together and keeps them as a list you can edit, which is what you want for an open-plan space or a room with an alcove: the area a unit has to condition is every part of the space the air moves through, not the rectangle you measured first.
- Electricity cost calculator — for what it costs once it is installed. Feed it the appliance’s rated input in watts, from the label or the spec sheet, not the kilowatt figure on this page: capacity is heat moved, and an air conditioner moves several times more heat than the power it draws.
What this calculator assumes
- It is a rule-of-thumb sizing estimate, not a load calculation. ACCA Manual J is the standard, it works from the building envelope rather than the floor area, and a contractor should run one before anyone buys equipment.
- The base rates — 20 BTU/hr per square foot for cooling, 30 to 60 for heating — assume an 8 ft ceiling and ordinary residential construction. Ceiling height is scaled proportionally from there; nothing else about the construction is modelled directly.
- Sun exposure, occupancy, and kitchen appliances are applied to cooling only. They are real gains, but heating is sized for a winter night with the room empty and the oven cold, so crediting them would undersize the heat.
- The insulation, glazing, and sun percentages are conventional trade allowances rather than measured values. The cooling side of the climate adjustment is the softest number here — it is a design-temperature nudge of at most 10%, and the case for it is weaker than for anything else on the page.
- Kilowatts are thermal capacity, not electrical draw. Only electric resistance heating consumes its full rating; a refrigeration cycle or a heat pump moves several times more heat than the power it uses.
- Nothing here models duct losses, infiltration or air changes, the sensible-versus-latent split, altitude, or a heat pump’s falling output as the outdoor temperature drops. Each belongs to Manual J, and adding a dropdown for one would make the answer look more authoritative than it is.
- No result is rounded up to the next catalogue size. Given how oversizing fails, that rounding is a decision to make deliberately rather than one to inherit from a calculator.
BTU calculator FAQ
What size air conditioner do I need for a 300 square foot room?
About 6,000 BTU/hr, which is half a ton and a very common window-unit size. That is the base rule — 20 BTU/hr for each square foot of living space — applied to an ordinary room with an 8 ft ceiling, average insulation, and two people in it. The adjustments move it from there: a 12 ft ceiling takes it to 9,000, poor insulation to 7,500, a kitchen adds a flat 4,000, and a heavily shaded north-facing room comes back down to 5,400. Change one answer at a time in the calculator and watch the contribution table; the point of showing each line is that you can see which of your answers is actually driving the number.
Is it better to buy an air conditioner that is too big or too small?
Too small, by a wide margin — and this is the single most counterintuitive thing about sizing. An oversized air conditioner does not simply cool faster and then idle. It drives the air temperature to the setpoint quickly, satisfies the thermostat, and shuts off long before the coil has run cold and wet for long enough to condense much moisture out of the room. You get air that is cold and clammy at the same time, which feels worse than air that is a degree warmer and dry, and it is why people describe an oversized system as making a room feel like a cellar. The compressor pays too: starting is where the mechanical and electrical stress lives, and a unit that short-cycles all afternoon does far more of it. An undersized unit fails gently by comparison — it runs longer, dehumidifies well while it does, and only falls behind on the handful of worst afternoons a year.
How many BTU are in a ton of air conditioning, and why is it called a ton?
12,000 BTU per hour, and the name is a fossil from the ice trade. Before mechanical refrigeration, cooling capacity was quoted against what a ton of ice could do: melting 2,000 pounds of ice absorbs roughly 286,000 BTU, and spread over 24 hours that is about 11,940 BTU per hour, rounded to 12,000. Central air conditioning is still catalogued in tons and half-tons — 1.5, 2, 2.5, 3 — while window and portable units are sold in raw BTU per hour. Dividing by 12,000 is the whole conversion, which is why this page shows both.
Why does my number differ from what the HVAC contractor quoted?
Because a contractor should be running ACCA Manual J, and this page is running a rule of thumb. Manual J works from the actual building: the U-value of each wall, window, door, floor, and ceiling assembly, the orientation and shading of every glazed opening, measured or estimated air infiltration, duct gains and losses through unconditioned space, internal gains hour by hour, and the local 1% and 99% design temperatures for your postcode. It also separates the sensible load from the latent load, which is what determines whether a unit will actually dehumidify. Two rooms of identical floor area in the same town can differ by a factor of two once all of that is counted. Use this page to know whether a quote is in the right postcode; use Manual J to buy equipment.
Does ceiling height really change the answer that much?
Yes, because equipment conditions a volume and the rule of thumb is written against a floor area. The 20 BTU per square foot figure quietly assumes an 8 ft ceiling, so a room with a 12 ft ceiling holds half again as much air and takes half again as much capacity — 9,000 BTU/hr instead of 6,000 for the same 300 square feet. Vaulted and cathedral ceilings make it worse than the arithmetic suggests, because the warm air that collects at the ridge is doing nothing useful for anyone standing on the floor. If your ceiling is anywhere near 8 ft, leave the field alone; if it is not, this is one of the two or three inputs on the page that genuinely moves the answer.
How many BTU do I need to heat a room?
Between 30 and 60 BTU/hr per square foot, depending almost entirely on where you live. This page uses 30 for a hot zone (IECC 1–2), 40 for mixed (3–4), 50 for cold (5–6), and 60 for very cold (7–8) — so the same 300 square foot room needs 9,000 BTU/hr in Florida and 18,000 in interior Alaska before anything else is considered. Nothing else on the page comes close to that spread, which is why climate zone is the first question asked. Insulation and glazing then modify it in both directions, and body heat, cooking, and sunshine are deliberately not credited: heating is sized for a winter night with the room empty and the oven cold.
Do the kilowatts on this page tell me what it costs to run?
No, and reading them that way is a common and expensive mistake. The kilowatt figures here are heat moved, not electricity drawn. A 6,000 BTU/hr air conditioner shifts 1.76 kW of heat while consuming perhaps 550 W from the wall, because a refrigeration cycle pumps heat rather than creating it — the ratio between the two is what EER and SEER2 measure. The same is true of a heat pump, whose output can be three or four times its input. The one case where the numbers coincide is electric resistance heating: a 1.5 kW panel heater draws 1.5 kW. For a real running cost, take the appliance's rated input in watts and put it through the electricity cost calculator.
Sources and review notes
- ACCA Manual J, Residential Load Calculation — the ANSI-recognised standard this page is explicitly not, and the procedure a contractor should run from the building envelope, infiltration, duct losses, and local design temperatures before sizing equipment
- ENERGY STAR — the room air conditioner sizing guidance behind four of the numbers used here: 20 BTU/hr per square foot, ±10% for heavy shade or full sun, 600 BTU/hr for each occupant past two, and 4,000 BTU/hr for a kitchen
The unit conversions on this page are exact — 12,000 BTU/hr to the ton, 0.29307107 W to the BTU/hr — and cannot go stale. Everything else is a convention: the per-square-foot base rates, the climate bands, and the percentage allowances for insulation, glazing, and sun are the figures the trade quotes, not measured properties of your building. Treat the answer as a sanity check on a quote, and ask the contractor for the Manual J.