Why use this calculator
Air conditioner sizing is the one decision that cannot be corrected afterwards, and being generous is not the safe option people assume. An undersized unit runs continuously, never reaches the set point on the hottest days and wears out early. An oversized unit is worse in a way that surprises most buyers: it cools the air quickly, satisfies the thermostat and shuts down before it has run long enough to pull moisture out of the room, leaving a space that is cold and clammy, with compressor short cycling that shortens equipment life and wastes electricity.
This calculator starts from the published capacity table rather than a single rule of thumb, then applies the documented adjustments for sun, occupancy and cooking, so the answer lands on a real purchasable size.
How to use it
- Measure the room's length and width in feet at floor level and enter the longer dimension as the length, treating an open plan space that the unit must cool as one room rather than several.
- Enter the ceiling height in feet, because the published table assumes eight feet and a raked or three metre ceiling adds volume the table does not know about.
- Choose the insulation level by considering the walls, roof and glazing together rather than the roof insulation alone, since single glazed windows will dominate the load in most older houses.
- Set the sun exposure by how much direct afternoon sun lands on the room's glass.
- Enter the normal occupancy, counting yourself, and select kitchen if the space contains a cooktop or oven.
What each result means
- Cooling capacity required
- The calculated hourly heat removal the room needs at design conditions, before rounding to a size you can buy.
- Nearest unit size to buy
- The next standard capacity at or above the requirement. Rounding down leaves a unit that runs flat out on hot days.
- Capacity in tons
- The same figure in tons of refrigeration, the unit used on North American split and ducted systems.
- Capacity in kilowatts
- The same capacity in kilowatts, which is how units are rated and advertised outside North America.
- Floor area
- The plan area the sizing is based on, given in both square feet and square metres for cross checking.
- Room volume
- The air volume the unit must condition, which is what a high ceiling changes even though the floor area does not move.
- Load per square foot
- The resulting intensity of the load. Twenty to thirty BTU per square foot is typical; far outside that range suggests an input error.
The formula
Capacity starts from the published table, which maps floor area A in square feet to a base capacity B in BTU per hour, stepping from 5,000 for rooms up to 150 square feet to 34,000 for rooms up to 2,500.
The adjusted load
Q = B × (H ÷ 8) × Fi × Fs + 600 × (P − 2) + K
Tons are Q ÷ 12,000 and kilowatts are Q × 0.000293071.
How this number is used in practice
Oversizing is the most common installation fault in residential cooling, and it is usually done deliberately by someone trying to be helpful. A unit with twice the required capacity reaches the thermostat set point in a few minutes, shuts off, and repeats all afternoon. Because dehumidification happens across the evaporator coil during sustained running, those short bursts remove far less moisture than a correctly sized unit running longer and steadier, which is why an oversized system produces a room that feels cold and damp at the same time. Short cycling also multiplies compressor starts, and compressor starts, not running hours, are what kill compressors.
A rule of thumb is a starting point and a proper load calculation is the real answer. Residential load calculation standards published by contractor associations work room by room from wall areas, glazing type and orientation, ceiling and floor construction, air leakage, occupancy and appliance load, and produce a figure that frequently differs from a square footage rule by twenty per cent or more in either direction.
Any contractor quoting a whole house system from floor area alone has skipped the step that determines whether the equipment will behave. For a single room unit, a table based estimate like this one is generally close enough to choose between adjacent sizes.
Units are sold in fixed capacity steps, which is why the recommended purchase size matters more than the calculated load. Between the calculated requirement and the next available size there is always spare capacity, and that is fine. What is not fine is jumping two sizes because the shop had a sale.
Where the calculation lands very close to a boundary, look at the room's worst case: a west facing wall of glass in a hot climate justifies rounding up, while a shaded south facing bedroom in a temperate one does not.
Worked examples
15 by 12 ft bedroom, 8 ft ceiling, average insulation, two people
6,000 BTU/hr required, which is exactly a 6,000 BTU unit, 0.50 tons or 1.76 kW.
20 by 25 ft living room, 10 ft ceiling, poor insulation, very sunny, four people
22,650 BTU/hr required, so a 24,000 BTU unit, 1.89 tons or 6.64 kW.
12 by 14 ft kitchen, 8 ft ceiling, good insulation, two people
9,100 BTU/hr required once the 4,000 BTU cooking load is added, so a 10,000 BTU unit.
When people use it
- Choosing between two adjacent air conditioner capacities for a single room
- Checking whether a contractor's proposed unit size is plausible before accepting a quote
- Converting a capacity quoted in kilowatts into BTU per hour or tons and back
- Understanding why an existing unit cools quickly but leaves the room feeling damp
- Estimating the extra capacity a raked ceiling or a wall of west facing glass demands
Tips
- Never round up two sizes. An oversized unit short cycles, removes less humidity and leaves the room cold and clammy.
- Add roughly 600 BTU per hour for each person beyond two, and 4,000 BTU for a kitchen with a cooktop, as the published adjustments specify.
- Ceiling height matters and floor area alone does not see it. A ten foot ceiling adds a quarter to the volume the unit must condition.
- Fix the envelope first. Sealing draughts, adding roof insulation and shading west facing glass usually cost less than the next unit size up.
- External shading beats internal blinds by a wide margin, because it stops the heat before it passes through the glass.
- For a whole house or a ducted system, insist on a room by room load calculation rather than a square footage rule.
- Match the unit to the room it serves, not the house. A single large unit cooling through a doorway will disappoint in both rooms.
What it assumes, and where it stops being accurate
- It is a room sizing estimate based on a published capacity table, not a full residential load calculation to a recognised standard.
- Ceiling height is scaled against an eight foot baseline, which is an accepted rule of thumb rather than part of the published table.
- It is not accurate for whole house or ducted systems, where duct losses, zoning and infiltration dominate and must be modelled properly.
- Climate is not an input. The same room needs materially more capacity in a hot humid climate than in a temperate one.
- Window area, orientation and glazing type are compressed into one three-step sun factor, which is a simplification of the dominant load in most rooms.
- Heating load is not calculated. A heat pump sized for cooling will not necessarily satisfy a cold climate heating requirement.
- Appliance and lighting loads beyond the kitchen allowance are ignored, which understates a room full of computers or servers.
Terms used on this page
- BTU
- British Thermal Unit, the energy to raise one pound of water by one degree Fahrenheit. Quoted per hour for cooling capacity.
- Ton of refrigeration
- 12,000 BTU per hour, historically the cooling produced by melting one ton of ice over a day.
- Short cycling
- Rapid on and off operation caused by oversizing, which wastes energy and shortens compressor life.
- Sensible and latent load
- The heat that changes air temperature versus the heat removed as moisture. Oversized units handle the first and neglect the second.
- Load calculation
- A room by room engineering assessment of heat gain, the proper basis for sizing a whole house system.
Questions people ask
- What is a BTU?
- A British Thermal Unit is the energy needed to raise one pound of water by one degree Fahrenheit. In air conditioning it is quoted per hour, so a 6,000 BTU unit removes six thousand BTU of heat every hour it runs. One ton of refrigeration is 12,000 BTU per hour, and one BTU per hour is about 0.293 watts, which is how the kilowatt ratings used elsewhere convert.
- Is a bigger air conditioner always better?
- No, and this is the most expensive misconception in residential cooling. An oversized unit reaches the set point quickly and shuts off before it has run long enough to dehumidify, leaving a room that is cold and clammy. The frequent starts also wear the compressor, since compressor starts rather than running hours are what shorten its life.
- How many BTU do I need per square foot?
- Twenty to thirty BTU per hour per square foot covers most rooms, with small rooms at the top of that range because the smallest unit sold is 5,000 BTU. Rather than applying a flat rate, this calculator uses the published capacity table, which is a step function, then applies the documented adjustments for ceiling height, insulation, sun, occupancy and cooking.
- How do I convert BTU to kilowatts?
- Multiply BTU per hour by 0.000293071, or divide by 3,412. A 12,000 BTU per hour unit is therefore about 3.52 kilowatts of cooling capacity, which is the figure used on Australian, New Zealand, British and European nameplates. Note that this is cooling output, not electrical input, which is several times smaller.
- Does ceiling height change the size I need?
- Yes, because the unit conditions a volume of air rather than a floor. The published capacity table assumes an eight foot ceiling, so a ten foot ceiling adds a quarter to the volume and this calculator scales the base capacity accordingly. Raked ceilings and warehouse style spaces need a proper load calculation rather than a proportional adjustment.
- Why does my room still feel humid with a new unit?
- Almost always because the unit is oversized for the room. Moisture is removed only while the compressor runs and air passes across a cold coil, so a unit that satisfies the thermostat in five minutes and stops has barely started dehumidifying. A correctly sized unit runs longer at lower intensity and leaves the room dry as well as cool.
Use this with
HVAC BTU Calculator answers one part of the question. These pick up where it stops: Insulation R-Value Calculator, Power Bill Calculator, Energy Calculator and Solar Panel Savings Calculator.
Where these figures come from
Figures and rules on this page follow United States federal rules.
- ENERGY STAR (US EPA). Published room air conditioner capacity table and sizing adjustments
- Air Conditioning Contractors of America. Manual J residential load calculation standard
- US Department of Energy. Home cooling, insulation and air sealing guidance
HVAC BTU Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.