AC BTU Calculator

Enter your room's square footage, ceiling height, climate zone, sun exposure, floor level and room type to find the exact air conditioner size you need. Get an instant result in BTU/h and tons, matched to the nearest standard AC capacity (9,000-60,000 BTU).

Use metric unitsSwitch inputs to square meters (m²) and meters (m)

Enter length × width insteadDon't know the square footage? Type the room's two side lengths and we'll multiply them for you

Room area

sq ft

Ceiling height

Pick the air conditioner capacity you have (or are considering), and we'll estimate how many square feet it can cool under your conditions:

AC capacity

Ceiling height

⚙️ Advanced settings — climate zone, sun, floor, room type, insulation, windows, occupants, devices

US climate zone

Sun exposure

Floor level

Room type

Insulation quality

Window / glass size

Number of occupants

Heavy electronicsTV, computer, gaming console, etc. add extra heat

Quick answer A common rule of thumb is roughly 20 BTU per square foot of room area, adjusted for climate zone, ceiling height, sun exposure, floor level, and room type. As a rough guide: 350-450 sq ft → 9,000 BTU, 450-600 sq ft → 12,000 BTU (1 ton), 600-900 sq ft → 18,000 BTU (1.5 tons), 900-1,200 sq ft → 24,000 BTU (2 tons). Use the calculator above for your exact conditions.

9,000 BTU350-450 sq ft

12,000 BTU450-600 sq ft (1 ton)

18,000 BTU600-900 sq ft (1.5 tons)

24,000 BTU900-1,200 sq ft (2 tons)

⚙️ Accuracy note: This calculator applies commonly used HVAC rule-of-thumb coefficients (roughly 14-25 BTU per square foot depending on climate zone, plus adjustments for sun, floor level, room type, insulation, windows, and occupants). It is not a substitute for a licensed HVAC contractor's Manual J load calculation, which accounts for local design temperatures, duct losses, and whole-house infiltration for precise, code-compliant central air sizing.

❄ AC BTU Calculator — ToolPico ↗

What is AC BTU sizing, and how do you calculate it?

A complete guide — with the formula, climate-zone coefficients, and every adjustment factor — to finding the right air conditioner size from a room's square footage.

AC BTU sizing is the process of finding the cooling capacity (BTU/h) an air conditioner needs to cool a room quickly and efficiently. BTU (British Thermal Unit) measures how much heat an air conditioner can remove per hour — the higher the number, the more powerful the unit. Choosing the right BTU rating matters for both comfort and your electric bill: an undersized unit never quite keeps up, and an oversized one wastes energy and struggles to control humidity.

How many BTUs per square foot do you need?

Quick answerFor a room with an 8-foot ceiling and medium sun exposure: 350-450 sq ft → 9,000 BTU, 450-600 sq ft → 12,000 BTU (1 ton), 600-900 sq ft → 18,000 BTU (1.5 tons), 900-1,200 sq ft → 24,000 BTU (2 tons). These ranges are a baseline; your climate zone, sun exposure, floor level, and room type shift the real number — see the full reference chart below (up to 60,000 BTU).

How do you calculate AC BTU? (formula and factors)

Quick answerBasic formula: BTU/h = Area (sq ft) × climate-zone coefficient × (ceiling height in feet ÷ 8). This figure is then adjusted by sun, floor, room type, insulation, and window multipliers, and topped up with fixed additions for occupants and electronics.

This calculator's engine applies these factors in order:

Climate-zone coefficient — Hot-Dry desert and Hot-Humid Gulf Coast zones use more BTU per square foot; Marine and Very Cold/Mountain zones use less.
Sun exposure — a room with south/west windows in sun most of the day needs about 12% more BTU; a shaded, north-facing room needs about 8% less.
Floor level — a top-floor room or one directly under the roof needs about 15% more BTU because of heat radiating down; a basement or semi-below-grade room needs about 10% less.
Room type — kitchens get a flat +1,000 BTU plus a 20% multiplier for cooking heat; home offices get +300 BTU and an 8% multiplier for equipment heat.
Insulation quality — good insulation (new construction, double-pane windows) cuts the requirement by about 10%; poor insulation (older home, single-pane windows) adds about 15%.
Window / glass size — large windows or sliding glass doors add about 10% more BTU for extra solar heat gain.
Occupants — each person radiates body heat; every occupant beyond two adds +600 BTU.
Electronics load — rooms with heavy TV, computer, or gaming-console use add a flat +1,000 BTU.

How does ceiling height and room volume affect AC size?

Quick answerStandard US ceiling height is 8 feet. Taller ceilings increase the room's volume (cubic feet) of air that must be cooled, so the calculation scales automatically by (height ÷ 8). A room with a 10-foot ceiling needs about 25% more BTU than the same floor area with an 8-foot ceiling.

What happens if AC capacity is too small or too big?

Quick answerAn undersized AC runs at full power nonstop and still can't reach the target temperature, wasting energy and wearing out the compressor. An oversized AC cools the room too fast and shuts off before it can properly remove humidity (short-cycling), leaving the room feeling cold but clammy. The most efficient choice is always the standard capacity closest to your calculated BTU/h figure.

BTU to tons conversion

Quick answer1 ton = 12,000 BTU/h. This is the standard unit US HVAC contractors use for central air and mini-split capacity — for example, 18,000 BTU/h = 1.5 tons and 36,000 BTU/h = 3 tons. This tool converts your BTU/h result to tons automatically.

How many BTUs does an X sq ft room need? (examples)

Reference values assuming Mixed-Humid climate zone, medium sun, ground floor, living room, average insulation, and an 8-foot ceiling. Use the calculator above for your own conditions.

Room area → required BTU/h (reference scenario)
Room area	Calculated BTU/h	Standard capacity	Tons

Popular calculations

Related calculators: operating cost & multi-unit spaces

Two practical calculations that round out AC sizing: an estimated monthly operating cost, and how many units a large open space needs.

⚡Operating cost estimator

Estimated monthly electricity cost from AC capacity, daily run hours, and your own price per kWh.

AC capacity

Hours/day

Price/kWh

Efficiency rating

🏢How many units do I need?

For large, open spaces (retail, office, workshop) — approximate number of units from total square footage.

Total area

sq ft

Climate zone

Single-unit capacity

Multi-room / whole-house BTU planner

Cooling more than one room? Add each room's area and climate zone below — the planner sums every room into a total BTU/h and tonnage requirement, useful for sizing a central air system or planning several mini-split heads.

Which type of AC system should you buy? (window, portable, mini-split, central)

Once you know your required BTU/h, the next decision is which kind of air conditioner to install. Calculate your BTU first — the recommendation below updates to match your result.

Recommended for you Calculate your BTU/h above to see which system type best fits your room and capacity.

AC system types — typical capacity range, best use case, and install complexity
System type	Typical BTU range	Best for	Install

Reference tables

Citable standard capacity chart, US climate-zone coefficients, and every adjustment multiplier.

Standard AC capacities — BTU/h, tons, and approximate coverage
BTU/h	Tons	Approx. sq ft

Approximate coverage assumes an 8-foot ceiling, medium sun exposure, and average insulation.

US climate-zone coefficients (BTU / sq ft, reference)
Climate zone	Coefficient

Hot and humid zones have a higher coefficient; mild and cold zones have a lower one. Local microclimates can shift these further.

Adjustment multipliers
Category	Option	Multiplier
Sun exposure	Low / Medium / High	×0.92 / ×1.00 / ×1.12
Floor level	Basement / Ground-mid / Top floor	×0.90 / ×1.00 / ×1.15
Room type	Bedroom / Living / Office / Kitchen	×1.00 / ×1.05 / ×1.08 / ×1.20
Insulation	Good / Average / Poor	×0.90 / ×1.00 / ×1.15
Windows	Small / Medium / Large	×0.97 / ×1.00 / ×1.10
Occupants	Each person beyond 2	+600 BTU
Electronics	Heavy electronics	+1,000 BTU

AC sizing terms glossary

Short definitions of the key terms you'll run into when choosing an air conditioner.

BTUBritish Thermal Unit — the amount of heat an air conditioner can remove per hour; the standard measure of AC cooling power.

Ton (AC)Standard US unit of cooling capacity; 1 ton = 12,000 BTU/h. Central air and mini-split systems are usually sold and quoted by tonnage.

EEREnergy Efficiency Ratio — cooling output divided by power draw; a higher EER means lower electricity use for the same capacity.

SEER / SEER2Seasonal Energy Efficiency Ratio — a system's average efficiency over a full cooling season; more comprehensive than a single EER figure.

Inverter / variable-speed ACA compressor that adjusts its speed to match demand, using less energy than a fixed-speed (single-stage) unit of the same capacity.

Ductless mini-splitA split system with an indoor unit (room) and outdoor condenser, without ductwork — one of the most common ways to add AC to a single room.

Room volume (cubic feet)Floor area × ceiling height; the underlying physical quantity that determines how much BTU capacity a room needs.

Short-cyclingAn oversized AC cooling a room quickly, then shutting off and restarting repeatedly — it hurts humidity control and lowers efficiency.

Manual J load calculationThe ACCA industry-standard method HVAC contractors use for precise, code-compliant sizing of whole-house central air systems.

Climate-zone coefficientA baseline BTU-per-square-foot value set by a region's average heat and humidity load, used to scale the calculation to your location.

Window AC unitA single self-contained unit that mounts in a window frame; lowest cost, DIY install, best for a single room up to about 12,000 BTU.

Portable AC unitA floor-standing unit on wheels that vents hot air through a window hose; movable between rooms but generally less efficient than a window unit of the same BTU rating.

Explore all ToolPico tools →

In-depth guides

Step-by-step answers to the most common AC sizing questions.

How many BTUs of AC should I buy? Quick practical guide

A standard bedroom (120-180 sq ft) usually needs about 9,000 BTU; a standard living room (250-400 sq ft) usually needs 12,000-18,000 BTU. If you're on a top floor, the room gets sun most of the day, or it's a kitchen/open-kitchen space, sizing up one step (say 18,000 instead of 12,000 BTU) is usually the more comfortable choice.

For an exact figure, enter your room's real square footage, ceiling height, and climate zone into the calculator above and press Calculate — the result is matched directly to the nearest standard capacity.

AC is cooling but the room still feels humid? The oversizing problem

If you installed an air conditioner that's too big for the room, it cools the space to the target temperature and shuts off before it has run long enough to pull much moisture out of the air. This short-cycling leaves a room that reads "cold" on the thermostat but still feels clammy or damp.

The fix is either to run the unit longer at a lower fan speed, or — on your next purchase — pick the standard capacity closest to your calculated BTU figure rather than the next size up. Use the BTU → Sq Ft tab above to check exactly how large a room your current unit is really sized for.

Does an inverter AC change the BTU calculation?

No — the BTU capacity you need is independent of whether the unit is an inverter (variable-speed) model; that's purely a function of the room's physical heat load. Inverter technology doesn't change the required BTU; it changes how efficiently the unit delivers that capacity (using less electricity, running at partial power instead of cycling on/off).

So find the right BTU size with the calculator above first, then decide between an inverter or single-stage unit at that capacity based on your budget and how much you value energy efficiency. Inverter models typically carry a higher EER/SEER rating — you can see that difference reflected in the "Operating cost estimator" mini-tool above.

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Frequently asked questions

How many BTUs per square foot do I need for an air conditioner?

A common rule of thumb is roughly 20 BTU per square foot of room area, though this varies by climate zone, ceiling height, sun exposure, and room type. As a general reference: 350-450 sq ft needs about 9,000 BTU, 450-600 sq ft needs about 12,000 BTU (1 ton), 600-900 sq ft needs about 18,000 BTU (1.5 tons), and 900-1,200 sq ft needs about 24,000 BTU (2 tons). These are baseline figures for an 8-foot ceiling and average sun exposure; your exact need depends on climate, ceiling height, floor level, and room type.

How do you calculate AC BTU / cooling capacity?

Basic formula: BTU/h = Room area (sq ft) × climate-zone coefficient × (ceiling height in feet ÷ 8). This baseline is adjusted by multipliers for sun exposure, floor level, room type, insulation quality, and window size, then increased by fixed additions for extra occupants (+600 BTU per person beyond two) and heavy electronics or a kitchen heat load.

How many square feet does a 12,000 BTU (1 ton) air conditioner cool?

A 12,000 BTU (1 ton) air conditioner typically cools about 450-600 square feet under standard conditions (average climate zone, 8-foot ceiling, medium sun exposure, average insulation). A top-floor room, one with heavy sun exposure, or poor insulation needs a smaller area for the same 12,000 BTU unit; a shaded, well-insulated room in a mild climate zone can handle a somewhat larger space.

How do I choose the right AC size (BTU rating)?

Start with room square footage and ceiling height, then factor in your climate zone, sun exposure, floor level, room type (a kitchen needs more capacity because of cooking heat), insulation quality, window size, and number of occupants. Round the calculated BTU/h figure up — never down — to the nearest standard AC capacity (9,000 / 12,000 / 18,000 / 24,000 BTU and up).

How does ceiling height affect air conditioner sizing?

Standard ceiling height in the US is assumed to be 8 feet. Taller ceilings increase the room's volume (cubic feet) of air that needs to be cooled, so the required BTU scales up proportionally with height divided by 8. A room with a 10-foot ceiling needs about 25% more BTU than the same floor area with an 8-foot ceiling.

How do you convert between BTU and tons?

1 ton = 12,000 BTU/h. So an 18,000 BTU unit is 1.5 tons, a 24,000 BTU unit is 2 tons, and a 36,000 BTU unit is 3 tons. Ton ratings are the standard way US HVAC contractors describe central air and mini-split capacity.

What happens if an air conditioner is undersized or oversized?

An undersized AC runs continuously at full power and still can't reach the target temperature on hot days, wasting energy and wearing out the compressor faster. An oversized AC cools the room too quickly and shuts off before it can properly remove humidity — a problem called short-cycling — leaving the room feeling cold but clammy. Choosing the standard capacity closest to your calculated BTU figure gives the best comfort and efficiency.

How do you increase AC size for a sun-facing room or top-floor room?

Rooms on the top floor or directly under a roof need roughly 15% more BTU capacity because of heat radiating down from above — select "Top floor / attic" in the floor level setting. Rooms with large south- or west-facing windows that get sun most of the day need roughly 12% more BTU — select "High" sun exposure. If both apply, rounding up to the next standard capacity size is a reasonable safety margin.

Should I get a window unit, portable AC, mini-split, or central air?

It depends mainly on your calculated BTU/h and how many rooms you need to cool. A window unit is the lowest-cost option for a single room up to about 12,000 BTU. A portable unit suits renters who can't mount anything in a window frame permanently. A ductless mini-split is the most efficient choice for 1-4 rooms without existing ductwork, generally 9,000-36,000 BTU. Central air is a whole-house, ducted system, typically sized 18,000 BTU and up. Use the calculator above to get your exact BTU/h figure, then check the system-type comparison table for the best fit.

Methodology & sources

ToolPico's AC BTU Calculator is a free, independent air-conditioner sizing tool. The calculation engine applies commonly used HVAC rule-of-thumb coefficients: a base BTU-per-square-foot value by US climate zone, a room-volume scale factor for ceiling height, multipliers for sun exposure, floor level, room type, insulation, and window size, and fixed additions for occupants and electronics.

Basis: General HVAC rule-of-thumb sizing methods (of the kind commonly published by AC manufacturers and retailers), cross-checked against publicly available square-footage sizing charts. This is a quick estimate, not a substitute for a contractor's Manual J load calculation. Last updated: July 21, 2026. Results are for informational purposes only.

What is AC BTU sizing, and how do you calculate it?

How many BTUs per square foot do you need?

How do you calculate AC BTU? (formula and factors)

How does ceiling height and room volume affect AC size?

What happens if AC capacity is too small or too big?

BTU to tons conversion

How many BTUs does an X sq ft room need? (examples)

Popular calculations

Related calculators: operating cost & multi-unit spaces

Multi-room / whole-house BTU planner

Which type of AC system should you buy? (window, portable, mini-split, central)

Reference tables

AC sizing terms glossary

In-depth guides

Add this calculator to your site (embed code)

Frequently asked questions

Methodology & sources

🔗 Add this tool to your site