How many BTUs do I need per square foot?

A common rule of thumb is about 20 BTUs per square foot for cooling, with 8-ft ceilings, average insulation, and typical sun exposure. A 400 sq ft room needs about 8,000 BTUs. Adjust for ceiling height, sun, and open layouts.

What size AC do I need for a 12×12 room?

A 12×12 room is 144 sq ft. At 20 BTUs per sq ft that's about 2,880 BTUs, so a 5,000–6,000 BTU window unit covers it comfortably. Measure the actual floor area, not the wall length.

How do I calculate BTU for a room?

Multiply the floor area in square feet by a base rate (about 20 BTUs/sq ft for cooling), then adjust for ceiling height, sun exposure, and open floor plans. The calculator does exactly that.

What does 1 BTU equal?

One BTU (British Thermal Unit) is the energy needed to raise one pound of water by one degree Fahrenheit. In practical terms, it measures how much heat an AC or heater can add or remove per hour.

Is higher BTU always better for a room?

No. An oversized unit short-cycles — it cools quickly, shuts off, and leaves the room humid and uneven. Right-sizing to the room's actual load keeps it efficient and comfortable.

BTU Calculator — Room Heating & Cooling Size

Published: August 5, 2026

Enter your room's size to get the BTUs you need to heat or cool it. No sign-up, no data stored.

Quick answer: About 20 BTUs per sq ft with 8-ft ceilings. A 12×12 room (144 sq ft) needs roughly 2,900–6,000 BTUs.

Multiply the room's square footage by ~20 for a rough BTU target with 8-ft ceilings.

Room length (ft)

Room width (ft)

Ceiling height (ft)

Sun exposure

Open floor plan?

Recommended BTUs

—

Room area

—

Base BTUs

—

Adjusted

—

Sq ft / BTU

—

How BTU is calculated

The estimate uses a widely used rule of thumb with adjustments you can follow by hand:

Base = room area (sq ft) × 20 BTUs/sq ft (cooling rule of thumb).
Ceiling = multiply by a height factor (1.125 for 9 ft, 1.25 for 10 ft, 1.5 for 12 ft).
Sun = add 10% for full sun, subtract 10% for heavy shade.
Open plan = add 15% when the room opens into adjacent space.

These are sizing estimates. For a precise number, a Manual J load calculation (done by an HVAC pro) accounts for windows, insulation, and orientation. The rule of thumb gets you in the right neighborhood for picking a unit size.

Worked example: A 12×12 ft room is 144 sq ft. At 20 BTUs/sq ft that's a base of 2,880 BTUs. With an 8-ft ceiling (×1), average sun (×1), and no open plan (×1), the recommended size is about 2,900 BTUs — so a 5,000–6,000 BTU window unit is the right class.

BTU sizing chart

Quick reference for common room sizes (8-ft ceilings, average sun).

Room size	Area	Approx. BTUs	Typical unit
10×10 ft	100 sq ft	~2,000	5,000 BTU
12×12 ft	144 sq ft	~2,900	5,000–6,000 BTU
12×15 ft	180 sq ft	~3,600	6,000 BTU
15×15 ft	225 sq ft	~4,500	6,000–7,500 BTU
18×18 ft	324 sq ft	~6,500	8,000 BTU
20×20 ft	400 sq ft	~8,000	10,000 BTU

Last updated: August 2026 · BTU sizing follows common residential rule-of-thumb values. Real loads vary with insulation, windows, and climate — confirm with a Manual J load or an HVAC contractor.

BTU calculator FAQ

How many BTUs do I need for a 400 sq ft room?

About 8,000 BTUs at the 20 BTUs/sq ft rule. If you have a 12-ft cathedral ceiling or heavy sun, you'll want closer to 9,000–10,000.

What size AC for a bedroom?

A standard bedroom around 120–180 sq ft needs a 5,000–6,000 BTU window unit. Measure the floor area and account for ceiling height and afternoon sun.

Does ceiling height change the BTU I need?

Yes — you're conditioning more air volume. A 10-ft ceiling adds about 25% over an 8-ft ceiling, and a 12-ft cathedral adds about 50%.

What if my unit is too big?

An oversized AC short-cycles: it cools fast, shuts off, and leaves the room humid and damp. Right-sizing is more important than raw power.

Do I need more BTU in a sunny room?

Usually yes. Full sun can raise your need by about 10%; heavy shade can let you subtract about 10%. East- and west-facing rooms warm most.

Related calculators & guides

Square Footage Calculator

Measure the room's floor area in feet or meters.

Use the calculator →

Concrete Calculator

Concrete for the same room's slab or pad.

Use the calculator →

TV Size & Viewing Distance

Pick the right screen size for the seating distance.

Use the calculator →