thermostat

BTU Calculator | Recommend Heating or Cooling Capacity for Rooms

Quick reference

Recommended cooling BTU by room size:

Room size	BTU/hr
150–250 sq ft	5,000–6,000
250–350 sq ft	7,000–8,000
350–550 sq ft	10,000–12,000
550–1,000 sq ft	14,000–18,000

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Responsive design tested down to a 320px-wide phone screen. The input fields use native number/date keyboards on iOS and Android, the calculate button is thumb-sized, and the result still reads big and bold on the smallest screen. No app to download, ever.

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Interpretation guide

Adjustments to base BTU:

Condition	Adjust BTU
Very sunny room	+10%
Heavily shaded	-10%
Kitchen	+4,000
Each extra occupant (>2)	+600

lightbulb Worked example — sizing a bedroom air conditioner

A bedroom is 12 ft × 15 ft = 180 sq ft. Base cooling load = 180 × 20 = 3,600 BTU/h. The room gets strong afternoon sun through a west-facing window: add 10% → 3,600 × 1.10 = 3,960 BTU/h.

Result: ≈ 4,000 BTU/h of cooling.

What this means: A 5,000 BTU window unit is the nearest standard size and covers it comfortably. A 6,000 BTU unit would short-cycle in this room — cooling fast but shutting off before it dehumidifies, leaving the air clammy.

Formula & methodology

Formula: Base cooling BTU = area (sq ft) x 20; then adjust for sun, occupants, and kitchens

How cooling load is estimated

The calculator starts from the standard rule of thumb — 20 BTU per hour per square foot of floor area — then adjusts for the room's real heat sources.

Base: floor area (sq ft) × 20.
Sun: +10% for strong direct sun through large windows.
Kitchen: +4,000 BTU for cooking heat.
Occupants: +600 BTU per person beyond the first two.

12,000 BTU/h equals one ton of cooling — the unit central systems are sized in. Oversizing wastes energy and humidity control; undersizing never catches up on hot days.

Authoritative source: timeanddate.com

Frequently asked questions

How many BTUs do I need to cool a room?

The BTU Calculator is free, private, and accurate: it runs entirely in your browser (no uploads, no accounts), applies the standard calculation, and explains each step so you can verify the result. Determines the heating or cooling capacity a room needs in BTUs. Enter the room's square footage, ceiling height, sun exposure, insulation quality, and number of occupants, and the tool applies standard BTU-per-square-foot factors to recommend the right There is no limit on usage, and it works on any device.

What happens if my AC is too big?

Yes — the BTU Calculator is fully responsive and works in any modern browser on a phone, tablet, or desktop, with touch-sized inputs and no app to install. The full calculation runs on the device, so you get the same speed and precision on mobile as on a computer, even with a weak connection.

Do I add BTUs for a sunny room?

Yes — add roughly 10% for a room with significant direct sun, especially through large south- or west-facing windows, because solar gain adds real heat load. Conversely, subtract about 10% for heavily shaded rooms. The calculator applies these adjustments as factors on top of the base BTU estimate by square footage, occupancy, and ceiling height.

How many BTUs for a kitchen?

Add about 4,000 BTUs to the room's base requirement for a kitchen, because cooking appliances, especially ovens, generate substantial heat. A typical 200 sq ft kitchen that needs ~10,000 BTUs by size jumps to ~14,000 BTUs with the kitchen adjustment. Bathrooms similarly add about 1,000 BTUs for steam and moisture.

How do BTUs relate to tons of cooling?

Enter the required values into the input fields and press the calculate button. The tool applies the formula Base cooling BTU = area (sq ft) x 20; then adjust for sun, occupants, and kitchens and returns the result with a short explanation of each step, so you can check the working yourself.

Does it size heaters as well as air conditioners?

Yes — it estimates both heating and cooling loads in BTU per hour. Heating needs are usually larger than cooling for the same room in cold climates, and insulation quality matters more. The room inputs (size, sun, occupants) feed both calculations.

Why start from 20 BTU per square foot?

It is the industry's standard rule of thumb for residential cooling with 8-foot ceilings and average insulation. A 200 sq ft room starts at 4,000 BTU, then the calculator adjusts up for sun exposure, kitchens (+4,000 BTU), and extra occupants, or down for shade.

Is my data sent anywhere when I calculate?

No — the sizing math runs on the page itself, so your room details never leave your device.

How precise is the recommendation?

It is a solid estimate for standard rooms, accurate enough to choose between common AC sizes (5,000 vs 6,000 BTU). Unusual cases — vaulted ceilings, huge glass walls, server heat — need a Manual J load calculation from an HVAC professional instead.

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