How is the BTU figure calculated?

It starts from about 20 BTU per square foot of floor area, then adjusts ±10% for sun exposure (×1.1 sunny, ×0.9 shaded) and adds roughly 600 BTU for each occupant beyond two. The total is rounded up to the nearest 100.

What size air conditioner do I need for my room?

As a rule of thumb, multiply the floor area in square feet by 20. A 12 m² (about 129 sq ft) room needs roughly 2600 BTU/hr before adjustments — enter your room to get a tailored figure.

How do I adjust for a sunny or shaded room?

A sunny, south-facing room gains more heat, so the tool raises the estimate by 10%. A heavily shaded room lowers it by 10%. Pick the option that matches your room.

Why add BTU for extra people?

Each person gives off body heat, so the calculator adds about 600 BTU for every occupant above the baseline of two, which adds to the cooling load.

Does it account for metres as well as feet?

Yes. Enter dimensions in metres and the tool converts to feet (÷ 0.3048) before applying the 20 BTU per square foot rule.

Is this a substitute for a professional load calculation?

No. It is a quick sizing guide for a single room. Ceiling height, insulation, climate and appliances all affect real cooling load, so confirm with an installer for a permanent system.

Is anything uploaded?

No — every calculation happens in your browser and nothing is sent to a server.

What is the BTU Air Conditioner Calculator?

Free BTU calculator for air conditioners — enter room size, sun exposure and occupants to estimate the cooling capacity (BTU/hr) you need. Runs entirely in your browser, nothing is uploaded. It runs free in your browser on Gera Tools, with nothing uploaded.

BTU Air Conditioner Calculator

Name: BTU Air Conditioner Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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BTU air conditioner calculator — size your unit

A BTU calculator estimates the cooling capacity an air conditioner needs for a room. Enter the room size, pick the sun exposure, and set how many occupants the room usually has. The tool returns a recommended capacity in BTU per hour — a practical starting point for choosing a window unit, portable AC, mini-split, or through-the-wall unit.

What BTU/hr means

BTU stands for British Thermal Unit — the amount of energy needed to raise one pound of water by one degree Fahrenheit. For air conditioners, the rating is in BTU per hour, which measures how much heat the unit can remove from a room each hour. A higher BTU/hr number means a more powerful unit capable of cooling a larger space.

Common window and portable units range from about 5,000 BTU/hr (small bedroom) to 24,000 BTU/hr (large open-plan room or small apartment). Mini-split systems go higher and cover multiple zones.

How the calculator works

The base figure uses the widely used rule of 20 BTU per square foot of floor area:

base BTU = floor area (sq ft) × 20

The tool then applies adjustments:

Sun exposure factor:

Sunny room (south or west facing, significant direct sun): ×1.1 (+10%)
Normal exposure: ×1.0
Shaded room (north facing, heavy shade): ×0.9 (−10%)

Occupants: For every occupant beyond two, add 600 BTU/hr. A room used by four people during the day needs 1,200 BTU/hr more than one used by two people, because each person releases body heat that the air conditioner must remove.

The result is rounded up to the nearest 100 BTU/hr.

Worked examples

Room	Dimensions	Exposure	People	Result
Small bedroom	3 m × 3.5 m (≈ 113 sq ft)	Shaded	1	≈ 2,000 BTU/hr
Standard bedroom	4 m × 3.5 m (≈ 151 sq ft)	Normal	2	≈ 3,000 BTU/hr
Sunny living room	5 m × 4 m (≈ 215 sq ft)	Sunny	4	≈ 6,000 BTU/hr
Home office	3 m × 4 m (≈ 129 sq ft)	Normal	1	≈ 2,600 BTU/hr

Factors this quick guide does not capture

The 20 BTU/sq ft rule is a shorthand approximation. A professional Manual J load calculation for a permanent system also accounts for:

Ceiling height — rooms taller than 8 ft have more air volume to cool
Insulation quality — a poorly insulated room leaks heat faster
Window area and glazing — large single-glazed south-facing windows add significant solar heat gain beyond a simple “sunny” adjustment
Climate — a room in Phoenix, Arizona needs substantially more cooling capacity than the same room in Portland, Oregon
Appliances — a home office with multiple computers and monitors adds meaningful heat

For a permanent installation (mini-split, central AC, or through-the-wall unit) in a climate where you will rely on it heavily, confirm the sizing with a certified HVAC installer who can run a proper load calculation for your specific space and location.

This tool is ideal for sizing portable and window units where a rough estimate is sufficient, and for quickly comparing how different rooms in the same building compare in their cooling needs. All calculations run in your browser; nothing is uploaded.