AC BTU Calculator - Air Conditioner Sizing Guide

How to Size an Air Conditioner in BTUs

This calculator starts from the common HVAC rule of thumb of about 20 BTU of cooling capacity per square foot of room area (assuming an 8 ft ceiling), then adjusts for a taller ceiling, how much direct sun the room gets, and how many people typically occupy the space.

Sunny rooms use a 1.1× multiplier and shaded rooms use 0.9× relative to an average room. Ceilings taller than 8 ft add 5% capacity per extra foot. Each occupant beyond the first two adds roughly 600 BTU, since each additional person in a room adds body heat the system has to remove.

Example

A 300 sq ft room (15 ft × 20 ft) with a standard 8 ft ceiling, average sun exposure, and 2 typical occupants needs about 300 × 20 = 6,000 BTU of cooling capacity — close to a common 6,000-8,000 BTU window unit size.

Common Use Cases

FAQs

What's the 20 BTU per square foot rule based on?

It's a widely used general guideline for a typical living space with a standard 8 ft ceiling and normal insulation. It's a solid starting point for most rooms, but rooms with poor insulation, lots of windows, or unusual layouts may need a more detailed load calculation from an HVAC professional.

Why does sun exposure change the recommendation?

A room with large west- or south-facing windows and lots of direct sunlight absorbs significantly more heat during the day than a shaded room, so it needs extra cooling capacity to keep up. Shaded rooms need somewhat less.

Should I round the result up or down?

Round up to the nearest common AC unit size (5,000, 6,000, 8,000, 10,000, 12,000, 14,000 BTU, and so on) rather than down — an undersized unit will run constantly without effectively cooling the room, while a modestly oversized one just cycles off sooner.