How to Calculate BTUs for Multiple Rooms

Published April 18, 2025

If you want to cool more than one room with a single air conditioning unit or central system, it's essential to calculate the total BTU requirement correctly. This guide will show you how to measure, sum, and balance your AC capacity across multiple spaces.

1. Measure Each Room

Start by measuring the length and width of each room in feet (or meters). Multiply to find the area of each room. For example, a 10 ft × 12 ft bedroom equals 120 sq ft.

2. Sum the BTU Requirements

Use the rule of thumb — 25 BTUs per square foot — and multiply each room’s square footage by 25. Add the results together for a total BTU estimate.

3. Adjust for Room Function and Sunlight

Increase BTU estimates for kitchens (by 4,000 BTU) or sun-exposed rooms (by 10–15%). Reduce for shady or low-use rooms. This fine-tunes your system to your home's real-world usage.

4. Use the Online Calculator

To save time and reduce math errors, use our free BTU & Tonnage Calculator. It lets you enter each room’s data and delivers the total cooling load instantly.

5. Choose the Right System

Once you have the total BTU number, you can decide if you need a ductless mini split, central air, or multiple window units. Proper sizing ensures comfort and avoids energy waste.