Plenty of home buying decisions depend on the size of the air in a room, not just the floor. This calculator gives you the room volume, a base cooling estimate in BTU, and the purifier strength (CADR) needed to cycle the air four times an hour, plus the common smoke rule figure.
Room Volume Calculator
Measure your room volume for cooling, air purifiers, or simply to know your space better.
Assumptions: the cooling estimate uses a base of 20 BTU per sq ft of floor area. Sunny rooms, rooms over a garage, kitchens and rooms with several people in them usually need more, and shaded rooms may need less. The purifier figures are sizing guides, and four air changes per hour is a common target for allergy and dust control.
How it works
- Measure the room length, width and ceiling height, in feet or metres.
- Read the volume in cubic feet and cubic metres, and the floor area.
- Compare the BTU figure with air conditioner sizes when you shop.
- Use the two CADR figures to shortlist air purifiers that are strong enough for the room.
Frequently asked questions
How many BTU do I need for a room?
A common starting point is 20 BTU per sq ft of floor area, which this calculator uses. Add more for rooms that get strong sun, sit above a hot garage, or hold several people, and expect a professional load calculation for whole house systems.
What does CADR mean?
CADR is the clean air delivery rate, a measure of how much filtered air a purifier pushes out. Higher is stronger. For general dust and allergy use, enough CADR to cycle the room air about four times an hour is a solid target.
Why are there two purifier numbers?
The four air changes figure is based on the room volume, so it suits taller rooms fairly. The two thirds rule is a quick floor area shortcut used for smoke sizing. If the two disagree, the volume based figure is the safer choice for high ceilings.