Learning Center · Air Quality Basics

How to Calculate Air Changes Per Hour (ACH)

Use the calculator below to find your ACH, or the CFM you'd need to hit a target. Switch modes depending on whether you're checking what you're currently getting, or sizing a unit for a room.

Delivered CFM*

CFM

Room Volume (L × W × H)

cu ft

ACH is a volume calculation, not a floor-space one — cubic footage, not square footage. Don't know your cubic footage? Work it out →

Length

Width

Height

Length × Width × Height fills in the cubic footage field above.

* Use delivered (post-filter) CFM, not the fan or pre-filter speed on the spec sheet — real airflow drops once the actual filters are seated. Full explanation ↓

Your air changes per hour

— ACH

Fill in the fields above to see the calculation.

Target ACH

ACH

Heavier contaminants call for more air changes — many people target 5+ ACH rather than the single change that's often used as a baseline.

Room Volume (L × W × H)

cu ft

Cubic footage, not square footage. Don't know your cubic footage? Work it out →

Length

Width

Height

Length × Width × Height fills in the cubic footage field above.

* The CFM figure below is the delivered (post-filter) number to look for on a spec sheet — not the fan or pre-filter speed, which reads higher. Full explanation ↓

Delivered* CFM you need

— CFM

Fill in the fields above to see the calculation.

ACH = (Delivered CFM × 60) ÷ Room Volume (cu ft) Multiply delivered CFM by 60 to get cubic feet moved per hour, then divide by cubic footage — not square footage.

The Two ACH Formulas

How to Calculate Air Changes Per Hour

ACH = (Delivered CFM × 60) ÷ Room Volume in cubic feet. Multiply your purifier's delivered CFM by 60 to get cubic feet moved per hour, then divide by the room's cubic footage.

How to Find the CFM You Need for a Target ACH

Required CFM = (Room Volume × Target ACH) ÷ 60. Multiply cubic footage by the ACH you're aiming for, then divide by 60 to get the delivered CFM your unit needs to move.

It's critical to use delivered CFM — not fan or pre-filter CFM

This is the part people tend to miss, and it's the part that throws the whole worksheet off if it is wrong. A unit's spec sheet will sometimes list a CFM number for each fan speed — but that number can be measured as the bare fan speed or pre-filter (before filter) speed. It is not always what the unit does once filters are installed and it is running in your home. With filters, we refer to this as delivered CFM or post-filter CFM. This is the true value of what the unit is capable of when it is in use.

Once the real filters are in the unit, and the unit runs with them installed, the CFM drops. This lower, real number is what's called post-filter or delivered CFM, and it's the number you should be plugging into the worksheet above. It's the only one that reflects what's actually moving through your room. This number will vary based on the filter configuration — carbon, medical-grade HEPA, or a combination of the two. Each filter is slightly different, so delivered or post-filter CFM truly is an estimate, which is exactly why some companies only quote fan speed, or pre-filter CFM.

Fan / pre-filter speed

400 CFM

on the spec sheet, filters not yet in play

→

Post-filter / delivered CFM

≈250 CFM

running with HEPA + carbon installed

Here's the example that makes it click: say a unit's fan speed or pre-filter speed is rated at 400 CFM. That's the number on the box. But once the unit runs with its actual filters in it, that CFM drops — down to something like 250–300 — because air has to work harder to push through dense HEPA and carbon media. Plug the 400 into the worksheet and you'll walk away thinking your room is getting a lot more air changes than it really is.

Since delivered CFM is an estimate due to the filter configurations and the filters themselves rather than a lab-exact figure, use this worksheet to land in the right range for your space and contaminant level — not to chase one exact number down to the decimal.

This math holds whether you're sizing a standalone unit for a single room or checking airflow on a whole-home HVAC system — the formula doesn't change, only the delivered CFM and cubic footage you plug in.