The cfm calculator turns air changes per hour (ach), ceiling height, length, width into airflow rate in other units, required airflow, floor area. It suits students checking a worked example, professionals sanity-checking a figure, and anyone who wants to see how the pieces of the calculation fit together.
What Is the CFM Calculator?
It is built around airflow rate in other units, required airflow, floor area, and it works the same way whether you are checking a homework answer, confirming a colleague's figure, or working something out for yourself. Everything runs in your browser, so nothing you enter is uploaded anywhere.
How the CFM Calculation Works
The calculator works through the relationship one step at a time, so a change in any input flows straight through to the result.
The relationship between air changes per hour (ach), ceiling height, length, width and airflow rate in other units, required airflow, floor area, volume is fixed, so the same figures always give the same answer. That makes it a good cross-check: run the numbers a second way and see whether the result agrees.
Getting a Result You Can Trust
The most common mistake is mixing units. Keep air changes per hour (ach) and ceiling height and length and width in the same system throughout, and make sure any rates or ratios are quoted per the same period. When a result looks far out of range, the units are nearly always the reason.
Frequently Asked Questions
What do I need to work out airflow rate in other units?
Enter air changes per hour (ach), ceiling height, length, width. The calculator then works through the relationship and reports airflow rate in other units, required airflow, floor area.
What does the cfm actually calculate?
It takes air changes per hour (ach) and ceiling height and length and width and converts them into airflow rate in other units, required airflow, floor area using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter air changes per hour (ach)?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Why does a small change in one value move the answer so much?
That happens when an output depends on a ratio, because dividing by a small number magnifies any change in it. Check the units first, then check whether the value you entered is in the range the relationship was designed for.
Can I use this for a quick estimate as well as a precise answer?
Yes. The calculator is built for quick checks, so round your inputs to a sensible number of significant figures and the result will be just as useful. If you need more precision, keep the same method and add digits to the inputs rather than adjusting the method itself.