HVAC
HVAC Filter Size Calculator
Check whether a filter is big enough for the air moving through it, and how often it needs changing.
Result
Change intervals depend far more on the house than on the filter: pets, building work, and running the fan continuously all shorten them dramatically. Treat the interval as a starting point and inspect rather than trust it.
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About this calculator
Is a higher MERV always better?
Not on an undersized filter. A denser filter on too small an area chokes air flow, which hurts capacity and can freeze the coil. Increase the filter area first, then the rating.
What face velocity should I aim for?
Around 300 feet per minute or lower for a pleated filter. Many systems run at double that, which is why filters load so fast and static pressure is so high.
Does a dirty filter really matter?
Yes. It raises static pressure, cuts air flow and in cooling can drop the coil below freezing. A blocked filter is one of the most common causes of a no-cooling call.
The formula
face velocity = CFM / filter area in square feet
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