HVAC
HVAC Filter Size Calculator
Check filter face velocity, which decides whether a filter chokes the system or loads at a sensible rate.
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.
Get a Free Business AuditWhat moves the number
- Filter area against airflow, which sets the face velocity.
- MERV rating, since higher filtration at the same area means more resistance.
- How quickly the filter loads, which is a function of area as much as of air quality.
Where this one goes wrong
Solving a filtration complaint by raising MERV in the same filter slot. On a system with little spare static pressure it reduces airflow, and reduced airflow causes the coil problems the customer will call about next.
Questions
About this calculator
How do I get better filtration without losing airflow?
More area at the same rating. A larger or deeper filter presents more surface, so the same MERV produces less resistance and loads more slowly.
What face velocity should I aim for?
Lower than most one inch filters in a return grille actually see. Lower face velocity means less pressure drop and longer filter life, which is why deeper media filters perform better.
How often should a filter be changed?
When pressure drop says so rather than on the calendar. A pressure measurement across the filter is the only honest answer, and it varies enormously with the building and the season.
The formula
face velocity = CFM / filter area in square feet
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How to use this calculator
- System air flow (CFM). Enter your own figure; the page starts at 1,200.
- Filter width (in) (in). Enter your own figure; the page starts at 20 in.
- Filter height (in) (in). Enter your own figure; the page starts at 25 in.
- Number of filters. Enter your own figure; the page starts at 1.
- Filter rating. Enter your own figure; the page starts at MERV 11, better.
The result updates as you type, so you can try a figure, see what it does to face velocity, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| System air flow (CFM) | 1,200 |
| Filter width (in) | 20 in |
| Filter height (in) | 25 in |
| Number of filters | 1 |
| Filter rating | MERV 11, better |
What comes out
| Result | Value |
|---|---|
| Face velocity main result | 346 |
| Filter area | 3.5 |
| Area for 300 FPM | 4.0 |
| Typical change interval | 3.2 |
Face velocity is 346 FPM. That is comfortable. Expect to change it about every 3.2 months.
How sensitive the answer is
Holding everything else steady and changing only system air flow (cfm), this is what happens to face velocity. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| System air flow (CFM) | Face velocity |
|---|---|
| 600 | 173 |
| 900 | 259 |
| 1,200 example above | 346 |
| 1,800 | 518 |
| 2,400 | 691 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- System air flow (CFM)
- A number, between 0 and 40000, starting at 1,200.
- Filter width (in) (in)
- Measured in in, between 0 and 60, starting at 20 in.
- Filter height (in) (in)
- Measured in in, between 0 and 60, starting at 25 in.
- Number of filters
- A number, between 0 and 20, starting at 1.
- Filter rating
- Choose from MERV 8, basic, MERV 11, better, MERV 13, fine particles, MERV 16, hospital grade.
What the results tell you
| Result | What it is |
|---|---|
| Face velocity | The headline result. A number. |
| Filter area | A decimal figure. |
| Area for 300 FPM | A decimal figure. |
| Typical change interval | A decimal figure. |
Before you rely on this number
Sizing, capacity and load figures here are planning estimates. A real installation depends on the manufacturer's data for the specific equipment, the duct system it is going into, and a proper load calculation for the building. Use this to sanity-check a number or price a job, not to select equipment.
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