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Box Fan Filter
Quick answer: A standard 20-inch box fan fits a 20x20x1 HVAC filter, which people attach to the intake side to make a do-it-yourself room air cleaner, sometimes called a Corsi-Rosenthal box when several filters form a cube. Use a newer fan with a UL or CSA listing, watch for motor heat, and never leave it unattended. We make no health claims.
What a box fan filter is
A box fan pushes a lot of air through a large square opening. Strapping a pleated HVAC filter to the fan's intake side draws room air through the filter. The idea became popular during wildfire smoke events and was refined into a five-sided version — four filters forming walls with a fan on top — that offers much more filter area. Researchers and public-health groups have tested such designs; consult those sources for findings. This page focuses on sizing and safe assembly, not on health outcomes.
Sizing the filter
Standard 20-inch box fans have a housing roughly 20 to 22 inches square. A 20x20x1 filter — actual about 19½ × 19½ × ¾ inches — covers the intake. Some fans are larger or smaller; measure the grille face. For a cube design, four 20x20 filters form the sides, with the fan on top and a cardboard base.
| Build | Filters needed | Relative filter area |
|---|---|---|
| Single filter on fan intake | One 20x20x1 | 1× |
| Two-filter wedge | Two 20x20x1 plus cardboard | about 2× |
| Five-sided cube | Four 20x20x1 (or 2-inch) plus fan | about 4× |
More filter area generally means lower resistance and more airflow through the filters, which is why the cube design is popular.
Which side does the filter go on?
Put the filter on the intake — the back of the fan, where air is drawn in — with its airflow arrow pointing toward the fan. Placing it on the outlet side gives poor sealing and can blow unfiltered air around the edges.
Choosing the filter
Many guides suggest MERV 13 pleated filters for these builds. A 1-inch MERV 13 adds resistance, which reduces fan airflow; a cube spreads that resistance over more area. Two-inch filters lower resistance further. MERV 8 or 11 filters restrict less but capture fewer small particles. We do not claim any setup will prevent illness. See MERV explained.
Fan safety
- Use a newer box fan with a safety listing such as UL or CSA; many guides recommend models made since 2012, which have thermal protection.
- Never cover the fan's motor housing or block its vents.
- Check the motor periodically for heat; a hot smell means turn it off.
- Do not run it unattended or while sleeping unless the fan is designed and listed for that use.
- Keep cords clear of walkways and away from water.
- Keep children and pets away from the blades.
Worked example
A household builds a five-sided cube using four 20x20x1 MERV 13 filters at a hypothetical $18 each and an existing 20-inch fan. Total filter cost is $72. They plan to inspect the filters monthly and replace them when the upstream faces turn evenly grey — likely every three to six months with occasional use, or sooner after heavy smoke. Yearly filter cost could range from roughly $144 to $288 depending on use.
Illustrative note (2026-10-29): This example was prepared on 2026-10-29 for illustration. Prices and replacement intervals are hypothetical and depend heavily on use; this is not a health recommendation.
Assembling a single-filter version
- Turn the fan off and unplug it.
- Place the filter on the back of the fan with the arrow pointing at the fan.
- Secure the edges with tape or straps so air cannot bypass the filter.
- Cover the gaps at the fan corners with cardboard if the filter is smaller than the housing.
- Plug in and run on low or medium; check that the filter is pulled flat, not sucked in.
Assembling a cube
A cube uses four filters taped into a box, arrows pointing inward, with the fan on top blowing outward and a cardboard base sealing the bottom. Add a cardboard shroud around the fan corners to reduce backflow. Many detailed build guides exist from universities and public-health groups; follow one carefully.
Airflow and noise
Higher fan speed moves more air through the filters, but box fans are noisy at full speed. Running a cube on a medium setting is a common compromise. Filters that are too restrictive can make the fan struggle; if airflow seems weak or the motor gets warm, try a less restrictive filter or a cube with more area.
Replacing the filters
Inspect the upstream faces. During smoke events, filters may darken within days or weeks. Mark the start date on the frame. Dispose of used filters in household trash, bagged to keep dust contained.
Not a replacement for HVAC filtration
A box fan filter cleans air in one room. It does not replace the HVAC filter, which protects your equipment and filters air through the whole system. Keep your central filter on schedule; see replacement schedule.
Placement in a room
Put the fan where it can draw air freely, a foot or more from walls and furniture, and away from curtains. Elevating it slightly can help it move air across a room. Keep doors and windows closed during smoke events if outdoor air is the source, following local public-health advice.
Storage between uses
If you only use the fan during smoke season, wrap the filters in a bag between uses to keep them from loading with household dust. Store the fan clean and dry.
Cost compared with buying a purifier
A DIY build is inexpensive up front, but filter replacements add up. Compare the yearly filter cost with a commercial unit's replacement filters before deciding.
Frequently asked questions
What size filter fits a box fan?
A standard 20-inch box fan fits a 20x20x1 filter. Measure your fan.
Is MERV 13 required?
Not required, but many guides use it. Higher MERV restricts more airflow.
Is a box fan air cleaner safe?
Use a listed fan, do not block the motor, and do not leave it unattended.