DIY box fan air filters – Corsi-Rosenthal box

Also known as a Corsi-Rosenthal box, this DIY method of building your own air filter with MERV13 furnace filters and a box fan are an easy and cost-effective way to help clear indoor air from airborne virus particles, wildfire smoke, pollen, dust, and more!

If you can seal a box, you can build one (or 100!) of these!

Image / Video Gallery

This gallery includes photos of builds, creative modifications, instructional images, as well as videos. Click through tags below to load more.

Video: How to effectively ventilate classrooms using fans, HEPA filters or Corsi-Rosenthal box fans and monitors

Cleaning indoor air properly greatly reduces infections by airborne viruses, like COVID-19. This video explains it all very well!


How to build a Corsi-Rosenthal box fan filter

Start here

A Variation on the “Box Fan with MERV 13 Filter” Air Cleaner Article by Jim Rosenthal

Rosenthal Corsi box - box fan filter made with a cube of MERV13 filters and a box fan

Read about it in WIRED:

Construction Guide:

More details, including filter brands to avoid, here: (by Philip Neustrom)
(hint: Filterbuy MERV-13, prevalent on Amazon, have not tested well)

Illustrated Guide

by Amanda H @ughberta

The Fan Shroud

An optional part of a box fan filter is the fan shroud, or covered fan corners. Covering corners of a square box fan closes gaps and improves efficiency of the fan. See Jim Rosenthal’s article about it here. Use cardboard or duct tape.

Testing to optimize fan shroud size. David Elfstrom found a 13.5 to 14.25 inch diameter opening to work best with a Utilitech brand fan. Lasko fans work best with a 15 inch fan shroud.

shroud size for box fan air filter
shroud size for box fan air filter

Choosing filters

David Elfstrom advises “keep an eye out for local sales on the 20x20x1 or 20x25x1 Filtrete 1900 and box fans”

You can order a full set of high-quality MERV13 filters to make this air purifier here:
These 2 inch deep filters (compared to typical 1 inch) provide greater surface area for filtration, better air flow, quieter operation, and longer filter life.

(hint: Filterbuy MERV-13, prevalent on Amazon, have not tested well- two people have independently compared with particle counters and come to the same conclusion about sub standard performance)


The Corsi-Rosenthal box (single filter tested, full cube would be even better!) performs very well! It is the most cost effective air cleaner on Marwa Zaatari’s chart!

Round Fan alternate design

Not all parts of the world have box fans available. This international team has worked out an alternate design using a round fan!

Modification: Screen as “prefilter”

This modification, adding fiberglass screen material, can keep pet hair out of filters and prolong their useful life, with barely noticeable flow reduction!

FAQ – Frequently Asked Questions

New comprehensive FAQ page here!

Also, tons of details and FAQs in this excellent comprehensive thread by Lazarus Long.

Video: Box Fan Filter build tutorial – Spanish

source: AleHSalcedo2

Video: A Fourth Grader Explains How to Build a DIY Corsi/Rosenthal Box Air Cleaner

source: @gbignami

Watch more videos

Mass production!

Check out how Rosie @BkPhilanthropy assembled and distributed 100 air filters!

Download the printable PDF of the insert they included with the filters!

Find more tweets about this:

Box fan filter
Corsi Rosenthal box

In the news:

WIRED: Could a Janky, Jury-Rigged Air Purifier Help Fight Covid-19?

DIY Air Filters For Classrooms? Experts Are Enthusiastic — And A Citizen Scientist Makes It Easy

UCSD students, staff build filtration boxes to prevent spread of COVID in classrooms