DIY box fan 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) – update – this appears to be resolved.

The Filtrete Store on Amazon features a discounted filters + tape bundle, just add your box fan. These are the recommended Filtrete 1900 MERV13 filters.


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!

From Bill Bahnfleth, Chair – ASHRAE Epidemic Task Force



CR Boxes in Johns Hopkins “Ventilation: Vital Tool to Reduce Spread”


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!

Travel DIY Air filter

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!

Dr. Prather with UCSanDiego

Brown University
Students unite to take a hands-on, DIY approach to improving air quality

An event hosted by Brown’s School of Public Health brought students together to build low-cost, highly effective Corsi-Rosenthal cubes as air filters that will augment existing filtration systems at Brown.

Common concerns addressed

Developed by @amandalhu, this interactive PDF addresses common points of confusion around air filters.

Rebuttal Matrix for Advocates of Air Filtration in Schools Based on Common Barriers

Find more tweets about this:

Box fan filter
Corsi Rosenthal box

In the news:

3M scientists: This Corsi-Rosenthal box movement is legit

Scientific American: We Need to Improve Indoor Air Quality: Here’s How and Why
Upgrading buildings’ ventilation, filtration and other factors would not only decrease COVID transmission but also improve health and cognitive performance in general

CBS News: New air purifiers filter at least 90% of COVID-carrying particles, researchers say

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

Smithsonian Magazine: The Homemade Air Purifier That’s Been Saving Lives During the Covid-19 Pandemic. Made from everyday items found in hardware stores, the Corsi-Rosenthal box is a testament to the power of grassroots innovation

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

NBC News: What to do if a family member tests positive? How to ‘Covid-proof’ your home. The best strategy for avoiding the Covid virus is to make indoor air as much like the outdoors as possible, experts say.

Brown University: Students unite to take a hands-on, DIY approach to improving air quality. An event hosted by Brown’s School of Public Health brought students together to build low-cost, highly effective Corsi-Rosenthal cubes as air filters that will augment existing filtration systems at Brown.

UCDavis: The Corsi-Rosenthal Box: DIY Box Fan Air Filter for COVID-19 and Wildfire Smoke