Corsi–Rosenthal box builder

A DIY box-fan air purifier that rivals machines costing 5× more. Parts list, cost, and estimated clean-air delivery below.

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Fire safety: use a fan with a UL/ETL-listed cord, don't run it on a damaged or extended cord, keep it clear of curtains and bedding, and do not run it unattended overnight or when you leave home. Replace filters when they look grey — a clogged filter makes the motor work harder.

Estimated CADR ≈ 300 cfm per box (community measurements vary). Affiliate links — see disclosure. Shopping for a commercial unit instead? Size the CADR you need first — the AHAM room rule applies to any purifier.

What this calculates

Two steps. First the airflow your room needs: required cfm = floor area × 8 ft ceiling × 5 air changes per hour ÷ 60. Then how many boxes deliver it: boxes = ceiling(required cfm ÷ 300), rounded up, never below one.

Both constants are deliberate. The 8-foot ceiling is the standard AHAM rating assumption, and 5 air changes per hour is the rate normally recommended for smoke rather than the 4 used for general filtration. If your ceilings are high, use the purifier sizer instead — it takes ceiling height and air changes as inputs, where this one fixes them.

Where the design comes from

The Corsi–Rosenthal box is a cube of four MERV-13 furnace filters with a 20-inch box fan on top, taped together, blowing outward. It was proposed by Richard Corsi and Jim Rosenthal in 2020 and has been measured repeatedly by independent groups since. The parts are a box fan, four 20×20 filters, tape and a cardboard base — which is where the roughly $70 total comes from.

The 300 cfm we assume per box is deliberately conservative. Published community measurements of five-filter cubes with a strong fan run well above that; four-filter builds with an older fan run lower. Rather than quote a best case, the tool sizes on a figure most builds should actually reach, so the answer errs toward one box too many rather than one too few. This is an estimate from community testing, not a certified AHAM rating — no CR box has one.

What this does not tell you

Whether your specific build hits 300 cfm. Fan model, filter MERV rating, how well the seams are sealed and how clogged the filters are all move it substantially. A box with a leaky base pulls unfiltered air in around the gap and can lose much of its benefit.

Whether the room is sealed. Air changes only help if the air being changed is the air you are breathing. A box in a room with an open window is filtering the outdoors. Pair it with the sealing guide.

Fan safety. Box fans are not designed with a filter's back-pressure in mind. Use a fan with a fused plug, do not leave one running unattended overnight for weeks, and stop if the motor is hot to the touch.

Filter life. During heavy smoke a filter can grey out in weeks. Airflow falls as it loads, so a box that met your requirement in June may not in September.

A worked example

A 15 × 20 ft living room is 300 sq ft. Required airflow: 300 × 8 × 5 ÷ 60 = 200 cfm. Divided by 300 cfm per box and rounded up: one box, at roughly $70.

Open that room to an adjoining 200 sq ft kitchen and the effective area is 500 sq ft: 500 × 8 × 5 ÷ 60 = 333 cfm, which needs two boxes. That step from one to two is why the tool asks for the area you actually leave open, not the room you are sitting in — and why closing an internal door is sometimes cheaper than building a second box.