DIY Box Fan Air Cleaner (CR Box) Calculator

A box fan strapped to MERV-13 filters cleans the air of an entire room for under $80. Enter your build and room to see its clean-air delivery rate and how fast it scrubs the air.

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What Is a DIY Box Fan Air Cleaner?

A DIY box fan air cleaner, popularized as the Corsi-Rosenthal Box (or CR Box), pairs an ordinary 20-inch box fan with one or more MERV-13 furnace filters. Air gets pulled through the filter media, leaving smoke, pollen, dust, and respiratory aerosols behind. The classic cube design tapes four filters into a box with the fan on top and a cardboard shroud, turning a $25 fan and $40 in filters into an air purifier that rivals units costing five times as much.

The reason it works is surface area. A single flat filter chokes a box fan with static pressure, but spreading the same airflow across four or five filters drops the face velocity, lowers resistance, and lets the fan move far more air. A well-built four-filter cube moves roughly 580 cubic feet of clean air per minute, enough to scrub a typical bedroom several times over every hour.

How the CADR and Air Changes Are Calculated

This tool estimates total airflow from your fan rating and filter count, then multiplies by the single-pass capture efficiency of your filter rating to get the Clean-Air Delivery Rate (CADR). It then compares that to your room volume to find air changes per hour (ACH).

CADR = FanCFM x (FilterArea / (FilterArea + 9.5)) x Efficiency
ACH = (CADR x 60) / (Length x Width x Ceiling)

Why 5 Air Changes per Hour Is the Target

Public-health guidance for reducing wildfire smoke and airborne disease points to at least 5 air changes per hour in occupied rooms. At 5 ACH the air is fully exchanged every 12 minutes, which keeps particle concentrations low even when a door opens or someone coughs. Hitting that number depends on both the build and the room: a strong four-filter cube covers around 870 square feet at an 8-foot ceiling, while a single flat filter handles closer to 330 square feet.

Frequently Asked Questions

Is a DIY box fan air cleaner as good as a store-bought HEPA purifier?
For particle removal, a four or five filter CR Box often beats consumer HEPA units on raw clean-air delivery because it moves so much more air. Independent testing has measured CR Boxes delivering 400 to 700 CFM of clean air, well above many $300 purifiers. It will not match HEPA on the very smallest particles, but MERV-13 captures the majority of smoke and respiratory aerosols.
What box fan and filters should I buy?
Use a standard 20-inch box fan rated around 2,000 CFM free-air and 20x20x1 or 20x20x4 inch MERV-13 furnace filters. Thicker four-inch filters last longer and flow slightly better. Avoid older fans without a thermal fuse, and never leave any DIY fan running unattended overnight.
How long do the filters last?
In a typical home, plan to swap MERV-13 filters every 3 to 6 months, or sooner during heavy wildfire smoke. A filter that looks gray is still working, but once it is visibly clogged or smells musty, airflow has dropped and it is time to replace it. The fan itself can run for years.
Does a higher MERV rating clean the air better?
Higher MERV ratings capture more small particles per pass, but they also add resistance that can cut a box fan's airflow. MERV-13 is the sweet spot for DIY builds: strong capture of smoke and aerosols without strangling the fan. MERV-14 and above are worth it only with a powerful fan and plenty of filter area.

Common Tip Percentages

ServiceTypical Tip RangeNotes
Restaurant (sit-down)15–20%20%+ for exceptional service
Food delivery10–20%Consider extra for long distances
Taxi / rideshare10–20%More for assistance with luggage
Hotel housekeeping$2–5/nightLeave daily, not just at checkout
Barber / hair salon15–20%On pre-discount total
Pizza delivery10–15% or $3–5 minimumHigher in bad weather

General Household Budget Ratios (50/30/20 Rule)

Category% of Net IncomeExamples
Needs50%Rent, groceries, utilities, insurance
Wants30%Dining out, entertainment, hobbies
Savings / Debt20%Emergency fund, retirement, extra payments

Common Measurement Conversions for Everyday Use

QuantityUS / ImperialMetric Equivalent
1 cup8 fl oz237 mL
1 pint16 fl oz473 mL
1 quart32 fl oz946 mL
1 gallon128 fl oz3.785 L
1 lb16 oz453.6 g
1 ft12 in30.48 cm

Practical Guide for DIY Box Fan Air Cleaner (CR Box) Calculator

The single biggest lever on performance is filter count. Going from one flat filter to a four-filter cube roughly doubles or triples the clean-air delivery from the same fan, because each added filter lowers the face velocity and the static pressure the fan has to fight. If your room comes back undersized in this calculator, adding filter area almost always beats buying a bigger fan.

Sealing matters as much as airflow. Any gap between the filters and the fan lets dirty air sneak past the media, so tape every seam tightly and add a cardboard shroud over the fan grille to keep air from short-circuiting back out the corners. A good shroud can add 20 to 50 CFM of effective clean air for the cost of a piece of cardboard.

Placement and runtime finish the job. Put the cleaner where air can reach all four filter faces, not jammed in a corner, and run it continuously on medium or high during smoke events or illness. Running 24/7 on a lower speed is quieter and still maintains the air changes you need, since particle removal depends on total volume moved over time.

Quick Checklist

  • Use MERV-13 filters with the airflow arrows pointing toward the fan.
  • Tape all seams airtight and add a cardboard shroud over the fan blades.
  • Build a four or five filter cube for any room over about 200 sq ft.
  • Run the cleaner continuously and replace filters every 3 to 6 months.