Not during fallout, smoke or a chemical incident. If you are told to "go in, stay in, tune in", keep windows and doors shut and turn off fans that draw in outside air. This calculator is for infections, damp and stale air in normal conditions.

The room

Length

Width

Height

People in the room

Windows Rough figures: wind and the outdoor temperature change them a lot.

Extractor fan in the room

Air purifier CADR (each)

The clean air delivery rate on the box (use the smoke figure). Real performance is often lower: allow for it.

Number of purifiers

DIY fan-and-filter boxes

A box fan with four good filters (Corsi–Rosenthal box). Counted at a cautious 500 m³/h each.

Clean air changes per hour –

02468+

Room volume–cubic metres

Clean air per person–litres a second (10+ is good)

To clear 90%–of airborne particles

To reach 5 an hour–more clean air

Check it with a CO₂ monitor

People breathe out carbon dioxide, so the CO₂ level shows how much "used" air is in a room. A £30–£80 NDIR monitor turns ventilation from guesswork into a number. (It doesn't measure what a purifier removes.)

CO₂ in an occupied room	Meaning	Action
Below 800 ppm	Likely well ventilated (not risk-free)	Keep going
800–1,500 ppm	Moderate	Open windows more, fewer people, less time
Over 1,500 ppm, sustained	Poor ventilation or overcrowding	Ventilate now; avoid sharing the room

Good habits

Open windows fully for a few minutes at a time; during a visit and for 10 minutes after.
Sick room: window slightly open, door closed, so air leaves the house rather than spreading inside it.
Winter: open a little, or the top sash only, and keep rooms at 18 °C or more for older or vulnerable people.
Avoid ionisers, "plasma" and ozone devices. Never run a fuel heater, barbecue or generator indoors: carbon monoxide kills.

Targets: CDC "Aim for 5" air changes an hour; the Lancet COVID-19 Commission rates 4 good, 5 better and 6 or more best; the WHO advises at least 10 litres a second per person. Window figures are typical values for UK rooms; real air changes vary with wind and temperature.

Room Ventilation and Air Purifier Calculator

The room

Check it with a CO₂ monitor

Good habits