What CADR do I need for a 300 square foot room?

At an 8-foot ceiling and 4.8 air changes per hour, a 300-square-foot room needs about 192 CFM of total smoke CADR.

Can I add the CADR of two air purifiers?

As a planning estimate, add the smoke CADR ratings of units serving the same room. Placement and airflow can make real performance differ.

Is CADR the same as fan airflow?

No. CADR combines airflow with particle-removal performance. A raw fan-airflow number does not show how much clean air the purifier delivers.

No product hype. Just clean-air math.

Buy the CADR your room actually needs.

Room-size claims hide the number of air changes. Enter your real room dimensions and compare purifiers on one consistent target.

Size my purifier

Calculator

Find your minimum smoke CADR

✓ Ceiling height

✓ CFM + m³/h

✓ Multiple units

✓ Actual ACH check

Method

Why room “coverage” can mislead

A purifier does not clean a square foot. It delivers a volume of filtered air each minute. That is why a 12-foot ceiling needs more CADR than an 8-foot ceiling over the same floor.

Start with volume

Length × width × ceiling height gives the amount of room air the purifier must process.

Choose one ACH target

ACH means equivalent room volumes of clean air per hour. Comparing every model at the same target makes coverage claims comparable.

Buy by smoke CADR

EPA guidance uses smoke CADR for smaller particles. Do not substitute a raw fan-airflow figure.

Imperial formula

required CADR (CFM) = length × width × height × target ACH ÷ 60

Metric formula

required CADR (m³/h) = room volume (m³) × target ACH

Before you buy

Read the box in this order

Smoke CADR
Use the smoke figure when small particles are the concern. Dust and pollen CADR can be different.
Speed used for the rating
Published CADR is commonly a maximum-speed result. A quieter setting will normally deliver less clean air.
Filter and noise cost
A correctly sized unit only helps when you run it. Check replacement-filter cost and noise at a usable speed.
Ozone information
EPA advises choosing a device that does not produce ozone. Avoid treating “ionizer” as a substitute for verified particle CADR.

Quick answers

CADR calculator FAQ

What CADR do I need for a 300 sq ft room?

At an 8-foot ceiling and 4.8 ACH, the calculation is 300 × 8 × 4.8 ÷ 60 = 192 CFM. A higher ceiling raises the requirement, which is why floor area alone is incomplete.

Can I add the CADR of two purifiers?

For planning, add the smoke CADR ratings of units serving the same room. Two 100 CFM units give about 200 CFM total. Real results still depend on placement, obstacles, open doors, and air mixing.

Is CADR the same as CFM?

CADR is often reported in CFM in the United States, but the terms are not interchangeable. CFM can describe unfiltered fan airflow; CADR represents delivered clean air after accounting for particle removal.

Why does this calculator default to 4.8 ACH?

AHAM states that its Verifide suggested room size is based on 4.8 air changes per hour. The dropdown also lets you compare at 5 ACH, use a higher planning target, or enter your own.

Does an air purifier remove gases and odors?

Particle CADR does not rate gas removal. EPA says gas removal needs an activated-carbon or other gas-specific filter, and there is no widely used equivalent performance rating for it.

Primary sources · checked September 11, 2026

Numbers you can audit

US EPA · Guide to Air Cleaners in the Home ↗US EPA · Indoor Air Filtration for wildfire smoke ↗AHAM Verifide · Air Changes per Hour FAQ ↗CDC/NIOSH · Aim for 5 or more ACH ↗

This is a sizing estimate, not medical or engineering advice. Building ventilation, leakage, room shape, filter loading, placement, and fan speed change real performance.