Air purifier sizing

What size air purifier do I need?

2-step calculator Based on AHAM AC-1 CADR standards Reviewed by the WhatSize editorial team

Last reviewed June 2026 • Sources: AHAM AC-1, EPA, ENERGY STAR

Quick answer

The right air purifier for your room is one whose CADR is at least two-thirds of your room's square footage (the AHAM 2/3 rule). A 300 sq ft bedroom needs ~200 CFM; a 1,000 sq ft living room needs ~660 CFM. Our 3-step wizard below does the math for your exact room and concern.

Step 1 of 2 — Room size

1 2

How big is your room?

We'll calculate the CADR (clean air delivery rate) needed for your space.

300 sq ft

1001,500

Primary concern

High-pollution environment (pets, smoking, or heavy dust)

For your 300 sq ft room

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CADR (CFM)

Based on AHAM AC-1, Section 4.2 (CADR Test Method)

AHAM certified 4.5 air changes/hr

How we picked these products

Each product below is verified to meet or exceed your calculated spec. We sort by verified specifications first, then by availability and customer reviews. We never recommend a higher-priced product for a higher commission. Read our product recommendation policy.

The sizing recommendations and product suggestions on this page are for general informational purposes only. They are based on simplified assumptions and may not account for site-specific factors (local building codes, actual load measurements, insulation, climate variations, safety requirements). Always consult a licensed professional for your specific situation.

Disclosure: WhatSize Appliance earns a commission when you buy through Amazon links. This doesn't affect which products we recommend. Read our full disclosure.

Got a result that doesn't match your situation?

Adjust your inputs above and re-run the calculator. Most users find their spec falls within 1-2 sizes of the recommendation.

Formula

Required CADR = Sq Ft × ²/₃ (AHAM 2/3 Rule) × Concern Factor(1.0-1.35) × Pollution Factor(1.0 or 1.2)
→ AHAM AC-1 Standard + ENERGY STAR CADR recommendation

About this calculation

This is a simplified sizing tool based on published industry standards, not a full professional audit.

Accuracy depends on your input — subjective factors like dampness level or insulation quality are estimates. Our formulas include a safety buffer that may result in oversizing for some situations (slightly higher cost, but safer than undersizing).

Local variables (climate zone, altitude, insulation levels, building codes) can affect real-world sizing needs. Standards are re-checked quarterly — see our Editorial Policy.

Common scenarios for air purifier sizing

If your situation matches one of these, jump straight to a detailed guide:

Bedroom — quiet sleep + allergen relief Large open space — 500+ sq ft living area Pet owners — dander + odor control Allergies or asthma — HEPA-grade CADR

Frequently asked questions

What CADR do I need for my room size?

Follow the AHAM AC-1 2/3 rule: your air purifier's CADR should be at least two-thirds of the room's square footage. A 300 sq ft bedroom needs CADR 200+, a 500 sq ft living room needs CADR 330+, and a 1,000 sq ft open space needs CADR 660+.

How do I calculate CADR from room size?

Per AHAM AC-1: CADR = room area (sq ft) × 0.67. For example, 250 sq ft × 0.67 = 168 CADR minimum. For allergies or smoke, add a 25% buffer. The wizard above runs this calculation for you.

Is higher CADR always better?

Not necessarily. Oversized CADR costs more and runs louder without improving air quality. Match CADR to room size per the AHAM 2/3 rule. Exception: allergy and asthma households benefit from a higher CADR targeting 4.8+ ACH.

What's the difference between CADR and ACH?

CADR (cfm) is the AHAM AC-1 rate at which a purifier delivers clean air. ACH is how many times per hour the room's full air volume is cleaned. ACH = (CADR × 60) ÷ room volume. Most homes should target 4-5 ACH.