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CFM Ventilation Calculator

Estimate the exhaust fan or ventilation airflow (CFM) needed for a room, based on typical air-changes-per-hour targets.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-25 · v1.0.0

Market
Imperial · sales tax
9.84 ft8.2 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Recommended airflow

84.76 CFM

Check your inputs

Based on general air-changes-per-hour targets, not a code-mandated minimum for your specific jurisdiction — check local building code for required bathroom/kitchen exhaust minimums.

Room volume
635.66 cu ft
Target air changes/hour
8 ACH

With the figures above, the recommended airflow comes to 84.8 CFM. The method behind this is well established, though site conditions and material batches will move it somewhat. This is calculated for United States and cites IRC 2024. Building in another market? Change the selector above so the units and code reference follow.

Add the equipment this sizes

This result is a specification — 84.755 CFM — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Formula: CFM = (room volume in cubic feet x air changes per hour) / 60
  • Typical air-changes-per-hour targets vary by room type — kitchens and bathrooms need substantially more than bedrooms due to moisture and odor

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Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Is this a code-required minimum?
No — building codes (e.g. the International Residential Code) often specify their own minimum exhaust fan CFM for bathrooms and kitchens directly. Check your local code for the actual legal minimum; this is a general ventilation-comfort estimate.
Should I round up when choosing a fan?
Yes — fans lose some effective CFM to ducting length, bends, and the vent cap, so choosing a fan rated somewhat above your calculated need is common practice.
What does ACH mean?
Air Changes per Hour — how many times the room's entire air volume is replaced with fresh/exhausted air in one hour. Higher-moisture or higher-odor rooms need more.