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Bathroom Exhaust Fan Calculator

Size a bathroom exhaust fan in CFM from the room volume and a target air-change rate.

Measurement units

Bathrooms are often planned around 8 ACH as a rule of thumb.

Free, runs entirely in your browser, and your numbers never leave your device. Results are estimates for planning only.

What this calculator does

A bathroom fan needs enough airflow to clear steam quickly and protect the room from mold. This tool sizes a bathroom exhaust fan in CFM from the room's volume and a target of about 8 air changes per hour, and flags the common 50 CFM minimum for small baths.

How to use it

  1. Enter the bathroom's length, width, and ceiling height.
  2. Keep about 8 air changes per hour, a common target for bathrooms.
  3. Read the required CFM, then choose a fan rated at or above it (and at least 50 CFM).

The formula

Volume = length × width × height. Airflow (CFM) = volume × ACH ÷ 60. A 50 CFM minimum is common for small bathrooms.

Example calculation

An 8 ft × 6 ft bathroom with an 8 ft ceiling at 8 air changes per hour:

  • Volume: 8 × 6 × 8 = 384 cu ft
  • Airflow: 384 × 8 ÷ 60 = 51.2 CFM → 52 CFM
  • Metric equivalent: about 87 m³/h

Result: About 52 CFM — just above the common 50 CFM minimum, so a 50–60 CFM fan suits this bath.

Buying and planning tips

  • Look at the fan's sone rating too; a quiet fan is one you'll actually leave running.
  • Run the fan during the shower and for 15–20 minutes afterward to clear moisture.
  • Keep duct runs short and straight; bends and long ducts cut the real airflow.

Common mistakes to avoid

  • Choosing a fan below 50 CFM for a small bathroom because the math suggested less.
  • Ignoring duct length and elbows, so the installed fan moves less than its rating.
  • Buying a loud fan that occupants avoid switching on.

Assumptions and limits

  • About 8 air changes per hour is a common rule of thumb for bathrooms, not a universal code value.
  • Duct length, bends, and the grille reduce real airflow below the rated CFM.
  • This is a planning estimate only — check your local code and the fan's rated airflow at realistic duct conditions.

Frequently asked questions

What size exhaust fan do I need for a bathroom?

Size by air changes per hour: volume × ACH ÷ 60. A small 8×6 ft bath needs about 52 CFM, so a 50–60 CFM fan works. Never go below the common 50 CFM minimum.

Is 50 CFM enough for a bathroom?

For a small bathroom, 50 CFM is a common minimum and is often enough. Larger or steamier bathrooms should be sized by air changes per hour, which can call for more.

How long should I run a bathroom fan?

Run it during the shower and for about 15–20 minutes after, to clear the moisture that leads to mold and peeling paint.

Does duct length affect fan size?

Yes. Long ducts and elbows reduce real airflow, so round up and check the fan's rated CFM under realistic duct conditions.

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