How many CFM for a bathroom fan?

1 CFM per sq ft of floor (minimum 50 CFM) is the common rule.

What ACH for a house?

ASHRAE 62.2 sets whole-house ventilation by floor area and bedrooms.

100 CFM per 10,000 BTU of gas burners, or 100–400 CFM for electric.

What fan for a 50 sq ft bathroom?

50 CFM minimum (1 CFM per sq ft).

What is static pressure?

Resistance from ducts and grilles; it reduces real airflow.

Natural or mechanical ventilation?

Mechanical guarantees the airflow; codes often require it in baths.

CFM airflow calculator

Room size and air changes per hour: you get airflow in CFM, m³/h and l/s to pick a fan or size ventilation.

Airflow

176.573 CFM

Airflow176.573 CFM
Airflow in litres per second83.33 l/s
Room volume65.398 yd³

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How it's calculated (visible formula)

CFM = room volume (ft³) × ACH ÷ 60; m³/h = volume × ACH

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Measure room length, width and height.
Work out the volume.
Pick air changes per hour for the use.
CFM = volume (ft³) × ACH ÷ 60.
Choose a fan rated for that CFM at your duct's static pressure.

Typical air changes per hour

Typical air changes per hour
Room	ACH
Bedroom, living room	1–2
Bathroom	6–10
Kitchen	10–15
Garage or shop	6–10
Laundry	8–12

Worked example: 16 × 13 × 8 ft room at 6 ACH

Volume ≈ 1,766 ft³ (50 m³)
1,766 × 6 ÷ 60 ≈ 177 CFM

→ About 177 CFM (300 m³/h).

Common mistakes

Choosing a fan by free-air CFM, ignoring duct losses.
No make-up air path.
Confusing ACH with CFM per person.

Pro tips

Provide make-up air (grille or undercut door).
Use duct at least the fan's outlet diameter.
In windowless baths, put the fan on a timer.
Range hoods should cover the whole cooktop.

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FAQs

How many CFM for a bathroom fan?: 1 CFM per sq ft of floor (minimum 50 CFM) is the common rule.
What ACH for a house?: ASHRAE 62.2 sets whole-house ventilation by floor area and bedrooms.
Kitchen range hood?: 100 CFM per 10,000 BTU of gas burners, or 100–400 CFM for electric.
What fan for a 50 sq ft bathroom?: 50 CFM minimum (1 CFM per sq ft).
What is static pressure?: Resistance from ducts and grilles; it reduces real airflow.
Natural or mechanical ventilation?: Mechanical guarantees the airflow; codes often require it in baths.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.