Restroom Exhaust Sizing Calculator

Calculator Inputs

Unit System

Room Length

Room Width

Ceiling Height

Peak Occupants

Target ACH

Toilets / WCs

Urinals

Basins / Sinks

Showers

Area Factor (CFM per ft²)

Manual Minimum CFM

Toilet Rate (CFM each)

Urinal Rate (CFM each)

Sink Rate (CFM each)

Shower Rate (CFM each)

Occupant Add-On (CFM each)

Duct Length

Number of Elbows

Base Static Pressure (in. w.g.)

Static per Foot (in. w.g.)

Static per Elbow (in. w.g.)

Leakage Allowance (%)

Safety Factor (%)

Static Reserve Factor

Fan Efficiency (0 to 1)

Fan Selection Step (CFM)

Target Duct Velocity (FPM)

Formula Used

1) Room Area
Area = Length × Width

2) Room Volume
Volume = Length × Width × Height

3) ACH Airflow
CFM_ACH = (Room Volume × ACH) ÷ 60

4) Area-Based Airflow
CFM_Area = Floor Area × Area Factor

5) Fixture and Occupancy Airflow
CFM_Fixture = (Toilets × Toilet Rate) + (Urinals × Urinal Rate) + (Sinks × Sink Rate) + (Showers × Shower Rate) + (Occupants × Occupant Add-On)

6) Base Required Airflow
Base CFM = max(CFM_ACH, CFM_Area, CFM_Fixture, Manual Minimum)

7) Static Pressure Estimate
Static = Base Static + (Duct Length × Static per Foot) + (Elbows × Static per Elbow)

8) Adjusted Design Airflow
Adjusted CFM = Base CFM × (1 + Leakage %) × (1 + Safety %) × (1 + Static × Reserve Factor)

9) Recommended Fan Selection
Recommended Fan = round adjusted airflow upward to the selected fan step

This tool is intended for planning and early sizing. Final fan selection should still be checked against local regulations, manufacturer data, sound limits, and installed system resistance.

How to Use This Calculator

Choose metric or imperial units.
Enter restroom length, width, and ceiling height.
Add expected occupants and the number of toilets, urinals, basins, and showers.
Set your target ACH and any area-based minimum rate.
Adjust fixture airflow rates if your project standard differs.
Enter duct length, elbow count, and pressure assumptions.
Add leakage and safety allowances for a more conservative result.
Press Calculate Exhaust Size to show the result panel above the form.
Use the CSV and PDF buttons to export the calculation summary.

Example Data Table

Project Type	Room Size	Fixtures	ACH	Recommended Fan
Home Ensuite	2.1 × 1.8 × 2.4 m	1 WC, 1 basin, 1 shower	10	85 CFM / 144 m³/h
Office Washroom	3.6 × 2.7 × 2.7 m	2 WC, 2 basins	10	140 CFM / 238 m³/h
Public Restroom	6.0 × 4.2 × 3.0 m	3 WC, 2 urinals, 3 basins	12	420 CFM / 714 m³/h

FAQs

1. What does this restroom exhaust sizing calculator estimate?

It estimates the airflow needed to ventilate a restroom. The calculator compares room-volume airflow, fixture-based airflow, and minimum design airflow, then recommends a rounded fan size.

2. Why does the tool use multiple sizing methods?

Different projects are driven by different factors. Small toilets may be governed by fixture count, while larger public washrooms often need airflow based on room volume, occupancy, or a minimum design threshold.

3. What is ACH in restroom ventilation?

ACH means air changes per hour. It describes how many times the room air is replaced each hour. Higher ACH values usually mean faster moisture and odor removal.

4. Should I rely only on the recommended CFM?

No. Use the output as a planning number. Final fan selection should also consider local code requirements, pressure losses, noise limits, grille resistance, and the fan manufacturer’s published performance curve.

5. Why are duct length and elbows included?

Longer ducts and more elbows increase resistance. Extra resistance can reduce delivered airflow, so the calculator applies a pressure-based reserve to avoid undersizing the fan.

6. Can I use metric and imperial dimensions?

Yes. Select the preferred unit system first. The calculator converts dimensions internally and reports airflow in both CFM and m³/h for easier comparison.

7. What is the suggested duct diameter used for?

It is an approximate round duct size based on the selected airflow and target duct velocity. It helps early design, but final duct sizing should be checked with full duct layout calculations.

8. Does this tool work for domestic and commercial restrooms?

Yes. The adjustable fixture rates, ACH target, airflow minimums, and pressure assumptions let you size anything from a small ensuite to a shared commercial washroom.