ASHRAE 62.1 Exhaust Calculator – Free HVAC CFM Tool
Free online calculator · No signup required
Calculate minimum exhaust airflow in CFM for toilets, kitchens, laboratories, parking garages and other common space types.
Unit system
Switching units converts your values — it never changes the requirement.
Minimum Exhaust Airflow
Select a space type
Your next design step
Check the duct that carries your exhaust airflow.
Use the free duct sizer for an individual exhaust branch. MEPFlow’s HVAC workspace is the next option for laying out supply and return systems on a floor plan.
Keep this check: Confirm space type, makeup air and the complete fan pressure path.
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Calculate exhaust CFM in four steps
First identify what must be exhausted. A public toilet room is counted by water closets and urinals; an area-based space uses floor area. Entering room area for every space can give the wrong answer.
- 1
Select the precise space type
Search for the room's actual function. The inputs change to match its calculation basis. A general room-exhaust entry does not replace a listed commercial cooking-hood design.
- 2
Enter the relevant area or count
Count water closets plus urinals for a public toilet room; do not count lavatories as toilet-exhaust fixtures. For an area-based space, use the served floor area in the selected units.
- 3
Choose the operating mode where available
Continuous and intermittent cases have different rates. Match the operating sequence, not simply the lower number.
- 4
Check replacement air and the complete pressure path
Review outdoor air, transfer air and room pressure. Then size the duct and fan for this airflow, including fittings, grilles, dampers and discharge components.
Worked example: public toilet exhaust
For a public toilet room with two water closets and one urinal, enter 3 fixtures. Using the calculator's continuous case of 50 CFM per fixture gives the result below. Switching to the intermittent case of 70 CFM per fixture gives 210 CFM. Check the standard edition and any applicable conditions for the project.
3 fixtures × 50 CFM/fixture = 150 CFM ≈ 71 L/s
This is the room-exhaust requirement for the stated example. It is not the required supply CFM for every neighbouring room and it is not the fan's static-pressure requirement.
Free ASHRAE 62.1 Exhaust Air Calculator
Use this HVAC exhaust calculator to estimate minimum exhaust airflow in CFM for common commercial and institutional spaces. Select the space type and the calculator applies the appropriate area-, fixture-, room-, showerhead-, or dwelling-unit-based exhaust method.
How the ASHRAE Exhaust Calculator Works
Exhaust requirements are prescriptive: each space type carries a minimum exhaust rate expressed either per unit of floor area or per fixture, showerhead, room or dwelling unit. Selecting a space type applies the correct method and the calculator asks only for the input that method needs.
Area-Based Exhaust Rates
Most commercial spaces — commercial kitchens, parking garages, janitor closets, locker rooms, copy rooms, laboratories and similar — use a rate per square foot:
Minimum Exhaust = Floor Area × Exhaust Rate
Exhaust Rates Based on Fixtures or Units
Some spaces are not sensibly sized from floor area. Public toilet rooms are exhausted per fixture — water closets plus urinals — while private toilet rooms are exhausted per room. Shower rooms are exhausted per showerhead, and dwelling-unit kitchens are exhausted per unit. In these cases the calculator multiplies the count by the applicable per-unit rate.
Continuous vs Intermittent Exhaust
Fixture-based and unit-based spaces have two operating cases. Continuous exhaust runs whenever the space is available for use and uses a lower airflow. Intermittent exhaust is activated on demand and uses a higher airflow while it operates. Choose the case that matches the intended control strategy for the system.
Exhaust Air vs Outdoor Ventilation Air
Exhaust airflow is a removal requirement for a specific space; outdoor ventilation airflow is a dilution requirement for occupied zones. A commercial kitchen, for example, has both an exhaust requirement and a separate outdoor-air requirement. Satisfying one does not satisfy the other, so both are normally scheduled during design.
Does Exhaust Air Need to Be Replaced?
Yes. Every cubic foot per minute removed by an exhaust system has to be replaced by outdoor air, transfer air from adjacent spaces, or a dedicated makeup-air unit. Replacement air strategy also determines the pressure relationship between the exhausted space and its neighbours, which matters for odour and contaminant control.
Frequently Asked Questions
Is exhaust air the same as outdoor ventilation air?
No. Exhaust airflow removes contaminated air from a specific space, while outdoor ventilation air is supplied to dilute contaminants in occupied zones. The two requirements are calculated separately and both must be satisfied.
Do I need to add floor area for every space type?
No. Area-based spaces use a rate per square foot, so floor area is required. Fixture-based and unit-based spaces such as toilet rooms, shower rooms and dwelling-unit kitchens use a rate per fixture or unit, so the calculator asks for a count instead.
Should I use continuous or intermittent operation?
Continuous applies when the exhaust fan runs whenever the space is available for use. Intermittent applies when exhaust is activated on demand, which generally requires a higher airflow while running.
Why do some spaces not return a number?
Paint spray booths and refrigerating machinery rooms are governed by additional applicable requirements and are not reduced to a simple prescriptive area rate in this reference dataset.
Does exhaust air need to be replaced?
Yes. Air removed by exhaust systems must be made up by outdoor air or transfer air so the space and building pressure relationships behave as intended.