HVACDirect.com Equipment Guide

How to Size Commercial Wall & Roof Exhaust Fans

Commercial exhaust fans are commonly sized by the airflow needed to replace room air or remove sensible heat. Use this guide to estimate the required CFM, understand when a wall or roof fan makes sense, and account for makeup air and system resistance before selecting equipment.

Estimate CFM from air changes per hour
Estimate CFM from sensible heat load
Select the fan at the required static pressure

This guide is for general comfort ventilation and heat removal—not hazardous exhaust design. Commercial kitchens, paint or spray operations, welding fumes, combustible dust, laboratories, battery rooms, chemical storage, parking garages, and other regulated processes may require source capture, listed equipment, special construction, interlocks, fire protection, discharge clearances, and code-specific airflow. Have a qualified engineer or mechanical contractor verify the application and local requirements.

Commercial Exhaust Fan CFM Calculator

Enter the information you know. The calculator estimates airflow by air changes and by sensible heat removal, then uses the larger completed result as the starting design CFM.

Method 1: Air Changes

Use this for general room ventilation when you know the dimensions and target air changes per hour (ACH).

Room length (ft.)

Room width (ft.)

Average height (ft.)

Target ACH

Method 2: Heat Removal

Use this when you know the sensible heat released into the room and how much warmer indoor air may be than incoming air.

Sensible heat load (BTU/h) Include sensible heat from equipment, motors, lighting, people, process and solar gain when known.

Allowed indoor-to-incoming air difference (°F) A smaller temperature difference requires more airflow.

— design CFM

Enter one or both sizing methods

Your estimated airflow will appear here automatically.

Air-change result—

Heat-removal result—

Select a fan from its certified performance data at the system’s required static pressure—not from free-air CFM alone.

Shop Commercial & Industrial Fans

Calculator assumptions: standard-density air near sea level and sensible-only heat removal using the common relationship BTU/h = 1.08 × CFM × ΔT. Elevation, air density, humidity or latent loads, outdoor design temperature, wind, stack effect, and system losses can change actual performance.

Method 1: Size by Air Changes per Hour

Air changes per hour describe how many times an airflow volume equal to the room volume is exhausted in one hour. First calculate the room volume, then multiply by the target ACH and divide by 60 minutes.

CFM = room length × room width × average height × ACH ÷ 60

General application	Planning starting range	Important consideration
Warehouse or general storage	2–4 ACH	Increase for heat, vehicle activity, odors or higher occupancy.
General workshop or light manufacturing	4–6 ACH	Process contaminants may require source capture rather than dilution alone.
Garage or service area	6–8 ACH	Vehicle exhaust and code requirements may govern.
Equipment, compressor or mechanical room	6–12 ACH	Use the heat-load method and equipment limits as an additional check.
Hot production or industrial area	10–20 ACH	High heat, stratification and process exhaust usually need engineered evaluation.

ACH ranges are planning aids, not universal code requirements. Many standards and codes prescribe airflow in CFM per square foot, CFM per person, CFM per fixture, or by a process-specific calculation instead of ACH. Always use the most demanding applicable requirement.

Air-change example

An 80-foot by 50-foot workshop with a 16-foot average ceiling has a volume of 80 × 50 × 16 = 64,000 ft³. At 6 ACH, the starting airflow is 64,000 × 6 ÷ 60 = 6,400 CFM.

Method 2: Size for Sensible Heat Removal

Ventilation can limit indoor temperature only when the incoming replacement air is cooler than the desired room temperature. Estimate the sensible heat entering the space, choose the acceptable temperature difference between indoor and incoming air, and calculate the airflow.

CFM = sensible heat load (BTU/h) ÷ [1.08 × allowed temperature difference (°F)]

Sensible heat load	10°F difference	15°F difference	20°F difference
50,000 BTU/h	4,630 CFM	3,086 CFM	2,315 CFM
100,000 BTU/h	9,260 CFM	6,173 CFM	4,630 CFM
200,000 BTU/h	18,519 CFM	12,346 CFM	9,260 CFM
300,000 BTU/h	27,778 CFM	18,519 CFM	13,889 CFM

Exhaust ventilation cannot cool below the incoming-air temperature. If outdoor or makeup air is already warmer than the desired indoor condition, more exhaust CFM will not provide mechanical cooling. Consider evaporative cooling where appropriate, tempered makeup air, spot cooling, or an engineered air-conditioning system.

Heat-removal example

A room with 100,000 BTU/h of sensible heat and an allowable 10°F rise above incoming air requires approximately 100,000 ÷ (1.08 × 10) = 9,260 CFM. If the same room’s ACH calculation produces 6,400 CFM, the heat-removal result is larger and becomes the preliminary design airflow.

Should You Use a Wall or Roof Exhaust Fan?

Wall Exhaust Fans

Often a practical choice for high-volume, low-pressure general ventilation with a short, direct path outdoors.

Typically simpler access and installation
Works well with wall louvers or motorized dampers
Useful for cross-ventilation when makeup openings are across the space
Discharge location must avoid doors, windows, intakes and occupied areas
Wind exposure and wall appearance may affect placement

Roof Exhaust Fans

Often useful when warm air stratifies near the roof, wall space is limited, or discharge should be carried above the building.

Can remove buoyant heat near the highest point
Offers downblast, upblast and hooded axial configurations
May provide a cleaner discharge path for some applications
Requires a compatible curb, flashing and weatherproof installation
Roof access, snow, wind and service clearances must be considered

Air path matters: locate replacement-air openings so fresh air sweeps through occupied and heat-producing areas before reaching the exhaust. Avoid placing the intake immediately beside the exhaust, which can short-circuit the airflow and leave much of the room poorly ventilated.

CFM Is Not Enough: Account for Static Pressure

A fan’s delivered airflow falls as resistance increases. Louvers, shutters, backdraft dampers, bird screens, guards, ductwork, elbows, transitions, filters, roof curbs and poor inlet or outlet conditions all add resistance. Select the fan from its performance curve at the estimated operating static pressure.

Map the complete air path

Include the makeup-air opening, room pressure, fan inlet, duct or curb, dampers, screens and discharge accessories.

Total the resistance

Use manufacturer pressure-drop data and a proper duct calculation. Free-air CFM assumes essentially no external resistance.

Select the operating point

Confirm CFM, static pressure, motor horsepower, voltage, phase, speed, sound and efficiency on certified performance data.

Every cubic foot exhausted must be replaced. Inadequate makeup air can depressurize the building, reduce fan airflow, increase door-opening force, cause drafts, pull contaminants from unwanted areas, and interfere with combustion equipment. Large systems commonly require powered and possibly tempered makeup air interlocked with the exhaust fans.

Controls and Layout Can Improve Performance

Thermostatic control

Starts or stages fans as room temperature rises. Use appropriate motor starters and controls rather than switching large motors directly from a light-duty thermostat.

Variable-speed control

Allows airflow to track heat or occupancy when the fan motor and drive are approved for speed control. Verify the safe operating range.

Staged fans

Multiple fans can provide turndown, redundancy and more even airflow across a large building. Divide total required CFM by the number of fans.

High-level exhaust

Warm air rises. Roof fans or high wall fans can remove stratified heat, while low-level makeup air promotes an upward sweep.

Interlocked makeup air

Coordinate supply and exhaust operation so the intended pressure relationship and airflow are maintained whenever fans run.

Commissioning

Measure actual airflow, rotation, current, vibration and room pressure after installation. Field conditions can differ from design assumptions.

Before You Select a Commercial Exhaust Fan

Verify	What to record	Why it matters
Application and contaminants	General air, heat, moisture, grease, fumes, dust or process exhaust	Determines construction, listing and whether dilution ventilation is appropriate.
Required airflow	_____ CFM	Use the greatest applicable code, ACH, heat or process result.
System resistance	_____ in. wg	The fan must deliver design CFM at this pressure.
Makeup airflow	_____ CFM	Supports fan performance and building pressure control.
Mounting and discharge	Wall / roof; upblast / downblast / hooded	Affects weather protection, air path, curb or opening, and clearances.
Electrical supply	Voltage, phase, frequency and available circuit	Must match motor and control requirements.
Environment	Air temperature, corrosion, wind, snow, seismic and hazardous classification	May require special motors, coatings, construction or certification.
Controls	Manual, thermostat, timer, sensor, VFD or staging	Determines starters, interlocks, dampers and operating sequence.

Commercial Exhaust Fan Terms to Know

CFM: Cubic feet of airflow delivered per minute.
ACH: Air changes per hour; airflow expressed relative to room volume.
Static pressure: Resistance the fan must overcome, commonly expressed in inches of water gauge.
Free air: Fan airflow with essentially no external system resistance.
Makeup air: Outdoor or transfer air supplied to replace exhausted air.
Sensible heat: Heat that changes dry-bulb air temperature without changing moisture content.
Fan curve: Published relationship between airflow and pressure for a fan at stated conditions.
System effect: Performance loss caused by poor airflow conditions near a fan inlet or outlet.

Need Help Selecting a Commercial Exhaust Fan?

Have the required CFM, estimated static pressure, mounting location, application, voltage and phase ready. An HVACDirect.com specialist can help you compare commercial wall and roof exhaust fans.

Shop Commercial & Industrial Fans