Makeup Air Unit Size Calculator

Enter your total exhaust CFM, natural infiltration, temperature rise, and site altitude to calculate the makeup air unit size, heating load, and pressure balance for your space.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Total Exhaust CFM
Input the combined airflow exhausted by all kitchen hoods or other exhaust fans in Cubic Feet per Minute (CFM).
2
Specify Natural Infiltration (CFM)
Estimate the CFM of air that naturally enters the building through leaks, doors, and windows without a dedicated makeup air unit.
3
Input Temperature Rise (°F)
Provide the temperature difference the makeup air unit must heat incoming outdoor air. For cold climates, a 60°F rise is typical.
4
Enter Site Altitude (ft)
Input the elevation above sea level. Higher altitudes result in lower air density, impacting CFM requirements for heating.
5
Review Your Makeup Air Unit Recommendations
The calculator will display the required makeup air CFM, recommended unit size, heating loads in BTU/hr and kW, and pressure balance ratio.

Example Calculation

A commercial kitchen exhausts 600 CFM, with 100 CFM natural infiltration. The makeup air unit needs to heat air by 60°F at sea level.

Total Exhaust CFM (CFM)

600

Natural Infiltration (CFM)

100

Temperature Rise (°F)

Site Altitude (ft)

Results

500 CFM

Tips

Verify Exhaust CFM Accurately

The most critical input is accurate exhaust CFM. Consult hood specifications or have an HVAC professional measure actual airflow, as underestimating this can lead to severe negative pressure issues.

Consider Seasonal Temperature Swings

While 60°F is a common design delta T, in regions with extreme winters, you might consider a larger temperature rise for peak heating load calculations, ensuring the unit can handle the coldest days.

Factor in Building Tightness

Modern, energy-efficient buildings are much tighter than older structures, meaning less natural infiltration. This often necessitates a larger makeup air unit to compensate for exhaust, as natural air leaks are insufficient.

Sizing Makeup Air Units for Optimal Indoor Environments

The Makeup Air Unit Size Calculator is an indispensable tool for HVAC professionals, architects, and building owners, enabling the precise determination of makeup air CFM requirements, heating loads, and recommended unit sizes. By factoring in total exhaust CFM, natural infiltration, desired temperature rise, and site altitude, this calculator ensures that commercial and residential spaces maintain healthy air quality and proper pressure balance. Accurate sizing prevents common issues like negative pressure and backdrafting, contributing to safe, comfortable, and energy-efficient buildings in 2025.

The Critical Role of Balanced Ventilation in Buildings

Balanced ventilation is paramount for maintaining healthy and comfortable indoor environments. When exhaust fans, particularly powerful commercial kitchen hoods, remove air from a building, an equal amount of fresh air must be introduced to prevent a negative pressure differential. Without adequate makeup air, buildings can experience significant issues, including cold drafts, difficulty opening doors, the drawing in of unfiltered outdoor air, and critically, the backdrafting of combustion gases from appliances like furnaces and water heaters, posing serious health and safety risks to occupants.

Calculating Makeup Air CFM and Heating Load

The makeup air requirement is determined by subtracting natural infiltration from the total exhaust CFM. This ensures that only the necessary amount of conditioned air is supplied.

Makeup Air CFM = Total Exhaust CFM - Natural Infiltration CFM

This raw CFM value is then adjusted for altitude, as air density decreases at higher elevations, meaning more volumetric flow is needed to deliver the same mass of air.

Adjusted Makeup Air CFM = Makeup Air CFM / Altitude Correction Factor

The heating load, expressed in BTU/hr or kW, is calculated using a standard HVAC formula based on the adjusted CFM and the desired temperature rise (ΔT):

Heating Load (BTU/hr) = Adjusted Makeup Air CFM × 1.08 × Temperature Rise (°F)
Heating Load (kW) = Heating Load (BTU/hr) / 3412.14

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Sizing a Makeup Air Unit for a Commercial Kitchen

Consider a commercial kitchen with a total exhaust of 600 CFM. It has an estimated natural infiltration of 100 CFM. The unit needs to heat incoming air by 60°F, and the site is at sea level (altitude 0 ft).

Calculate Makeup Air Required:
- Makeup Air CFM = 600 CFM (exhaust) - 100 CFM (infiltration) = 500 CFM.
Adjust for Altitude: At sea level, the altitude correction factor is 1, so the adjusted makeup air is still 500 CFM.
Calculate Heating Load (BTU/hr):
- Heating Load = 500 CFM × 1.08 × 60°F = 32,400 BTU/hr.
Convert Heating Load to kW:
- Heating Load (kW) = 32,400 BTU/hr / 3412.14 = 9.495 kW.

Based on these calculations, a makeup air unit capable of delivering 500 CFM and providing approximately 32,400 BTU/hr (or 9.5 kW) of heating capacity would be recommended. A standard unit size of 500 CFM would be appropriate.

💡 Ensuring proper ventilation extends to other building elements. Our Stack Vent Size Calculator helps determine correct pipe sizing for plumbing systems to prevent pressure imbalances.

Maintaining Indoor Air Quality and Pressure Balance

Beyond simply replacing exhausted air, makeup air units contribute significantly to overall indoor air quality (IAQ) and pressure balance. By introducing fresh, filtered outdoor air, they dilute indoor pollutants, odors, and humidity, creating a healthier and more pleasant environment. Maintaining a slight positive pressure within a building (where slightly more air is supplied than exhausted) can also prevent the infiltration of unconditioned air and pollutants through cracks and openings, further enhancing IAQ and energy efficiency. ASHRAE Standard 62.1 (Ventilation for Acceptable Indoor Air Quality) provides guidelines for minimum ventilation rates to ensure adequate air changes and pollutant dilution in various building types.

Typical Makeup Air Unit Benchmarks

In commercial settings, makeup air unit (MAU) sizing and performance benchmarks are critical for effective HVAC design. For restaurant kitchens, exhaust hoods typically operate between 200 and 600 CFM per linear foot of hood, requiring substantial makeup air. Total exhaust volumes for a full-service restaurant can range from 2,000 to 10,000+ CFM. Residential applications for powerful range hoods often require smaller MAUs, typically 300-1200 CFM. Heating loads for MAUs can vary dramatically based on climate, from a few thousand BTU/hr in mild regions to over 200,000 BTU/hr (approx. 60 kW) for large commercial units in cold climates, where a 60°F to 90°F temperature rise is common. Achieving a balance ratio of 90-100% is the industry standard to prevent negative pressure and ensure optimal system performance.

Frequently Asked Questions

What is a makeup air unit (MAU) and why is it needed?

A makeup air unit (MAU) is an HVAC system designed to introduce fresh, conditioned outdoor air into a building to replace air exhausted by ventilation systems, particularly kitchen hoods. It prevents negative pressure, which can cause backdrafting of combustion appliances, cold drafts, and inefficient exhaust fan operation, maintaining indoor air quality and comfort.

What happens if a building has negative pressure?

Negative pressure occurs when more air is exhausted from a building than is supplied, creating a vacuum effect. This can lead to several problems: backdrafting of furnaces or water heaters (pulling dangerous combustion gases indoors), cold drafts from exterior cracks, difficulty opening doors, and reduced efficiency of exhaust systems like kitchen hoods.

How does altitude affect makeup air requirements?

Altitude affects makeup air requirements because air density decreases at higher elevations. A fan moving a specific volume of air (CFM) at sea level will move less mass of air at higher altitudes. Therefore, a makeup air unit designed for higher altitudes needs to be larger or run at a higher speed to deliver the same effective amount of air mass to balance exhaust.

What is a good balance ratio for makeup air?

An ideal balance ratio for makeup air is typically between 90% and 100% of the exhausted air. A ratio close to 100% ensures that almost all exhausted air is replaced, preventing negative pressure. Some slight positive pressure (100-110%) can be desirable in certain commercial settings to prevent infiltration of unfiltered air.