Makeup Air Balance Calculator

Concepts behind this calculation

Calculate additional makeup airflow across a defined boundary using volume-flow balance or explicit actual-density mass balance.

Makeup air is a boundary accounting problem: identify simultaneous outgoing exhaust and the intentional incoming streams that can offset it. Volume balance is appropriate only when the streams have a compatible density basis. A selected surplus adds an explicit incoming-flow objective rather than predicting the pressure it will create.

When densities differ, each actual volume flow must first become a total-air mass flow. The remaining mass deficit is converted back to a required volume at the proposed makeup-air density. This separates conservation of mass from volume accounting; recirculation within the boundary and transfer already counted elsewhere must not be credited again.

How to use this calculator

1. Choose Balance method after defining the boundary and simultaneous operation.
2. For volume balance, enter Total scheduled exhaust, Existing intentional supply, Explicitly allowed incoming transfer, and Selected supply surplus target.
3. For mass balance, document Density and mass-balance source, confirm Mass-balance basis confirmation, and enter each airflow with its matching total-air density, Makeup-air total density, and Selected mass-surplus target.
4. Check that every credited stream crosses the boundary once.

Calculation method and applicability

Volume: Qmakeup=max(0,Qexhaust+target−Qsupply−Qtransfer). Mass: m=60×Q×ρ; Qmakeup=max(0,mexhaust+targetMass−msupply−mtransfer)/(60×ρmakeup).

Input definitions and units

Balance method

Use volume-flow balance when all streams share a compatible density basis, or use actual-density mass balance when they do not. Neither method predicts pressure. Choices: Volume-flow balance; Actual-density mass balance.

Total scheduled exhaust (CFM)

Sum the exhausts simultaneously operating across the same defined boundary. Applies to Balance method: Volume-flow balance.

Existing intentional supply (CFM)

Count only intentional supply crossing that boundary; avoid double-counting recirculated air. Applies to Balance method: Volume-flow balance.

Explicitly allowed incoming transfer (CFM)

Use only a documented incoming transfer allowance; do not assume unlimited infiltration. Applies to Balance method: Volume-flow balance.

Selected supply surplus target (CFM)

Zero requests neutral arithmetic flow balance. A positive value is a user-selected flow target, not a predicted pressure. Applies to Balance method: Volume-flow balance.

Density and mass-balance source

Identify the measurement, psychrometric state, design basis or other source for each total-air density and confirm that the streams cross the selected boundary once. This note is not authenticated. Applies to Balance method: Actual-density mass balance.

Mass-balance basis confirmation

Confirm stream direction, common boundary, total-air density basis and non-duplication of transfer air. The calculator checks arithmetic inputs only. Choices: Not confirmed; I confirmed the mass-balance basis. Applies to Balance method: Actual-density mass balance.

Exhaust airflow (ACFM at exhaust state)

Actual exhaust volume at the density entered below. Count only the simultaneous streams crossing this boundary. Applies to Balance method: Actual-density mass balance.

Exhaust total-air density (lbm/ft³)

Positive total-air density matching the exhaust airflow state; no universal default or lookup is applied. Applies to Balance method: Actual-density mass balance.

Intentional supply airflow (ACFM at supply state)

Actual intentional supply crossing the same boundary, at its entered density. Do not count recirculation twice. Applies to Balance method: Actual-density mass balance.

Supply total-air density (lbm/ft³)

Positive total-air density matching the supply airflow state. Applies to Balance method: Actual-density mass balance.

Allowed transfer airflow (ACFM at transfer state)

Actual incoming transfer at its entered density. Use only a documented transfer path and count it once. Applies to Balance method: Actual-density mass balance.

Transfer total-air density (lbm/ft³)

Positive total-air density matching the transfer airflow state. Applies to Balance method: Actual-density mass balance.

Makeup-air total density (lbm/ft³)

Positive total-air density at the proposed makeup-air state. The required CFM is reported at this density. Applies to Balance method: Actual-density mass balance.

Selected mass-surplus target (lbm/h)

User-selected positive incoming mass surplus target. Zero requests a neutral mass balance; this is not a pressure or code criterion. Applies to Balance method: Actual-density mass balance.

How to interpret the result

Read the requirement as an additional incoming stream on the selected boundary and density basis. Pressure control and conditioning that stream remain separate design tasks.

Worked examples

Assumptions and limits

• Volume mode assumes compatible density basis. Mass mode conserves the user-supplied total-air mass flows with m = 60 × actual CFM × density; it does not infer density from temperature, humidity, altitude or weather.
• The result is a boundary accounting calculation. It does not predict building pressure, envelope leakage, pressure coefficients, controls, combustion safety, hood capture, legal/code makeup-air fractions or equipment performance.
• The schedule must use simultaneous streams crossing one defined boundary. The calculator cannot detect omitted paths, duplicate transfer air, unequal measurement conditions or an inapplicable density source.
• No default density, infiltration credit, psychrometric lookup, outdoor-air conditioning load or mass-to-volume conversion outside the entered makeup density is applied.

Sources

• ASHRAE 2025 Fundamentals, Chapter 21: Duct DesignAir-system boundary and pressure-design context. The flow schedule calculation is an explicit continuity relationship, not a hood-code calculator.
• ASHRAE Handbook—Fundamentals, Chapter 3: Fluid FlowFluid density and volumetric/mass-flow relationship context. User-supplied densities remain explicit; no lookup or default is embedded.
• ASHRAE Handbook—Fundamentals, Chapter 13: Environmental ControlSteady zone air-mass conservation context. This calculator is a boundary accounting method, not a pressure or comfort model.