Makeup Air CFM Calculator

Inputs

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Rated (or measured) hood exhaust at the speed actually used.
cfm
Dryer (~100-200 CFM), bath fans, central vac — whatever can run at the same time as the hood.
cfm
Natural leakage the house can supply (from the ACH50/N screen). Tight homes have little to give — use 0 to be conservative.
cfm

Results

Default result
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Required makeup air(Q_mua)
980cfm
Out of tol.

Large shortfall: expect strong depressurization — hood underperformance, door-whistle, fireplace and water-heater backdraft risk. Provide interlocked makeup air.

Exhaust the envelope cannot supply — to be provided deliberately (damper, inlet, interlocked supply).

Also computed

Total exhaust(ΣQ_ex)1,050cfm

Hood exhaust(Q_hood)900cfm

Over the ~400 CFM residential kitchen-hood trigger (IRC M1503 family): dedicated makeup air provisions typically required — and combustion-appliance backdrafting must be checked regardless.

Method notes 4 notes
  • Balance screen: air out must equal air in. The ~400 CFM hood trigger is the widely adopted residential threshold (IRC M1503 family) — the adopted local code and the hood listing govern.
  • Natural-draft combustion appliances (water heaters, fireplaces, older furnaces) are the real stake: depressurization reverses their flues. Any home with them deserves a combustion-safety test when big exhaust is added.
  • Makeup air wants to arrive near the kitchen, tempered or at least directed — a January 900 CFM of raw outdoor air through one grille is its own complaint.
  • Commercial kitchens follow IMC 508-style dedicated makeup requirements sized to the hood schedule — different rules entirely.

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