How to use this calculator
- Inventory simultaneous exhaust. Hood at the speed actually used, plus dryer, bath fans — whatever can run together.
- Establish the real infiltration credit. From the ACH50/N screen; use zero for tight construction to be conservative.
- Read the requirement. The shortfall is what an interlocked damper or tempered supply must provide.
- Check combustion safety. Natural-draft appliances get a worst-case depressurization test whenever large exhaust is added — non-negotiable.
How it works
A fan cannot remove air a building does not replace. Every CFM exhausted must re-enter somewhere, and the balance decides whether that happens through a deliberate, tempered path or through flue pipes and rim joists:
required makeup ≈ Σ exhaust − infiltration credit
Modern tight envelopes made this arithmetic urgent: the same 900 CFM hood that "worked fine" in a leaky 1970s house starves and backdrafts a 3 ACH50 one. The infiltration calculator supplies the honest credit; this page does the balance and flags the ~400 CFM residential hood trigger. For the fresh-air side of the same envelope conversation, the ACH calculator covers ventilation rates.
Worked example
Verified against the live calculator
A 900 CFM hood with a 150 CFM dryer running,
in a house whose blower-door numbers give about 70 CFM of
natural infiltration:
makeup ≈ (900 + 150) − 70 = 980 CFM
The hood is over the ~400 CFM trigger and the shortfall is nearly the whole exhaust — this kitchen needs an interlocked makeup-air damper (tempered, delivered near the kitchen), and if a natural-draft water heater shares the house, a combustion-safety test before the hood ever runs on high.
Frequently asked questions
How much makeup air does a range hood need?
Roughly its exhaust minus what the house can leak in: required makeup ≈ total simultaneous exhaust − infiltration credit. A 900 CFM hood plus a 150 CFM dryer in a house leaking only ~70 CFM naturally needs about 980 CFM provided deliberately.
At what CFM is makeup air required?
The widely adopted residential trigger is kitchen hoods over ~400 CFM (IRC M1503 family) — above that, dedicated makeup air provisions are typically required. The adopted local code and the hood listing govern; combustion safety matters at any size.
What happens without makeup air?
The house depressurizes until the fan finds its air anyway: hood performance collapses, doors whistle and slam, and — the dangerous one — natural-draft water heaters and fireplaces backdraft their flue gases into the house.
Can infiltration count as makeup air?
Only as much as actually exists, which in tight modern construction is very little (run the blower-door numbers through the infiltration calculator). Counting generous phantom leakage is how 1,000 CFM hoods end up moving 400.
Method & assumptions
- Steady-state balance screen; the ~400 CFM trigger is the common residential threshold (IRC M1503 family) — adopted code and listings govern.
- Worst-case simultaneity is the honest input: the inventory is what CAN run together, not what usually does.
- Delivery design (tempering, location, interlock) and commercial-kitchen rules (IMC 508 family) are outside this screen.
- Combustion appliance zone safety is a test, not a calculation — this page only tells you when to insist on it.