You don’t need to be an engineer to size home ventilation. A ventilation CFM calculator uses a clear rule (ASHRAE 62.2) to decide how much fresh air your home needs. Think of it like portion sizes for air: enough to stay healthy, not so much that you waste energy or make things noisy. In this guide, we’ll walk through the math, show a realistic example, and share field-tested tips from real installs so you can choose an HRV/ERV, pick duct sizes, and set it up quietly and correctly.
When you’re ready for equipment help or quotes, our team at The Furnace Outlet is here like a good neighbor, with straight answers and solid parts. Need broader equipment planning alongside ventilation? See our Sizing Guide or visit the Design Center for layout help.
Gather the facts (the info your calculator needs)
Before you punch numbers, grab a tape measure and a notepad. The calculator asks for three things:
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Conditioned floor area (ft²): heated/cooled space only.
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Bedrooms: a quick proxy for typical occupancy.
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Actual occupants (if higher): who sleeps here most nights.
Walk the home and note attic knee walls, bonus rooms, and finished basements they count if they’re conditioned. If you don’t have ducts (or have limited ductwork), that’s okay; HRV/ERV systems can be ducted independently or tied into an air handler. Planning other upgrades too?
Browse Air Handlers or, for ductless homes, see Ductless Mini-Splits so your fresh-air plan matches your comfort system.
Do the math (ASHRAE 62.2 made easy)
ASHRAE 62.2 gives a simple formula for continuous ventilation:
Qtot (CFM) = 0.03 × A Floor (ft²) + 7.5 × (Bedrooms + 1)
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The “+1” assumes one more person than bedrooms.
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If actual occupants exceed (Bedrooms + 1), add 7.5 CFM per extra person.
Example: 2,000 ft² home with 3 bedrooms
Qtot = 0.03×2,000 + 7.5×(3+1) = 60 + 30 = 90 CFM continuous.
If 6 people actually live there (2 more than 4 assumed), add 15 CFM: 105 CFM.
If you run the fan intermittently (say 50% of the hour), deliver twice the CFM when on: 105 ÷ 0.5 = 210 CFM on-cycle. Most online calculators handle this for you; we simply want you to understand the “why.”
Blower-door credit and runtime settings (fine-tuning)
Homes leak a little through cracks and joints. If you’ve had a blower door test, a calculator based on ASHRAE 62.2-2016 can apply an infiltration credit mechanical ventilation can be reduced by the amount of reliable, measured fresh air leakage (within limits). No test? Skip the credit and size to the full formula.
How pros fine-tune:
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Infiltration credit: Enter CFM50 and weather/height data in a 62.2-compliant tool to compute the allowable credit.
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Duty cycle: Decide continuous (quietest) vs intermittent (higher on-cycle CFM).
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Bedrooms vs occupants: If your household is larger than “bedrooms + 1,” bump the CFM now don’t wait.
HRV vs ERV (pick for your climate and lifestyle)
Both devices exchange indoor air with outdoor air while recovering energy so you don’t “dump” heating or cooling. Choose based on climate and humidity goals:
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HRV (Heat Recovery Ventilator): Best in cold, dry climates; excels at saving heat.
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ERV (Energy Recovery Ventilator): Better where summers are humid; helps control moisture and avoid over-drying in winter.
Also consider cooking, showers, pets, and allergies. If moisture is a regular battle, an ERV usually wins. Pairing new ventilation with system upgrades? Explore heat pump and AC options you might be evaluating alongside fresh air: R-32 Heat Pump Systems, and Air Conditioner & Air Handler Systems.
Pick the right unit size (CFM capacity that fits)
Match the unit’s rated CFM to your Qtot with a little headroom for filters, seasons, and balancing:
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Choose a model whose low/medium speed meets your continuous CFM target.
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If you’ll use intermittent mode, confirm the unit’s high speed can hit the on-cycle CFM.
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Check external static pressure (ESP) ratings; your ducts and filters add resistance.
If you’ll run MERV 11–13 filters, expect more pressure drop than coarse screens. Want help comparing options while you plan the rest of your system? Our team can sanity-check selections through the Design Center or a quick Quote by Photo.
A unit with ECM variable-speed fans gives you “trim” control to dial in quiet, balanced flows.
Quiet ducts 101 (diameter and materials that behave)
Ventilation works best when air moves slowly and smoothly. Follow manufacturer tables or standard duct charts. As a quick rule of thumb for flex/round branches:
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< 15 CFM → 3-inch
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15–25 CFM → 4-inch
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Higher flows → 5–6-inch branches as needed
Keep static pressure low (target ≤ 0.25" w.c. on both intake and exhaust runs). Use round metal where possible, keep runs short, and avoid extra elbows. If you must use flex, pull it tight and support every 4 feet.
Step up one size when a run is long or has several elbows big, slow air is quiet air. Need parts? See Accessories for grilles and hardware and Line Sets if you’re pairing with a mini-split system.
Place grilles smartly (so air doesn’t “short-circuit”)
Supply and exhaust should not face each other across the same small room. Spread them out:
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Exhaust: kitchens (near, not inside, the range hood), baths, laundry.
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Supply: bedrooms, living areas, basement rec rooms.
Aim supplies air across the room, not straight down a short hallway to the return or exhaust. Keep outdoor hoods well separated (fresh air intake away from dryer, flue, and bathroom exhausts).
Use adjustable grilles (with low-noise patterns) for final tweaks during balancing day. If you’re working in rooms served by ductless gear, see our Wall-Mounted Ductless Systems for coordinated layouts.
Balance the system (keep supply and exhaust within 20%)
A healthy system is balanced, the supply and exhaust CFM should be within ±20%. Too much exhaust can pull in dusty or humid air from the envelope; too much supply can push warm, moist air into walls.
How we balance like pros:
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Set the unit to your target speed.
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Measure branches flow with a flow hood or an anemometer + duct ring.
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Use balancing dampers at tees and adjust grilles last.
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Verify whole-house totals: supply vs exhaust.
Filters, drains, and simple maintenance (so it stays quiet)
Ventilation units are low-maintenance, but not no-maintenance:
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Filters: Check every 3 months; wash/replace as needed.
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Cores: HRV/ERV cores need seasonal inspection and gentle cleaning per the manual.
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Condensate (ERVs): Confirm drains are pitched and clear.
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Outdoor hoods: Keep them lint- and leaf-free.
Label damper positions and jot down “balanced flow” speeds on a sticker inside the unit door. Future you will thank me.
Real-world example (numbers you can copy)
Let’s size a typical home together.
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Home: 1,800 ft², 3 bedrooms, 5 occupants
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Formula: Qtot = 0.03×1,800 + 7.5×(3+1) = 54 + 30 = 84 CFM
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Extra occupants: Actual is 5 vs assumed 4 → add 7.5 CFM → 91.5 CFM (round to 92 CFM)
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Operation: Continuous low speed at ~95–110 CFM gives margin.
Ducting plan:
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Bedrooms (×3) supplies: 15–20 CFM each → 4" branches.
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Living area supply: 30–40 CFM → 5" branch.
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Baths (×2) exhaust: 20–25 CFM each → 4" branches.
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Laundry exhaust: 15 CFM → 3" branch.
Total supply ≈ 95 CFM; exhaust ≈ 95 CFM. Longest branch adds an elbow? Bump that branch one size for quiet.
If you must run intermittently 50%, set an on-cycle target ≈ 184 CFM (92 ÷ 0.5).
Tying ventilation into real systems (ducted, ductless, PTAC)
Every home’s layout is different. Here’s how we make ventilation play nice:
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Central ducted homes: Tie fresh air to an air handler return with a motorized damper and separate exhaust paths, or keep a dedicated HRV/ERV duct system for best balance.
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Ductless homes and additions: Run a small dedicated duct system for HRV/ERV alongside Mini-Splits.
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Apartments/retrofits: Look at compact options if you rely on PTACs.
Keep ventilation separate from dryer, kitchen hood, and bath fans those are local exhausts with different jobs.
your next step (and where we can help)
You now have the recipe: measure, calculate, choose HRV/ERV, size ducts, place grilles, and balance within 20%. If you want a second set of eyes, send us photos and room sizes through Quote by Photo. we’ll sanity-check your ventilation CFM calculator results and suggest parts that fit your layout.
We’ll keep it practical, code-aware, and quiet. When the air feels clean and the house stays calm, you’ll know the numbers and the install are right.
