Introduction
Sizing one room is arithmetic; estimating a whole house is a workflow. A proper ventilation estimate walks every room, assigns each space its target rate, totals the spot-ventilation loads, checks the whole-house baseline against ASHRAE 62.2, and reconciles the two into an equipment list. Done in order, the process takes under an hour for a typical home and produces numbers a contractor can quote against.
The Ventilation Calculator handles the per-room math — volume × ACH → CFM and m³/h. This guide provides the workflow around it: what to measure, which rate applies where, how to total the house, and how to sanity-check the result before it becomes a purchase order.
Step 1: Survey and Measure Every Room
Walk the house with a tape measure and record for each room: length, width, ceiling height, and purpose. Note vaulted ceilings (record wall height and peak height separately), open archways between rooms (they share air — treat connected open spaces as one volume or split proportionally), and rooms with special loads: bathrooms, kitchen, laundry.
Record conditioned spaces only. Garages, unfinished attics, and unconditioned crawlspaces are excluded from ventilation estimates — ventilating them wastes energy and can draw contaminants indoors. A simple table keeps the survey organized:
| Room | L × W × H | Volume | Notes |
|---|---|---|---|
| Primary bedroom | 14 × 12 × 8 ft | 1,344 ft³ | — |
| Bedroom 2 | 12 × 10 × 8 ft | 960 ft³ | — |
| Living room | 18 × 14 × 9 ft | 2,268 ft³ | Vaulted section averaged |
| Kitchen | 12 × 10 × 8 ft | 960 ft³ | Gas range, 40,000 BTU |
| Bath 1 | 8 × 6 × 8 ft | 384 ft³ | Shower, no window |
Metric surveyors: the same table works in meters (e.g., 4.27 × 3.66 × 2.44 m = 38.1 m³). Consistency within one system is the only rule — the common mistakes guide covers what goes wrong when systems mix.
Step 2: Assign a Target Rate to Each Room
Apply the planning rates per room purpose. Bedrooms, living rooms, and offices take ACH targets (5–6 ACH); bathrooms and kitchens take direct CFM rules:
| Room | Volume | Target | Required airflow |
|---|---|---|---|
| Primary bedroom | 1,344 ft³ | 5 ACH | 5 × 1,344 ÷ 60 = 112 CFM (190 m³/h) |
| Bedroom 2 | 960 ft³ | 5 ACH | 5 × 960 ÷ 60 = 80 CFM (136 m³/h) |
| Living room | 2,268 ft³ | 5 ACH | 5 × 2,268 ÷ 60 = 189 CFM (321 m³/h) |
| Kitchen hood | — | 100 CFM / 10k BTU | 400 CFM (680 m³/h) |
| Bath 1 | — | 50 CFM intermittent | 50 CFM (85 m³/h) |
Run each room through the calculator rather than hand-computing — the point of the workflow is systematic coverage, not mental arithmetic. The formula-explained guide derives every number in the table above.
Step 3: Compute the Whole-House Baseline (ASHRAE 62.2)
Separately from the room-by-room spot loads, compute the continuous whole-house requirement:
Q = 0.03 × conditioned floor area (ft²) + 7.5 × (bedrooms + 1)
For a 2,000 ft² (186 m²), 3-bedroom home: 0.03 × 2,000 + 7.5 × 4 = 90 CFM (≈ 153 m³/h) continuous. This baseline is about fresh air for the whole dwelling — building pollutants plus occupants — and is typically delivered by an ERV/HRV, a supply fan, or a central-fan-integrated system.
Step 4: Reconcile Spot Loads with the Baseline
The two calculations serve different operating modes and must not be blended into one number:
- Continuous baseline (90 CFM): runs 24/7 via the whole-house system. Size the ERV/HRV or supply fan to this.
- Intermittent spot loads (bath 50 CFM, kitchen 400 CFM): run on demand. Size each fan/hood to its own target.
- Interaction check: the 400 CFM kitchen hood is the dominant intermittent load — verify make-up air provisions, since it can overwhelm the envelope and starve the continuous system.
A frequent estimating error is adding spot CFM into the whole-house total (90 + 50 + 400 = 540 CFM continuous — wrong and enormously oversized). Keep the columns separate on your estimate sheet.
Step 5: Convert to an Equipment List
Translate each airflow target into purchasable equipment, stepping one size up for duct losses:
| Need | Calculated | Select |
|---|---|---|
| Bath 1 exhaust | 50 CFM | 80 CFM quiet fan (≤ 1.5 sones), timer |
| Kitchen hood | 400 CFM | 400–600 CFM ducted hood + make-up air damper |
| Whole-house continuous | 90 CFM | 120 CFM ERV with boost mode |
Bedrooms and living rooms in this example have no dedicated exhaust — they’re served by the whole-house system’s supply air plus natural air movement, which is the standard approach. If a bedroom is chronically stuffy, that’s a distribution problem (closed doors, no transfer grilles), not a sizing problem. Equipment selection detail lives in the equipment guide; installed costs in the cost guide.
Step 6: Sanity-Check the Estimate
Before the estimate becomes an order, run three checks:
- Order-of-magnitude check: whole-house continuous rates for typical homes fall in the 50–150 CFM range. A result of 900 CFM continuous means a decimal or unit error somewhere.
- Cross-check one room by hand using the metric path (m³/h = ACH × m³) — if the two unit systems agree, the arithmetic is sound.
- Reality-check against existing equipment: if the house currently has a 50 CFM bath fan that performs adequately, an estimate demanding 200 CFM for the same room deserves a second look at the inputs.
Special Cases
- Open-plan spaces: treat the connected area as one volume with the ACH of its primary use, or split proportionally at the archway — don’t double-count the shared air.
- Home gyms and workshops: 8–12 ACH planning range; consider dedicated exhaust rather than loading the whole-house system.
- Finished basements: 4–5 ACH plus attention to dehumidification — ventilation alone rarely controls basement moisture.
- Tight new construction: the envelope can’t be assumed to supply make-up air — mechanical ventilation and dedicated make-up air are design requirements, not options.
Related Guides
- Ventilation Complete Guide — the overview behind this workflow.
- Formula Explained — derive every calculation in the estimate.
- Ventilation Equipment Guide — turn airflow targets into purchases.
- Ventilation Cost Guide — budget the equipment list.
- Ventilation Checklist — verify the estimate before ordering.
Frequently Asked Questions
How long does a whole-house ventilation estimate take?
Under an hour for a typical home once you’re organized: 20–30 minutes measuring, 15 minutes running the calculator per room, 10 minutes on the ASHRAE 62.2 baseline and reconciliation. The survey table is what keeps it fast.
Do I include the garage in the estimate?
No — garages are unconditioned and excluded from ventilation estimates. Ventilating a garage wastes energy and can draw vehicle exhaust and stored-chemical fumes toward the house.
Should spot and whole-house CFM be added together?
No. They’re separate operating modes: continuous baseline (whole-house system) versus intermittent on-demand (bath fans, hoods). Size each independently and check their interaction through make-up air.
What if different rooms suggest conflicting equipment?
They shouldn’t — each room gets its own device sized to its own target. Bedrooms and living areas without dedicated exhaust are served by the whole-house supply system. Conflicts usually trace to double-counted open-plan volumes.
How accurate is a DIY estimate versus a professional load calculation?
For exhaust-fan and hood sizing, a careful DIY estimate using this workflow matches professional results — the inputs are simple and the formulas are standard. Whole-house balanced system design (duct layouts, pressure balancing) is where professional design adds value beyond the estimate.
From Estimate to Order
A complete ventilation estimate is a room table, a whole-house baseline, a reconciled equipment list, and three sanity checks — all reproducible from your measurements. Build it with the Ventilation Calculator, verify the totals against the checks above, price the equipment list with local suppliers, and confirm code requirements with your local building authority before you commit.
Disclaimer: This workflow produces planning estimates based on standard industry methods. Actual requirements are set by your local building code and vary by climate, construction, and occupancy. Verify targets with your local building authority (AHJ) and consult a licensed HVAC professional for system design decisions.
