Introduction
Every ventilation plan — a bathroom fan, a range hood, or a whole-house fresh-air system — starts with the same question: how much air does this room need to move? Undersize the airflow and moisture and odors linger; oversize it and you pay for a fan that roars. Ventilation estimating answers with arithmetic: measure the room, look up the target air-change rate, and convert it to a required airflow.
The Ventilation Calculator handles the core calculation: enter room dimensions and a target air changes per hour (ACH) to get the required airflow in CFM and m³/h. This guide explains everything behind that number — terminology, formulas, a worked example, room-by-room targets, and the judgment calls it can’t make.
What Ventilation Rates Are and Where They’re Used
Ventilation is the deliberate exchange of indoor air with outdoor air to control moisture, odors, pollutants, and carbon dioxide. Spot ventilation targets a single source — a bath fan clearing shower steam, a hood capturing cooking fumes. Whole-house ventilation provides baseline fresh air across the home, usually via an ERV, HRV, or supply/exhaust system sized to ASHRAE 62.2.
The people who run these numbers include:
- Homeowners and DIYers replacing a bath fan, sizing a range hood, or checking a stuffy bedroom.
- Remodeling contractors specifying fans and duct runs for bath and kitchen renovations.
- HVAC professionals designing whole-house systems and balancing airflow.
- Inspectors and energy auditors verifying rates against local code requirements.
Bathroom targets are usually stated directly in CFM rather than ACH — 50 CFM intermittent or 20 CFM continuous is the widely used baseline.
Key Terms and Variables
The calculator’s core inputs and what they mean:
Room volume
Length × width × ceiling height, in ft³ or m³. Measure to the finished walls — open alcoves count, sealed closets don’t; use average height for vaulted ceilings.
Air changes per hour (ACH)
How many times per hour the room’s air volume is replaced. A 1,000 ft³ bedroom at 5 ACH needs 5,000 ft³ of air moved per hour. ACH encodes the room’s purpose — kitchens and workshops need far more air changes than bedrooms.
CFM and m³/h
The airflow output. CFM (cubic feet per minute) is the US convention; m³/h is the metric convention on most fan nameplates outside North America (1 CFM ≈ 1.7 m³/h). Fans are sold by rated CFM or m³/h — this is the number you match to a product.
Intermittent vs continuous
Intermittent fans run when needed (a bathroom fan on a switch or timer); continuous fans run all day at low speed. Code minimums differ between the two — the bathroom baseline of 50 CFM intermittent drops to 20 CFM continuous, because 24-hour operation moves far more total air.
Make-up air
The replacement air that flows in as the fan exhausts stale air out. An exhaust fan can’t move its rated CFM if the house is sealed tight — the calculation assumes make-up air has a path in.
The Formulas Involved
The calculator runs a short chain: volume from dimensions, airflow from volume and ACH, then unit conversion — three steps.
Formula 1: Room volume
Volume = Length × Width × Height, all in consistent units: 12 ft × 10 ft × 8 ft = 960 ft³, or 3.66 m × 3.05 m × 2.44 m = 27.2 m³.
Formula 2: Required airflow
CFM = ACH × Volume (ft³) ÷ 60; m³/h = ACH × Volume (m³). The ÷ 60 converts hourly air changes to per-minute airflow — leaving it out inflates the answer sixtyfold.
Formula 3: Unit conversion
| Conversion | Factor |
|---|---|
| CFM → m³/h | × 1.699 (planning: × 1.7) |
| m³/h → CFM | ÷ 1.699 |
| ft³ → m³ | ÷ 35.315 |
| m³ → ft³ | × 35.315 |
| ft² → m² | ÷ 10.764 |
The formula-explained guide derives each step, including the ASHRAE 62.2 whole-house formula.
Worked Example: From Measurements to Fan Size
A full pass from tape measure to fan rating: a bedroom 12 ft × 10 ft with an 8 ft ceiling at 5 ACH.
Step 1 — Volume. 12 × 10 × 8 = 960 ft³.
Step 2 — Airflow. 5 × 960 ÷ 60 = 80 CFM.
Step 3 — Metric check. 80 × 1.7 ≈ 136 m³/h.
Step 4 — Select equipment. Choose a fan rated at least 80 CFM — an 80 CFM nominal fan on a long duct run may deliver less, so a 90–100 CFM model is the common real-world pick.
Metric cross-check: 27.2 m³ × 5 ACH = 136 m³/h — matching the imperial result.
Practical takeaway: the arithmetic gives the minimum; duct losses and noise preference push real-world selection one step up.
Room-by-Room Target Rates
Target ACH (or CFM) varies by room purpose — confirm these planning values against your local code:
| Room | Target rate (planning) | Notes |
|---|---|---|
| Bedroom | 5–6 ACH | Comfort and CO₂ control for sleeping occupants |
| Living room / family room | 5–6 ACH | Similar occupancy logic to bedrooms |
| Bathroom | 50 CFM intermittent / 20 CFM continuous | Moisture control; timer recommended |
| Kitchen (range hood) | 100 CFM per 10,000 BTU | Based on cooktop output, not room volume |
| Laundry room | 5–8 ACH | Moisture and lint; consider make-up air |
| Home office | 5–6 ACH | Same as bedroom for one occupant |
| Basement (finished) | 4–5 ACH | Moisture and radon dilution support |
Whole-house ventilation uses a different formula — ASHRAE 62.2’s floor-area-plus-occupants method — covered in the estimating guide and code-compliance guide.
Spot Ventilation vs Whole-House Ventilation
Spot ventilation answers “how much air for this room”; whole-house ventilation answers “how much fresh air for the entire dwelling” via the ASHRAE 62.2 area-plus-occupants formula. A house typically has both: bath and kitchen fans for source control plus a continuous low-rate system for baseline fresh air. Size each fan for its room, then verify the envelope can supply make-up air for the largest intermittent load — a 400 CFM range hood in a tight house can starve a 50 CFM bath fan on the same envelope.
Mistakes That Distort Results
Most errors are measurement errors, not arithmetic:
- Using floor area instead of volume. Forgetting ceiling height is the ACH equivalent of the classic estimating blunder.
- Mixing CFM and m³/h. A 100 m³/h fan is only about 59 CFM — ordering by the wrong unit leaves the room under-ventilated.
- Ignoring duct losses. Rated CFM is measured at a standard test pressure; a long or kinked duct run delivers less.
- No make-up air path. The calculation assumes replacement air can enter. In a tight house, it may not.
- Targeting the minimum in a high-moisture room. Bathrooms with no window or heavy shower use benefit from sizing above the baseline.
The full catalogue, with fixes, is in the common mistakes guide.
Related Guides
- Ventilation Estimating Guide — step-by-step whole-house estimation workflow.
- Ventilation Calculator FAQ — quick answers to the most asked questions.
- Ventilation Equipment Guide — fans, hoods, ERVs, and how to read nameplates.
- Pro Tips — noise, ducting, and placement advice from the trade.
- Ventilation Checklist — everything to verify before you buy and install.
Frequently Asked Questions
How many CFM do I need for a bedroom?
A typical 12 ft × 10 ft bedroom with 8 ft ceilings (960 ft³) at 5 ACH needs 80 CFM (≈ 136 m³/h). Round equipment selection up to the next standard fan size.
What is the formula for calculating ventilation CFM?
CFM = ACH × Room Volume (ft³) ÷ 60. In metric: m³/h = ACH × Room Volume (m³). Convert with 1 CFM ≈ 1.7 m³/h.
How many CFM does a bathroom fan need?
The widely used baseline is 50 CFM intermittent or 20 CFM continuous. Larger bathrooms or heavy shower use may justify sizing up.
How do I size a kitchen range hood?
Plan on 100 CFM per 10,000 BTU of cooktop output — a 40,000 BTU gas range suggests about 400 CFM, before duct length and make-up air.
What is a good ACH for a living room?
5–6 ACH is the standard planning range for living rooms and bedrooms — enough for comfort and pollutant dilution without excessive energy loss.
Should I use CFM or m³/h?
Use whichever unit your equipment is rated in — US fans are labeled CFM, most others m³/h. The calculator reports both.
Get Your Airflow Number
Ventilation estimating is straightforward arithmetic built on careful measurements: measure the room volume, pick a target rate suited to the space, apply the ACH formula, and select equipment one step above the minimum to cover duct losses. The Ventilation Calculator does the arithmetic — your job is the tape measure and the judgment calls. Run your numbers, sanity-check them against the room-by-room table, and confirm requirements with your local building authority before you buy.
Disclaimer: Calculator results are planning estimates based on the dimensions and rates you enter. Actual ventilation requirements vary with climate, construction, occupancy, and local code. Verify airflow targets with your local building authority (AHJ) and consult a licensed HVAC professional for system design decisions.
