Whole-House Ventilation Calculator
This free whole-house ventilation calculator answers a question tight modern homes can no longer leave to chance: how much mechanical fresh air the house needs. It computes the ASHRAE 62.2-2019 total rate — 0.03 times conditioned floor area plus 7.5 times bedrooms-plus-one — and, honestly, shows it next to the IRC’s Equation 15-1 number, which uses a 0.01 coefficient and a 30 CFM floor. Those two are not the same, and most online tools quietly pick one; this one surfaces the disagreement so you can size to whichever code your jurisdiction has adopted, defaulting to the stricter ASHRAE figure.
It also handles the details that matter for a real system. Pick a balanced HRV or ERV, exhaust-only, or supply-only system — balanced is the gold standard because it supplies and exhausts equally without pressurizing the house. Run the fan continuously or intermittently, and the calculator boosts the required capacity by the IRC run-time factor (25 percent run time means four times the flow). If you have a blower-door test, turn on the infiltration credit and enter your CFM50 result: the tool applies the correct 2019 superposition method (Equation 4-2 with the Equation 4-3 infiltration estimate), not the deprecated 2010 shortcut that so many sources still quote.
The output is the continuous CFM to design for, the intermittent fan capacity if you run part-time, and the infiltration credit if you took it — plus notes on the balanced-system reduction and the ASHRAE de minimis threshold, below which no system is required. No pricing, no signup. It is part of a growing HVAC and ventilation section alongside the bathroom exhaust fan and range hood calculators.
Whole-House Ventilation Calculator
How much fresh air does your house need? Enter the floor area and bedroom count and this calculator returns the continuous ventilation rate under ASHRAE 62.2-2019 — and, honestly, next to the IRC’s different number, because the two codes use different coefficients (0.03 vs 0.01) and most tools quietly pick one. Optionally apply a blower-door infiltration credit using the correct 2019 method. Free, no signup.
Home
Whole-house ventilation is sized to conditioned floor area and bedroom count (occupants are assumed to be bedrooms + 1).
System & operation
A balanced HRV/ERV supplies and exhausts equally; exhaust-only can be a continuous fan. Run a fan part-time by boosting its capacity.
Infiltration credit (optional)
If you have a blower-door test, ASHRAE 62.2-2019 lets you subtract natural infiltration (Eq 4-2). Leave off for the full rate.
Calculation Formulas
The continuous whole-house rate in CFM. Floor area is in square feet; occupants are assumed to equal bedrooms + 1. This is the stricter, and default, number.
Example:
2,000 sq ft, 3 BR → 0.03 × 2000 + 7.5 × 4 = 60 + 30 = 90 CFM.
The IRC uses a smaller floor-area coefficient (0.01 vs 0.03) because it bakes in an assumption of more natural infiltration — a genuine conflict with ASHRAE that this tool shows rather than hides.
Example:
2,000 sq ft, 3 BR → 0.01 × 2000 + 7.5 × 4 = 20 + 30 = 50 CFM.
Estimates natural infiltration in CFM from a blower-door leakage result (Q50, the CFM at 50 Pa), a weather/shielding factor (wsf), and the building height (H). The 2019 method, not a flat percentage.
Example:
Q50 = 1,500, wsf = 0.55, 1 story → about 43 CFM.
Subtracts infiltration from the total rate by SUPERPOSITION, not simple subtraction. Φ = 1 for a balanced system; Qinf/Qtot for unbalanced (exhaust- or supply-only). Aext = 1 for a detached home.
Example:
90 − (43/90) × 43 ≈ 70 CFM for an exhaust-only system.
A fan that runs only part of each hour must move more air when it runs. IRC Table M1505.4.3(2): 50% run time → ×2, 33% → ×3, 25% → ×4.
Example:
90 CFM continuous at 25% run time → 360 CFM fan.
The IRC allows a 30% reduction when the system is balanced AND ducted to each bedroom plus a main living space — a reward for good air distribution.
Example:
50 CFM × 0.70 = 35 CFM.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| ASHRAE coefficient | 0.03 / sq ft | Floor-area term in the ASHRAE 62.2-2019 total-rate formula. |
| IRC coefficient | 0.01 / sq ft | Floor-area term in IRC Equation 15-1 (assumes more infiltration). |
| Occupant term | 7.5 × (Nbr + 1) | Per-person airflow; occupants are assumed to be bedrooms + 1. |
| IRC minimum | 30 CFM | The IRC floor for a whole-dwelling rate. |
| ASHRAE de minimis | 15 CFM | If the required fan flow after credit is ≤ 15 CFM, no system is required. |
| Blower-door reference | 50 Pa | Q50 is the measured leakage in CFM at 50 pascals of pressure. |
| Reference height | 8.2 ft | Hr in the infiltration equation (Eq 4-3). |
| Run-time factor | ×2 / ×3 / ×4 | Multipliers for 50% / 33% / 25% intermittent operation. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
ASHRAE 62.2-2019 — Residential Ventilation(ASHRAE 62.2)
View StandardThe residential ventilation standard that defines the whole-house rate and the optional blower-door infiltration credit.
Key Requirements:
- •§4.1.1 total rate: Qtot = 0.03 × Afloor + 7.5 × (Nbr + 1).
- •§4.1.2 infiltration credit by superposition (Eq 4-2/4-3), not a flat percentage.
- •De minimis: no system required if the credited fan flow is ≤ 15 CFM.
IRC M1505.4.3 — Whole-House Mechanical Ventilation(IRC M1505.4.3)
View StandardThe International Residential Code whole-dwelling ventilation requirement, using Equation 15-1 and intermittent run-time multipliers.
Key Requirements:
- •Qr = 0.01 × Afloor + 7.5 × (Nbr + 1), not less than 30 CFM.
- •Intermittent operation multiplied per Table M1505.4.3(2).
- •30% reduction allowed for a balanced, fully ducted system.
ASHRAE 62.2 Appendix B — Weather & Shielding(ASHRAE 62.2 App B)
View StandardProvides the weather/shielding factor (wsf) used in the infiltration estimate, which varies by climate and site exposure.
Key Requirements:
- •wsf is location-specific; this tool uses a 0.55 default.
- •Building height enters as (H / 8.2)^0.4.
- •A blower-door (Q50) test is required to take the credit.
California Title 24 — Ventilation(Title 24)
View StandardCalifornia adopts ASHRAE 62.2 with amendments, including removing the blower-door requirement for the ventilation calculation.
Key Requirements:
- •Adopts 62.2-2019 with state amendments.
- •Eliminates the blower-door step for the Qfan calculation.
- •Balanced ventilation encouraged in tight new construction.
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
ASHRAE vs. IRC Coefficient
0.03 vs 0.01 — a real conflict
ASHRAE multiplies floor area by 0.03; the IRC by 0.01, because the IRC assumes more natural infiltration is already present. On a 2,000 sq ft home that is a 40 CFM difference. Size to whichever your jurisdiction enforces.
Regional Examples:
The Infiltration Credit
Superposition, not a flat percentage
The 2019 credit uses a blower-door Q50 in a superposition equation (Eq 4-2/4-3). The popular "reduce by half the excess above 2 CFM per 100 sq ft" shortcut is from the 2010 edition / the existing-building path and must not be used for 2019 new-construction compliance.
Regional Examples:
System Type
Balanced vs. unbalanced
A balanced HRV/ERV supplies and exhausts equally and does not pressurize the house, taking full credit (Φ = 1). Exhaust- or supply-only systems interact with infiltration and take a reduced credit, and a balanced fully ducted system earns the IRC 30% reduction.
Regional Examples:
Climate & Tightness
Tight homes need more mechanical air
A leaky older home may already meet much of the rate through infiltration; a tight, energy-efficient new home has almost none and needs the full mechanical rate. Humid climates favor balanced ERV to manage moisture with the fresh air.
Regional Examples:
California Title 24
Amended 62.2
California adopts ASHRAE 62.2-2019 but removes the blower-door requirement for the calculation, effectively using the full total rate, and pushes balanced ventilation in tight new construction.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
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How to Use This Calculator
- Enter the conditioned floor area in square feet and the number of bedrooms (occupants are assumed to be bedrooms + 1).
- Pick your ventilation system: balanced HRV/ERV, exhaust-only, or supply-only.
- Choose continuous operation, or an intermittent run time (25/33/50%) to see the boosted fan capacity.
- If you have a blower-door test, turn on the infiltration credit and enter your CFM50 result and number of stories.
- Click Calculate to see the ASHRAE rate next to the IRC rate, the recommended continuous CFM, and the intermittent fan size.
- Size your fan or HRV/ERV to the recommended number for the code your jurisdiction enforces, and read the notes on distribution and credits.
ASHRAE vs. the IRC — why two numbers, and which to use
The two codes disagree on purpose. ASHRAE 62.2-2019 multiplies floor area by 0.03; the IRC multiplies by 0.01 and adds a 30 CFM floor, because the IRC assumes a home already gets more air through natural infiltration. On a 2,000 sq ft, three-bedroom home that is 90 CFM (ASHRAE) versus 50 CFM (IRC) — a real difference that changes the fan or HRV you buy. This calculator shows both and defaults to the stricter ASHRAE figure, but you should size to whichever code your building department has adopted. The optional infiltration credit is where most sources go wrong: the correct 2019 method uses a blower-door Q50 measurement in a superposition equation (Eq 4-2 with the Eq 4-3 infiltration estimate), not the old "reduce by half the excess above 2 CFM per 100 sq ft" rule, which belongs to the 2010 edition and the existing-building path. This tool sizes the ventilation rate; it does not design the ductwork or select a specific HRV/ERV model, and California Title 24 removes the blower-door step, so always confirm your local code.
Frequently Asked Questions
How much fresh air does my house need?
Under ASHRAE 62.2-2019, the continuous whole-house rate is 0.03 times the conditioned floor area plus 7.5 CFM per occupant, where occupants are assumed to be bedrooms plus one. A 2,000-square-foot, three-bedroom home needs 0.03 × 2000 + 7.5 × 4 = 90 CFM. The IRC gives a lower number (50 CFM for the same house) because it uses a smaller coefficient. Tight modern homes need this mechanical fresh air because they no longer leak enough on their own to dilute indoor pollutants and moisture.
Why do ASHRAE and the IRC give different ventilation rates?
Because they use different floor-area coefficients — ASHRAE 62.2 multiplies by 0.03, the IRC by 0.01. The IRC's smaller number bakes in an assumption that the house already receives more air through natural infiltration, so it asks for less mechanical ventilation. For a 2,000-square-foot, three-bedroom home that is 90 CFM (ASHRAE) versus 50 CFM (IRC). Neither is 'wrong' — you size to whichever code your jurisdiction has adopted. This calculator shows both and defaults to the stricter ASHRAE figure so you don't accidentally under-ventilate.
What is the ASHRAE 62.2 infiltration credit, and how does it work?
It lets you subtract the fresh air your house already gets through leakage, measured by a blower-door test, from the required mechanical rate. The correct 2019 method estimates infiltration from your CFM50 result (Qinf = 0.052 × Q50 × weather-shielding factor × height term) and then subtracts it by superposition, not simple subtraction (Qfan = Qtot − Φ × Qinf). A very common error is the old 'reduce by half the excess above 2 CFM per 100 square feet' rule — that belongs to the 2010 edition and the existing-building path, not 2019 new construction. A tight home has little infiltration, so the credit is small.
What is the difference between a balanced, exhaust, and supply ventilation system?
A balanced system (an HRV or ERV) supplies and exhausts equal amounts of air, so it does not pressurize or depressurize the house and it can recover heat or moisture from the outgoing air — the gold standard. Exhaust-only just pulls stale air out (a continuous bath fan is the simplest version) and lets replacement air leak in wherever it can. Supply-only pushes filtered fresh air in. Unbalanced systems interact with natural infiltration, so ASHRAE gives them a smaller credit, and the IRC rewards a balanced, fully ducted system with a 30 percent rate reduction.
Can I run a ventilation fan intermittently instead of 24/7?
Yes — you just have to move more air when it runs. The IRC (Table M1505.4.3-2) lets you run a fan part-time if you multiply the continuous rate by a run-time factor: 50 percent run time needs twice the flow, 33 percent three times, and 25 percent four times. So a 90 CFM continuous requirement becomes a 360 CFM fan running 25 percent of each hour. Continuous operation at a lower rate is usually quieter and steadier for indoor air quality, but intermittent operation on a timer or control lets a single larger fan do the job.
Does a tight, energy-efficient home need more ventilation?
Yes — that is exactly why whole-house ventilation exists. Older leaky homes exchange a lot of air through gaps in the envelope, which is wasteful for energy but does dilute indoor pollutants. A tight, well-sealed modern home has almost no natural infiltration, so it needs a deliberate mechanical system to supply fresh air and remove moisture, cooking and cleaning byproducts, and carbon dioxide. The tighter the house (a lower blower-door number), the smaller the infiltration credit and the more the mechanical system has to carry — a balanced HRV or ERV is the efficient way to do it.