Ventilation / ACH Helper

Calculate air changes per hour for any room, estimate the CFM needed to hit a target ACH, and compare against ASHRAE 62.2 ventilation minimums.

Room & Airflow Inputs

Works in both directions — enter CFM or target ACH

Results

ACH, required CFM, and ASHRAE 62.2 comparison

Enter room dimensions to see results

Educational estimate only. ASHRAE 62.2 values shown are simplified residential guidelines. Actual ventilation requirements depend on building envelope, occupancy patterns, and local code. Consult a qualified IAQ professional for design or compliance work.

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What ACH (Air Changes per Hour) Means

ACH tells you how many times the entire volume of air in a room gets replaced with fresh (or recirculated) air every hour. A room with 1 ACH has its full air volume swapped once per hour; a room with 4 ACH swaps it four times. Ventilation ACH specifically tracks outdoor air brought in — it's a proxy for how quickly indoor pollutants, CO2, and excess humidity get diluted and removed.

How Much Fresh Air Does a Home Need?

ASHRAE 62.2, the residential ventilation standard, sets a baseline target of roughly 0.35 ACH for whole-house mechanical ventilation, with a floor that scales by house size and occupant count. Older, leakier homes often hit this target through uncontrolled air leaks alone (drafts around windows, gaps in the envelope). Newer, tightly sealed homes — built to modern energy codes — often fall short of 0.35 ACH naturally, which is exactly why they need a dedicated mechanical ventilation system (like an ERV or HRV) to stay within healthy air quality guidelines.

Natural ACH by Home Age vs. the ASHRAE Target

Older homes often clear the ventilation target through drafts and leaks alone; newer, tighter homes usually need mechanical help to get there.

Bar chart 0.75 Pre-1980 (leaky) 0.35 1990s-2000s 0.15 2015+ (tight) 0.35 ASHRAE 62.2 Target

Illustrative natural air-change rates by construction era, vs. the ASHRAE 62.2 mechanical ventilation target (highlighted).

Balancing Ventilation and Energy Efficiency

More ventilation isn't automatically better — every cubic foot of outdoor air you bring in has to be heated or cooled to match the rest of the house, which costs energy. The goal is hitting the ASHRAE target, not exceeding it by a wide margin. Energy and heat recovery ventilators (ERVs/HRVs) solve this by transferring heat (and in ERVs, some humidity) between the outgoing stale air and the incoming fresh air, so you get the air quality benefit without the full energy penalty of a simple exhaust fan.

Quick check: if your home was built or majorly renovated after 2010 and doesn't have a dedicated ventilation system, it's worth confirming your ACH against the ASHRAE 62.2 target using the calculator above.

Frequently Asked Questions

What is ACH in HVAC?
ACH stands for Air Changes per Hour — the number of times the total air volume in a space is replaced in one hour. Calculated as: ACH = (CFM × 60) ÷ room volume (ft³). A bedroom getting 75 CFM with a volume of 1,080 ft³ gets 75×60÷1,080 = 4.2 ACH. Building codes and ASHRAE standards specify minimum ACH for different occupancy types.
How many air changes per hour does a home need?
ASHRAE 62.2 (residential ventilation) requires a minimum of 0.35 ACH of mechanical ventilation for the whole house, or 7.5 CFM per person + 0.01 CFM/sqft — whichever is greater. Typical residential HVAC systems deliver 4–8 ACH during operation, but run intermittently. For continuous ventilation in tight homes, an ERV or HRV is recommended.
What is the minimum ACH for a bedroom?
ASHRAE 62.2 doesn't set a per-room minimum for sleeping areas in residential buildings — it applies to the whole dwelling unit. However, building scientists often target 4+ ACH in bedrooms during operation to maintain comfort. Bathrooms require 50 CFM or 20 ACH (whichever is less) for spot exhaust ventilation per ASHRAE 62.2.
How does ACH affect indoor air quality?
Higher ACH dilutes indoor pollutants (CO₂, VOCs, allergens) and reduces odors. In very tight modern homes (0.1–0.3 ACH natural infiltration), mechanical ventilation is essential to maintain acceptable CO₂ levels. ASHRAE 62.2 targets keep CO₂ below 1,100 ppm and dilute other pollutants to safe levels. More ACH is not always better — over-ventilating wastes energy and can cause humidity problems in humid climates.
What is the difference between ACH and ACHN?
ACH (or ACH50) is measured during a blower door test at 50 Pascals of pressure — it's a measure of envelope airtightness. ACHN (natural ACH) adjusts the blower door result to estimate actual natural infiltration under normal conditions using a climate-specific N factor (typically 15–20). Modern energy codes target ACH50 ≤ 3 for new construction, which translates to roughly 0.15–0.20 ACHN.