How to use this calculator
- Take ACH50 from the report. Air changes at 50 Pa, already normalized by the tester to the conditioned volume.
- Pick the N factor. 20 to screen; nudge smaller for tall/exposed buildings and harsher climates, larger for short/sheltered ones.
- Enter the volume. The same conditioned volume the test used, so CFM50 and natural CFM stay consistent.
- Use the natural airflow. ACHnat and natural CFM feed load calculations and ventilation balances; the tightness band tells you whether sealing or equipment is the better next dollar.
How it works
A blower door measures leakage at an artificial 50 Pa — handy for testing, but no house lives at 50 Pa. The working conversion to real-weather average infiltration is a single division:
ACHnat ≈ ACH50 / N (N ≈ 10–30; 20 = the classic rule)
N packs climate severity, building height and wind
shielding into one empirical divisor from the LBL infiltration work.
The result drives two practical numbers this page also returns: the
natural airflow in CFM (what a load calculation or a
makeup-air balance wants) and
CFM50 (what the tester quoted).
Downstream: the ACH calculator covers the general room-air-change arithmetic, the CFM/ΔT calculator turns infiltration air into load, and the mini-split sizing calculator consumes the envelope verdict directly.
Worked example
Verified against the live calculator
A 1,500 ft² house with 8 ft ceilings (12,000 ft³) tests at
7 ACH50. With the screening N = 20:
ACHnat = 7 / 20 = 0.35 CFM50 = 7 × 12,000/60 = 1,400 natural ≈ 70 CFM
So the envelope leaks an average of about 70 CFM of outdoor air around the clock — roughly what a bathroom fan moves, except unfiltered, unheated and through the rim joist. Air-seal it to 3 ACH50 and the average drops to 30 CFM, which is when code mechanical ventilation stops being optional and starts being the design.
Frequently asked questions
How do I convert ACH50 to natural air changes?
Divide by an LBL-style correction factor: ACHnat ≈ ACH50 / N. The classic screening rule uses N = 20, so a 7 ACH50 house averages about 0.35 natural air changes per hour. Taller, wind-exposed homes use a smaller N (more natural infiltration); short sheltered ones a larger N.
What is a good ACH50 score?
Common anchors: ~3 ACH50 is the modern energy-code tier in much of the US, 5-7 is typical existing housing, above 7 is leaky, and 0.6 is the Passive House benchmark. The verdict bands here follow those anchors.
Why does the N factor change with height and shielding?
Natural infiltration is driven by stack effect and wind. A tall house in open country sees bigger driving pressures than a one-story house among trees, so the same 50 Pa leakage area leaks more air naturally — which means dividing by a SMALLER N.
Is natural infiltration the same as ventilation?
No. Infiltration is uncontrolled, weather-dependent and unevenly distributed. Tight homes (roughly under 3-5 ACH50) need code mechanical ventilation (ASHRAE 62.2-style), and infiltration credit toward that requirement is taken cautiously precisely because it disappears on mild days.
Method & assumptions
- LBL-style single-factor screening; hour-by-hour infiltration models supersede it in detailed energy work.
- ACHnat is an annual average — winter design infiltration runs higher and is handled inside the load-calculation method, not here.
- The N guidance (smaller for tall/exposed) is directional; programs that prescribe N tables (weatherization, code compliance) govern when they apply.
- Ventilation code credit for infiltration follows the adopted standard's rules, not this screen.