Enter project data
Example data table
| Project | Mode | Volume (ft³) | Airflow (CFM) | N-factor | Test ACH | Estimated Natural ACH |
|---|---|---|---|---|---|---|
| Townhouse shell | Blower door | 18,000 | 950 | 20 | 3.167 | 0.158 |
| Primary classroom | Blower door | 32,400 | 2,100 | 18 | 3.889 | 0.216 |
| Warehouse bay | Direct airflow | 120,000 | 900 | Not used | 0.450 | 0.450 |
Formula used
These equations support common airtightness reviews, blower door interpretation, and envelope quality checks during construction planning and verification.
Building Volume = Length × Width × Average Height
Test ACH = (Input Airflow CFM × 60) ÷ Building Volume
Estimated Natural Airflow = Blower Door CFM50 ÷ N-factor
Estimated Natural ACH = (Estimated Natural Airflow × 60) ÷ Building Volume
Minutes per Air Change = 60 ÷ Estimated Natural ACH
Leakage per Floor Area = Estimated Natural Airflow ÷ Floor Area
Test Flow per Envelope Area = Input Airflow ÷ Envelope Area
When direct operating airflow is entered, the calculator treats that airflow as the natural airflow condition and skips blower door conversion.
How to use this calculator
- Choose Blower door conversion when your airflow value is measured at a test pressure, usually 50 Pa.
- Choose Direct operating airflow when your airflow already represents expected in-service leakage conditions.
- Select whether building volume should be calculated from dimensions or entered directly from design documents.
- Enter airflow, pressure, N-factor, floor area, envelope area, and your target ACH.
- Press Calculate ACH to show the result block above the form.
- Review test ACH, estimated natural ACH, minutes per air change, and improvement guidance.
- Use the export buttons after calculation to save a CSV summary or a PDF report.
- Compare the outcome against project specifications, energy goals, or airtightness acceptance thresholds.
Frequently asked questions
1. What does ACH mean in an air leakage review?
ACH means air changes per hour. It shows how many times the building air volume is replaced in one hour because of infiltration or measured test airflow.
2. Why does blower door data need an N-factor?
Blower door testing often occurs at an artificial pressure like 50 Pa. The N-factor converts that pressurized airflow into an estimated natural leakage rate for normal operation.
3. Should I use direct volume or dimensions?
Use direct volume when design documents already provide reliable conditioned volume. Use dimensions when you need the calculator to derive volume from basic geometric inputs.
4. What is a good ACH value?
A lower natural ACH usually means a tighter envelope. Acceptable levels depend on code, occupancy, ventilation strategy, and project performance targets.
5. Why include floor area and envelope area?
These optional values normalize leakage intensity. They help compare projects of different sizes using leakage per floor area or test flow per envelope area.
6. Can this calculator replace a certified field test?
No. It is a planning and interpretation tool. Certified compliance decisions should still rely on approved test procedures, calibrated equipment, and project-specific requirements.
7. What if my calculated ACH is above target?
The result section shows how far the estimate exceeds your target and approximates the airflow reduction needed. Use that information to prioritize sealing work.
8. Does pressure difference affect the formulas here?
Pressure is recorded for reporting clarity. In blower door mode, the entered airflow and N-factor drive the ACH conversion, while pressure documents the test basis.