Formula Used
The calculator first finds the orifice area:
A = π × d² / 4
Then it estimates measured leakage airflow through the orifice:
Q = Cd × A × √(2 × ΔPo / ρ)
It normalizes leakage to the selected reference pressure:
Qref = Q × (Pref / Ptest)n
The leakage index divides normalized flow by duct surface area:
Leakage index = Qref × 1000 / Surface area
The leakage percentage compares normalized leakage with system airflow:
Leakage % = Qref / System airflow × 100
For the statistical estimate, pressure repeatability is converted to approximate flow repeatability:
Relative flow deviation ≈ 0.5 × Pressure standard deviation / Orifice pressure difference
How to Use This Calculator
- Enter the orifice diameter used during the leakage test.
- Enter the pressure difference measured across the orifice.
- Set the discharge coefficient for the orifice device.
- Enter air density, duct surface area, and test pressure.
- Choose a reference pressure for normalized leakage reporting.
- Add repeated reading count and pressure deviation for uncertainty.
- Press the calculate button to view results above the form.
- Download the CSV or PDF report for records.
Example Data Table
| Duct Section |
Diameter |
Orifice ΔP |
Test Pressure |
Surface Area |
Leakage Result |
| Main supply |
50 mm |
120 Pa |
250 Pa |
40 m² |
About 0.42 L/s/m² |
| Return branch |
45 mm |
95 Pa |
250 Pa |
35 m² |
About 0.32 L/s/m² |
| Riser duct |
65 mm |
180 Pa |
300 Pa |
55 m² |
About 0.63 L/s/m² |
| Zone duct |
40 mm |
70 Pa |
200 Pa |
28 m² |
About 0.27 L/s/m² |
Understanding Oriflow Duct Leakage
Duct leakage testing checks how much air escapes from sealed ductwork during a controlled pressure test. An oriflow duct leakage calculator uses an orifice reading to estimate the airflow passing through leaks. It is useful for balancing reports, commissioning records, and quality checks. The method needs measured pressure, orifice size, air density, surface area, and a correction pressure. These values convert a field reading into a normalized leakage rate.
Why the Result Matters
Leakage affects energy use and comfort. Air that escapes before reaching rooms increases fan load. It can also cause poor ventilation in distant zones. A normalized result helps compare different duct sections. Large ducts and small ducts can be judged fairly when leakage is divided by surface area. The percentage result also shows how much leakage exists compared with design airflow.
Statistical View
Field readings can move because gauges fluctuate. The calculator includes reading count and pressure deviation. It estimates repeatability uncertainty from those values. This is not a laboratory certificate. It is a practical field confidence range. A narrow range means the readings were stable. A wide range means the test should be repeated or checked for loose tubes, unstable fans, or changing test pressure.
Best Practice Notes
Use sealed access panels before testing. Confirm the orifice plate is clean. Keep hoses straight and dry. Measure air temperature if density is adjusted. Enter the same pressure units across the form. Use the pressure exponent supplied by your test method when available. A common value is entered by default for typical duct leakage behavior. Save the report after each section. The CSV file helps spreadsheets. The PDF file helps project records. Always compare results with project specifications, not only with a generic label. Good notes make future inspections easier and faster.
Interpreting Results
Treat the leakage class as a screening guide. Review the numeric rate first. A small duct may show a large percentage while still leaking little total air. A large duct may look acceptable by percentage but waste more air overall. Check both views together. Repeat the test after repairs. Record every pressure setting, reading count, and correction used. Store technician initials and site notes with each exported file for traceability.
FAQs
What does this calculator estimate?
It estimates duct leakage airflow from orifice pressure data. It also reports normalized leakage, leakage index, percentage leakage, and a simple confidence range.
What is orifice pressure difference?
It is the pressure drop across the orifice device during the test. The calculator uses this value to estimate airflow through the test setup.
Why is air density included?
Air density affects the flow equation. Lower density can increase calculated flow for the same pressure difference. Use a project value when available.
What does pressure exponent mean?
It describes how leakage changes with pressure. A value near 0.65 is often used for duct leakage estimates when project guidance is unavailable.
What is leakage index?
Leakage index is normalized leakage divided by duct surface area. It helps compare duct sections of different sizes using one practical rate.
Why add reading count?
Reading count supports a repeatability estimate. More stable repeated readings usually reduce the standard error and narrow the confidence range.
Can I export the result?
Yes. Use the CSV button for spreadsheet records. Use the PDF button for a simple project report that can be saved or shared.
Is the leakage class a final standard?
No. It is a quick screening label. Always compare the final leakage rate with your contract, code, commissioning plan, or test specification.