Duct Leakage Test: Flow ↔ % Leakage Calculator

Advanced duct leakage calculator for field and lab workflows with clear results. Enter system airflow, measured leakage, or target percentage to compute instantly values. Scale flows between pressures using a selectable leakage exponent for realistic modeling. Built-in presets, validations, and unit switching reduce errors during testing. Export CSV and PDF, share links instantly anywhere.

Inputs

Quick Presets

one-click field population
Airflow (CFM)
Test Pressure
Target Leakage (%)
Leakage Exponent n
Bundles (pressure + target % + limit)
Bundles are illustrative presets. Use your project specification for compliance.
Total design or measured fan airflow.
%
Optional threshold to show pass/fail.
Measured by tester at the indicated pressure.
%
Percentage of system airflow considered leakage.
Pressure Scaling (optional) Q ∝ ΔPn
Pressure at which leakage flow was measured.
Common examples: 25 Pa, 50 Pa, 1 in. w.g.
Typical range 0.5–0.65; 0.60 default.
decimals: 2

Results

Leakage Percentage
—
of system airflow at standard/target pressure.
Leakage Flow @ Target Pressure
—
Shown in CFM and L/s.
Leakage Flow @ Measured Pressure
—
If scaling inputs were provided.
Computation Notes
Provide inputs and press Calculate.

Example Data

Sample systems to demonstrate calculations. Adjust numbers to your context.

# System Airflow % Leakage (target) Leakage Flow (target pressure) Test Pressure
1 AHU-1 Supply 4000 CFM 5% — 25 Pa
2 RTU-2 Supply 2500 CFM 8% — 25 Pa
3 Exhaust-3 1200 CFM 6% — 50 Pa
4 Zone-4 Return 10000 CFM 3% — 1 in. w.g.
5 Branch-5 1800 CFM 10% — 25 Pa
6 Shaft-6 7000 CFM 4% — 50 Pa

Formulas Used

  1. % Leakage = (Qleak / Qsystem) × 100
  2. Qleak = Qsystem × (% Leakage / 100)
  3. Pressure scaling: Q2 = Q1 × (ΔP2/ΔP1)n where n is the leakage exponent.
  4. Unit conversions: 1 CFM = 0.471947 L/s, 1 in. w.g. ≈ 249.0889 Pa.
  • Use target/standard pressure relevant to your specification or test method.
  • Set n between 0.5–0.65 for typical duct leakage scaling.

How to Use This Calculator

  1. Enter the total system airflow and choose units.
  2. Select the mode: compute percentage from measured flow, or vice versa.
  3. (Optional) Enter measured pressure, target pressure, and leakage exponent to scale flows.
  4. Set rounding and an optional compliance limit to show pass/fail.
  5. Press Calculate. Results appear with dual-unit outputs and notes.
  6. Export CSV for logs, download a PDF report, or copy a shareable link.

FAQs

It is the fraction of total airflow that leaks through unintended openings. It is computed as leakage flow divided by system airflow, multiplied by 100.

Follow your project specification or standard. Common values are 25 Pa or 50 Pa in residential tests and up to 1 in. w.g. in some commercial verifications.

The exponent n shapes how flow changes with pressure. Higher n increases sensitivity. A common assumption is 0.60, but some ducts may be closer to 0.5–0.65.

Yes. Select CFM or L/s for flow and choose Pa or inches water gauge for pressure. The calculator converts automatically and keeps results consistent.

Limits vary by code, standard, and project. Enter your required limit in the input to see a pass/fail indicator for your particular system.

It treats leakage as total unless you specifically test “to outdoors.” Provide the appropriate measured flow and system airflow corresponding to your test scope.

Yes. Use the CSV export for logs and the PDF button to capture a printable summary that includes inputs, results, and computation notes.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.