Duct Traverse Airflow Calculator

Concepts behind this calculation

Calculate area-weighted airflow from multiple measured duct velocities or Pitot velocity-pressure readings, with an explicit density basis.

A duct traverse estimates cross-sectional airflow from many local velocities. Each reading represents part of the open area, so the area-weighted mean velocity is multiplied by the finished inside area. The weights describe relative sampled areas, not confidence scores or the apparent quality of individual readings.

Pitot pressure must be converted to velocity point by point because velocity depends on the square root of velocity pressure. Averaging pressures first generally biases the resulting mean upward for nonuniform positive readings. The documented common probe coefficient K multiplies velocity outside the square root; direct velocity readings must already contain their applicable corrections.

How to use this calculator

1. Select Measured quantity and enter Finished inside duct area and Actual air density.
2. For Pitot mode, document Probe calibration and coefficient reference and enter Pitot velocity coefficient K.
3. Enter Measurement rows as each reading with its positive relative area weight, then review weighted velocity and airflow.

Calculation method and applicability

Pitot vᵢ = K × 4005√(VPᵢ × 0.075/ρ) FPM; v_mean = Σ(wᵢvᵢ)/Σwᵢ; actual CFM = area × v_mean. K is the documented common probe velocity coefficient, outside the square root; direct velocities are already corrected.

vᵢ = point velocity (FPM); VPᵢ = point velocity pressure (in. w.g.); ρ = total air density (lb/ft³); K = common probe velocity coefficient; wᵢ = relative sampled-area weight. K multiplies the square-root velocity, not VP inside the square root.

Input definitions and units

Measured quantity

Pitot mode converts each point before averaging; direct mode accepts already-corrected local velocity. Choices: Pitot velocity pressure (in. w.g.); Direct velocity (FPM).

Finished inside duct area (ft²)

Use clear inside cross-sectional area after any liner, not exterior sheet area.

Actual air density (lb/ft³)

Pitot conversion uses the stated density. Direct readings must already be corrected by the instrument.

Probe calibration and coefficient reference

Document the probe and velocity-coefficient source applicable to every reading. K multiplies velocity, not pressure. A coefficient of 1 requires an established standard-Pitot method; this field does not validate calibration. Applies to Measured quantity: Pitot velocity pressure (in. w.g.).

Pitot velocity coefficient K (dimensionless)

Positive velocity coefficient from the applicable probe calibration/manufacturer method. One common coefficient must apply across this traverse. It is not used on already-corrected direct velocities. Applies to Measured quantity: Pitot velocity pressure (in. w.g.).

Measurement rows (reading, area weight)

One reading and positive relative area weight per line, separated by a comma. Pitot readings are in in. w.g.; direct readings in FPM. Use weight 1 for equal-area points. Decimal point only; up to 500 rows.

How to interpret the result

The result is actual CFM for the entered open area and measurement basis. Point placement, probe alignment, flow disturbances, and instrument resolution still control how representative it is.

Worked examples

Assumptions and limits

• This interprets a supplied measurement set; it does not choose point locations or certify a traverse to AMCA/ASHRAE procedures.
• Weights must represent relative sampled areas. Do not use arbitrary statistical confidence weights or centerline velocity as an area average.
• Pitot mode requires an established common velocity coefficient K valid for this probe and every reading. Unity is illustrative, not a universal default. A varying calibration curve, different probes, compressibility or unsuitable pressure/velocity ranges require the instrument-specific method. Direct velocities must already be corrected; K is never applied twice.
• Actual CFM at the stated density is not standard CFM. Low pressure readings are especially sensitive to instrument resolution.

Sources

• DOE Nuclear Air Cleaning Handbook, Chapter 7, Ventilation System DesignHistorical DOE handbook, Chapter 7 Eq. 7.6: V=4005 K sqrt(pv) for the stated standard-air units. K is the tube calibration velocity coefficient. This is mathematical measurement support only; no nuclear/glovebox design, current standard certification or assumed calibration value is imported. Density correction retains the existing D14 basis.
• Dwyer Series 160 instruction manual, Pitot traverse methodManufacturer instructions require representative traverses, density correction where necessary and conversion of each pressure reading to velocity before averaging. The manufacturer method and probe calibration govern actual applicability; no instrument accuracy certificate is inferred.
• ASHRAE 2025 Fundamentals, Chapter 38: Measurement and InstrumentsMeasurement principles, multipoint traverse and instrument limitations. The tool does not reproduce a point-location standard or certify a field test.
• DOE/PNNL: measuring mechanical ventilation airflowGovernment field-measurement context; actual field procedure and instrument instructions govern.