Airflow measurement and conversion

Airflow K-Factor Calculator

Apply a documented pickup coefficient and pressure signal to calculate airflow, required signal, or apparent coefficient.

Calculation inputs

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An example is loaded automatically. Replace it with your project values; the result updates as you edit.

Pickup calculation Three directions use one documented square-root pickup relation. The coefficient check is a single observation, not a calibration procedure.

Coefficient density treatment Use the already-applicable mode when K or controller output already matches the required conditions. Select density correction only when the actual method permits the square-root density model; avoid double correction.

Documented source airflow coefficient (CFM/√(in. w.g.)) Use the dimensional flow coefficient for the exact pickup and flow convention. It is not a dimensionless fitting-loss K, velocity multiplier, nominal diameter or arbitrary material constant.

Measured pickup differential pressure (in. w.g.) Use a positive differential from the specified pickup ports, within the documented signal range. Do not substitute terminal pressure drop, duct static pressure or a generic Pitot reading. Negative readings and zero are not treated as valid pickup flow measurements.

Minimum valid pickup signal (in. w.g.) Strictly positive lower bound established for the actual pickup/instrument/method. The formula does not establish low-flow resolution, uncertainty or control stability. This illustrative range is not a manufacturer rating.

Maximum valid pickup signal (in. w.g.) Upper bound from the same source and conditions, at least the lower bound. Measured or required signals outside this interval are withheld, not clipped or extrapolated.

Pickup, coefficient and range reference Identify pickup/controller model, revision, ports, units, flow convention, coefficient conditions and valid signal interval. For density correction, also document its permitted model. For a coefficient check, identify the independent airflow reference and matched state. This note is not authenticated.

Worked-example data and reference

Constructed pickup example: K=900 CFM/sqrt(in. w.g.) already applicable to the stated flow convention and air conditions; valid signal window 0.01–1 in. w.g. Replace with the exact pickup/controller source,units,configuration and valid range.

These example values are for the worked calculation. Replace them with the source and conditions for your own case.

Pickup calculation basis Confirm exact configuration and units, matched flow/signal conditions, the allowed density treatment and independently established signal range. This does not certify field calibration.

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Result

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Formula and source basis

Concepts behind this calculation

Convert a documented differential-pressure pickup signal to airflow, find a target signal, or check a single-point apparent airflow coefficient, with an explicit valid signal range and optional source-permitted density correction.

An airflow pickup coefficient connects the specified port differential to airflow through a square-root relation. The dimensional coefficient carries CFM per square root of pressure, unlike a dimensionless fitting-loss coefficient. Doubling a target flow therefore requires four times the differential when the effective coefficient is unchanged. The usable signal window comes from the actual pickup and instrument, rather than from the square-root equation.

A coefficient may already apply to the stated flow convention and air condition. Where the source permits it, the optional density factor adjusts the coefficient using the square root of reference density divided by actual density. Applying that factor again to an already corrected coefficient would double-correct it. The apparent-coefficient route uses one independently established flow and matching signal, which provides a single comparison point rather than a calibration curve.

How to use this calculator

Choose Pickup calculation and Coefficient density treatment, then document Pickup, coefficient and range reference and establish Pickup calculation basis.
Enter Documented source airflow coefficient with Measured pickup differential pressure or Target airflow as required; for a coefficient check, supply independently measured airflow and matching differential.
Enter Source coefficient reference density and Actual pickup inlet density only for the permitted density correction. Supply Minimum and Maximum valid pickup signal for all methods.
Review supported signals and the source/effective coefficient distinction; obtain matching evidence rather than clipping an out-of-range result.

Calculation method and applicability

fρ=1 for an already-applicable coefficient,or √(ρreference/ρactual) for a permitted correction. Keffective=Ksource fρ; Q=Keffective√ΔP; ΔP=(Q/Keffective)²; Ksource,apparent=Qobserved/(√ΔP fρ). K units:CFM/√(in. w.g.).

Ksource/Keffective = pickup coefficient (CFM/√in. w.g.); Q = airflow on the selected basis (CFM); ΔP = positive pickup signal (in. w.g.); fρ = dimensionless permitted density correction.

Input definitions and units

Pickup calculation: Three directions use one documented square-root pickup relation. The coefficient check is a single observation, not a calibration procedure. Choices: Airflow from pickup signal; Signal for a target airflow; Single-point apparent coefficient.
Coefficient density treatment: Use the already-applicable mode when K or controller output already matches the required conditions. Select density correction only when the actual method permits the square-root density model; avoid double correction. Choices: K already applies to stated conditions; Documented source-to-actual density correction.
Documented source airflow coefficient (CFM/√(in. w.g.)): Use the dimensional flow coefficient for the exact pickup and flow convention. It is not a dimensionless fitting-loss K, velocity multiplier, nominal diameter or arbitrary material constant. Applies to Pickup calculation: Airflow from pickup signal or Signal for a target airflow.
Measured pickup differential pressure (in. w.g.): Use a positive differential from the specified pickup ports, within the documented signal range. Do not substitute terminal pressure drop, duct static pressure or a generic Pitot reading. Negative readings and zero are not treated as valid pickup flow measurements. Applies to Pickup calculation: Airflow from pickup signal or Single-point apparent coefficient.
Target or independently measured airflow (CFM on selected basis): Target mode: flow desired on the selected basis. Coefficient mode: independently established airflow matched to the same pickup operating point. Density-corrected mode uses actual inlet CFM; already-applicable mode uses its stated source convention. Applies to Pickup calculation: Signal for a target airflow or Single-point apparent coefficient.
Source coefficient reference density (lbm/ft³): Explicit reference total-air density associated with the original coefficient and permitted correction model. The example is not a selected standard or default. Applies to Coefficient density treatment: Documented source-to-actual density correction.
Actual pickup inlet density (lbm/ft³): Independently established actual total-air density at the measurement state. No weather, elevation, temperature or humidity lookup is applied. Applies to Coefficient density treatment: Documented source-to-actual density correction.
Minimum valid pickup signal (in. w.g.): Strictly positive lower bound established for the actual pickup/instrument/method. The formula does not establish low-flow resolution, uncertainty or control stability. This illustrative range is not a manufacturer rating.
Maximum valid pickup signal (in. w.g.): Upper bound from the same source and conditions, at least the lower bound. Measured or required signals outside this interval are withheld, not clipped or extrapolated.
Pickup, coefficient and range reference: Identify pickup/controller model, revision, ports, units, flow convention, coefficient conditions and valid signal interval. For density correction, also document its permitted model. For a coefficient check, identify the independent airflow reference and matched state. This note is not authenticated.
Pickup calculation basis: Confirm exact configuration and units, matched flow/signal conditions, the allowed density treatment and independently established signal range. This does not certify field calibration. Choices: Not established; Pickup conditions and signal range documented.

How to interpret the result

Read the calculated flow or required signal on its selected basis and within the documented pressure interval. Endpoint model flows describe that interval only. A one-point apparent source coefficient should remain attached to its observed operating condition.

Worked examples

Airflow from pickup signal

Inputs: Constructed K=900 CFM/√(in. w.g.), already applicable at the stated conditions; pickup differential 0.25 in. w.g.; documented illustrative window 0.01–1 in. w.g.

Calculation: Q=900√0.25=450 CFM. Model flows at the stated lower/upper signals are 90 and 900 CFM. These endpoints are not equipment control or capacity ratings.

Result: 450 CFM at stated basis; the 0.25 in. w.g. signal is inside the entered range.

Signal for a target airflow

Inputs: Target 450 CFM on the stated coefficient basis; K=900 CFM/√(in. w.g.); valid signal window 0.01–1 in. w.g.

Calculation: ΔP=(450/900)²=0.25 in. w.g. Compare with the independently established signal window.

Result: 0.25 in. w.g. required signal; no damper position or achievable equipment operating point is inferred.

Single-point apparent coefficient

Inputs: Independent matched observation: 450 CFM and 0.25 in. w.g., without additional density correction; illustrative valid signal window 0.01–1 in. w.g.

Calculation: Apparent K=450/√0.25=900 CFM/√(in. w.g.). One matched point does not establish calibration over a range or authorize controller changes.

Result: 900 CFM/√(in. w.g.) apparent coefficient from one observation.

Assumptions and limits

Use the dimensional coefficient for the exact pickup and flow convention. Fitting-loss coefficients, dimensionless Pitot corrections, nominal duct area and transducer electrical scaling are different quantities. No generic size-to-K table is used.
The already-applicable mode preserves the stated source flow convention; no ACFM/SCFM conversion is inferred. Density mode applies the documented square-root reference/actual model to actual inlet volume only when permitted for that sensor/method. Entered densities are not automatically standard air, and an already corrected coefficient must not be corrected again.
The actual pickup/instrument method must establish a strictly positive valid pressure interval. Zero, negative or out-of-range signals are withheld. Passing the entered interval does not verify sensor accuracy, installation quality, swirl, straight-run conditions, low-flow stability or a controller range.
The apparent coefficient uses one independent simultaneous or otherwise matched flow/signal observation. It does not establish repeatability, linearity, uncertainty, a multi-point calibration or validity elsewhere; no controller setting or adjustment is prescribed.
No leakage correction, thermal measurement, pressure-port subtraction, unit inference, nominal terminal selection, fan/system operating point, compressibility/variable-density field solver or field/code certification. Product-specific coefficients and all reference conditions remain user-established.

FAQs

Can I use terminal pressure drop as the pickup differential?

Use the differential from the specified pickup ports. Terminal pressure loss and sensing-port differential describe different physical measurements, even when both are reported in inches of water.

Can I update the controller K-factor from one apparent coefficient?

A single matched observation does not establish repeatability, signal linearity or validity over the operating range. Review configuration and independent measurement evidence through the applicable calibration procedure before making any control adjustment.

Sources

Titus AeroCross K-Factors, Installation Manual KFACTOR-IOM-1.0, February 11, 2016Primary pickup relation Q=K√ΔP and inverse, with pressure in in. w.g. and dimensional CFM coefficient. Actual sensor type, size and application determine the coefficient; none of its catalog values is a generic default.
Nailor Airflow Sensor K-Factors for VAV Terminal Units, IOM-VAVK, January 23, 2024Distinguishes sensor differential pressure, standard-air flow coefficient, amplification factor and nominal area. Coefficients vary by exact model/application; the tool does not transfer a Nailor table to another pickup.
Trane Performance Climate Changer Air Handler, CLCH-SVX010E-EN, p. 57Fan-inlet measurement section gives Q=K√ΔP and a specific standard-to-actual density correction; it also distinguishes field-applicable K and other measuring systems. The optional general reference-density form requires the actual source/method to permit that same square-root model; it is not automatic permission for every VAV sensor.

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