Pressure loss and static pressure

Duct Static Regain Calculator

Use upstream and downstream air speeds, density, and total-pressure loss to calculate net static pressure change.

Calculation inputs

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Upstream average velocity (FPM) Use representative area-average velocity at the upstream measurement section.

Downstream average velocity (FPM) Use the matching downstream section; different flows or leaking sections need an explicit mass balance.

Air density (lb/ft³) This illustrative value corresponds approximately to 1.2 kg/m³. Use the actual air-density basis.

Actual total-pressure loss (in. w.g.) Measured or source-based loss for the real geometry and flow; no fitting K or recovery efficiency is invented.

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Result

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Formula and source basis

Concepts behind this calculation

Calculate the change in static pressure between two duct sections from velocity-pressure change and an explicitly supplied total-pressure loss.

When air slows through a duct section, some velocity pressure can become static pressure. The ideal available recovery is the decrease in velocity pressure between the two representative sections. Real total-pressure loss must be subtracted to find the net static change.

Static regain does not mean total pressure is created in a passive fitting. Total pressure falls through loss even when static pressure rises. A sufficiently lossy transition can therefore produce a negative static change despite lower downstream velocity. The calculation requires actual loss data rather than inferring fitting quality from velocity reduction alone.

How to use this calculator

Enter Upstream average velocity and Downstream average velocity for compatible sections.
Supply Air density and Actual total-pressure loss for the real geometry and flow.
Compare the available velocity-pressure recovery with the loss and the resulting signed static change.

Calculation method and applicability

ΔP_static = ρ(V₁²−V₂²)/2 − ΔP_total-loss, with consistent units

V₁ and V₂ = section-average velocities; ρ = air density. The kinetic-pressure expression requires consistent units; fields use FPM and lb/ft³ and the result is converted to in. w.g. Total loss is a positive magnitude; static change remains signed.

Input definitions and units

Upstream average velocity (FPM): Use representative area-average velocity at the upstream measurement section.
Downstream average velocity (FPM): Use the matching downstream section; different flows or leaking sections need an explicit mass balance.
Air density (lb/ft³): This illustrative value corresponds approximately to 1.2 kg/m³. Use the actual air-density basis.
Actual total-pressure loss (in. w.g.): Measured or source-based loss for the real geometry and flow; no fitting K or recovery efficiency is invented.

How to interpret the result

Read net static change separately from total-pressure change. This two-section balance can explain a transition, but it does not size a complete static-regain network.

Worked examples

Worked example

Inputs: Velocities approximately 10 and 5 m/s (1,968.504 and 984.252 FPM), air density 1.2 kg/m³, total-pressure loss 10 Pa.

Calculation: Ideal velocity-pressure recovery is 45 Pa. Subtract the 10 Pa total-pressure loss.

Result: About 35 Pa (0.141 in. w.g.) static rise, while total pressure falls by 10 Pa.

Assumptions and limits

This is a two-section pressure relationship, not a complete static-regain duct-sizing method.
Geometry-specific losses must come from the actual fitting or measurement; a velocity reduction alone does not fix recovery efficiency.
Use consistent total/static definitions and representative velocities. Do not add a static recovery term again if it is already included in supplied measured static data.

FAQs

Why is static change negative even though downstream velocity is lower?

The entered total-pressure loss exceeds the available decrease in velocity pressure. The flow decelerates, but that potential recovery does not overcome the loss.

Can I add the predicted regain to an already measured static-pressure change?

That would count the same effect twice if the measurement already spans these sections. Use the relationship to interpret compatible measurements or predict from independently supplied losses.

Sources

ASHRAE 2025 Fundamentals, Chapter 21: Duct DesignPressure definitions, velocity pressure, duct losses and design context.

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