Flue Gas Flow Calculator

Inputs

Enter values, then press Calculate. Fields are arranged as 3 columns on large screens, 2 on tablets, and 1 on mobile.

Example data table

These examples demonstrate typical flue gas sizing checks.

Formula used

• Flow area (circular): A = π·D²/4
• Actual volumetric flow: Qₐ = V·A
• Standard (wet) volumetric flow (ideal gas): Qₛ = Qₐ·(Pₐ/Pₛ)·(Tₛ/Tₐ)
• Standard (dry) flow (optional): Qₛ,dry = Qₛ·(1 − xw)
• Density estimate (ideal gas): ρ = P·MW/(R·T)
• Mass flow: ṁ = ρ·Qₐ
• Reynolds number (indicative): Re = ρ·V·D/μ

Where: T is absolute temperature (K), P is absolute pressure, MW is molecular weight, R is the universal gas constant, and μ is dynamic viscosity.

How to use this calculator

1. Choose Diameter or Area as your geometry method.
2. Enter the average velocity using your measurement or design value.
3. Provide gas temperature and either absolute pressure or gauge + barometric.
4. Optionally enter moisture and a reasonable viscosity for Reynolds estimation.
5. Set your preferred standard conditions (temperature and pressure).
6. Press Calculate. Results appear under the header and above the form.
7. Use Download CSV or Download PDF for reports.

If your gas is far from ideal or highly condensable, confirm results with site-specific methods.

Notes for construction applications

• Use measured traverse averages for compliance-grade reporting.
• Confirm whether your project requires wet-basis or dry-basis standard flow.
• Large temperature gradients can make single-point measurements misleading.
• Reynolds number here is indicative, not a guarantee of turbulence.

Accurate flue gas flow supports safer, cleaner project decisions.

Professional guidance article

1) Why flue gas flow matters on site

Flue gas flow is a practical control variable for many construction tasks: verifying draft and venting performance, validating exhaust capacity, sizing stacks, selecting fans, and documenting commissioning results. When flow is underestimated, combustion appliances may spill products of combustion, while oversized systems can increase noise, energy use, and installation cost. A consistent calculation method also makes it easier to compare field readings with design intent and regulatory reporting formats.

2) What “actual” versus “standard” flow means

In ducts and stacks, instruments typically measure velocity at the operating temperature and pressure. That yields an actual volumetric flow, expressed in m³/s or m³/h. Many specifications, permits, and equipment curves use standard volumetric flow, normalized to a defined temperature and pressure. This calculator converts actual flow to standard flow using an ideal-gas relationship, helping you present results in a consistent “Nm³/h” basis.

3) Geometry and measurement quality

Accurate area is the foundation of accurate flow. Use inside diameter for circular stacks, or enter a known area from drawings when available. For field work, velocity should be an average from a traverse (multiple points) rather than a single-point reading. If velocities vary across the section, the average can shift notably, especially downstream of bends, dampers, or transitions.

4) Moisture and reporting basis

Flue gas often contains significant water vapor. Some standards require wet-basis standard flow; others require dry-basis values. This tool reports both by applying the moisture fraction to the standard flow. If your project specification defines oxygen correction or dry gas at a reference moisture, you can adapt the inputs to match that reporting basis.

5) Example data walkthrough

Consider a stack with D = 0.60 m, V = 12 m/s, T = 180°C, P = 101.325 kPa abs, and moisture = 8% vol. The calculator produces an actual flow near 12,215 m³/h, a wet standard flow around 7,764 Nm³/h, and a dry standard flow around 7,143 Nm³/h. These values align with the example table and illustrate how hot gases can have a much larger actual volume than the equivalent standard volume.

6) Using density and Reynolds number

Density is estimated from pressure, temperature, and molecular weight. That enables a mass-flow estimate, which is useful for heat balance checks and emissions mass-rate calculations. Reynolds number is included as an indicator of flow regime; it can help you judge whether a given duct condition is likely to be fully turbulent, which affects mixing and measurement repeatability.

7) Practical checks before you finalize

Confirm that pressure is absolute, temperature is representative of the cross-section, and units match your instrument outputs. If condensate or particulate loading is high, instrumentation and flow assumptions may require additional corrections. For critical compliance reporting, follow the applicable test method and document sampling locations, traverse points, and calibration details.

Reliable flow calculations improve design coordination and commissioning outcomes.

FAQs