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Calculating Airflow Using Speed, Pipe Diameter: Methods & Formulas for Accurate Results

stefbut 27534 14
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  • #1 17762596
    stefbut
    Level 14  
    Is it possible to somehow calculate the amount of air knowing the speed and diameter of the pipe?
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  • #2 17762627
    any56
    Level 39  
    Yes
    1. Use the distance parameter d (m) from the set speed (m / s)
    2.Calculate the volume of air flowing in 1 sec (Pi * r ^ 2 * d)
    - where r is half the diameter of the pipe
    For the pipe diameter given in meters, we get the flow in m ^ 3 / sec
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  • #3 17762681
    stefbut
    Level 14  
    I'm getting some nonsense
    Let us assume that the velocity is 8 m / s
  • #4 17762746
    any56
    Level 39  
    And the diameter of the pipe?
  • #5 17762754
    stefbut
    Level 14  
    The diameter is 100mm
  • Helpful post
    #6 17762816
    any56
    Level 39  
    Within 1 sec, air flows through 8 meters of pipe.
    So, its volume:
    3.1415 * 0.05 * 0.05 * 8 = 0.0628 m ^ 3
    And this is the value of the flow in m ^ 3 / sec
    Using the conversion to hours, we get:
    226.188 m ^ 3 / hour
  • #7 17762950
    Anonymous
    Level 1  
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  • #8 17763008
    any56
    Level 39  
    Of course!
    Everything is correct...
    I assumed that the fan does not compress the air and therefore the temperature remains constant :-)
  • #9 17764240
    gag70
    Level 24  
    Under typical conditions (domestic ventilation) and for typical measuring instruments (thermoanemometer, damming tube), the accuracy of the air velocity measurement is lower than the error resulting from the lack of temperature and pressure compensation.
  • #10 17764434
    Chris_W
    Level 39  
    Without philosophizing - you multiply the speed by the cross-section - of course you have to in the same units, i.e. for example [m / s] and [m2]
    In your case, 8m / s x 0.0079m2 = 0.0632m3 / s
    (we multiply by 3600 - the number of seconds per hour)
    0.0632m3 / sx 3600s / h = 227m3 / h
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  • #11 17764489
    stefbut
    Level 14  
    I know colleague any56 solved the problem.
  • #12 20850907
    intechoze
    Level 2  

    >>17764434 And in the case where I have an anemometer in m/s (such a windmill with a display) and to it I have a sealed funnel, which I apply to the anemostat, when calculating m3/h should I give the diameter of the pipe because I have 125 or the diameter of the anemometer itself. In my opinion, the diameter of this windmill on the anemometer.
  • #13 20851006
    gag70
    Level 24  
    Pipe radius. We need to make it so that throughout the pipe the flow is laminar and the velocity the same at every point. So you need a sufficiently long straight section, for example, 6xDn.

    In the anemostat (plate with airflow around the perimeter), an additional limitation is the minimum allowable diameter of the pipe you apply to the plate. The dn125 pipe is not suitable for measuring from any anemostat, because the flow of air directly on its cylindrical wall causes additional resistance and will most likely disturb (reduce) the flow compared to that which will be without the measuring pipe. A large cardboard box (with an anemometer, for example, mounted centrally in the bottom) instead of a pipe, will be a better idea. It is important to seal the box in the part pressed against the ceiling.
  • #14 20851185
    intechoze
    Level 2  

    Ok this I understand. I have an anemometer in m/s. Above is an example of calculation with a circle field, I don't know whether to use a windmill circle from an anemometer for calculation.
  • #15 20851361
    Chris_W
    Level 39  
    Cross-section of the opening from which the air exits.
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Topic summary

✨ The discussion centers on calculating airflow in a pipe using the speed of air and the diameter of the pipe. The primary formula involves determining the cross-sectional area of the pipe (using the radius derived from the diameter) and multiplying it by the air velocity to find the volumetric flow rate in cubic meters per second. For a pipe with a diameter of 100mm and an airspeed of 8 m/s, the calculated flow rate is approximately 0.0632 m³/s, which translates to about 227 m³/h. Participants note that while this method provides a basic calculation, it does not account for variations in pressure and temperature, which can significantly affect airflow measurements. Additionally, considerations regarding the use of anemometers and the importance of maintaining laminar flow in the pipe are discussed.
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FAQ

TL;DR: Multiply air speed by duct area; 8 m/s in a 100 mm duct ≈ 227 m3/h. "you multiply the speed by the cross-section". [Elektroda, Chris_W, post #17764434]

Why it matters: For HVAC DIYers and technicians, this FAQ shows how to compute airflow from velocity and diameter and avoid measurement pitfalls.

Quick Facts

How do I calculate airflow (m3/h) from air speed and duct diameter?

Airflow Q equals velocity v times cross-sectional area A. For a round duct, A = π(D/2)^2. Example: v=8 m/s, D=0.1 m gives Q ≈ 0.063 m³/s = 227 m³/h. As one expert said, “you multiply the speed by the cross-section”. How-To: 1. Convert diameter to meters and compute A = π(D/2)^2. 2. Multiply by velocity to get m³/s. 3. Multiply by 3600 for m³/h. [Elektroda, Chris_W, post #17764434]

Which units should I use so the math works?

Use SI units: speed in m/s and diameter in meters. Compute area in m², then multiply v × A to get m³/s. Convert to m³/h by multiplying by 3600. Converting millimeters to meters before squaring prevents large numeric errors. [Elektroda, Chris_W, post #17764434]

What area do I use when measuring with a funnel on an anemostat?

Use the cross-section of the opening where the air exits. That could be the anemostat outlet or the funnel outlet if sealed. Do not use the anemometer’s fan diameter, because it does not define the flow area. [Elektroda, Chris_W, post #20851361]

Does temperature and pressure change the result?

Yes. The v × A method assumes constant air density. Pressure and temperature shifts change density and therefore the volume flow. “This calculation method is basically errors” if you ignore those factors in sensitive cases. For precision work, include compensation. [Elektroda, Anonymous, post #17762950]

How much straight duct do I need before measuring velocity?

Use about six duct diameters of straight run (≈6×Dn) to promote a uniform velocity profile. This reduces swirl and separation from fittings, bends, or grilles. With steadier flow, your single-point reading better represents the average velocity. [Elektroda, gag70, post #20851006]

Is a DN125 pipe okay for measuring at an anemostat?

Often no. Pressing a DN125 pipe to an anemostat adds wall friction and can reduce flow, giving low readings. A sealed cardboard box with the anemometer mounted centrally is better. Seal the box perimeter against the ceiling to prevent leakage. [Elektroda, gag70, post #20851006]

What’s the cross-sectional area of a 100 mm duct?

D=100 mm means D=0.1 m. Area A = π(0.05)² ≈ 0.00785 m². At 8 m/s, Q ≈ 0.063 m³/s or 227 m³/h. Multiply by 3600 to convert seconds to hours. These values align with the example calculations provided. [Elektroda, Chris_W, post #17764434]

How do I convert m3/s to m3/h?

Multiply by 3600, the number of seconds in an hour. Example: 0.063 m³/s × 3600 ≈ 227 m³/h. This unit conversion is standard in airflow reporting and matches the worked example. [Elektroda, Chris_W, post #17764434]

Can I compute using distance traveled in one second instead of velocity?

Yes. Distance per second equals velocity. Volume per second equals pipe area times that distance: V = πr²·d. Here r is half the diameter. Use meters consistently for r and d to get m³/s. [Elektroda, any56, post #17762627]

My 8 m/s, 100 mm result looks wrong—what am I missing?

Convert 100 mm to 0.1 m, then use radius r = 0.05 m. Compute V = π·0.05²·8 ≈ 0.0628 m³/s. That equals about 226.2 m³/h after multiplying by 3600. Unit mismatches or using diameter instead of radius cause errors. [Elektroda, any56, post #17762816]

What instruments work best for home ventilation airflow checks?

Use a thermoanemometer or a pitot (damming) tube. In typical domestic conditions, compensation error can exceed the instrument’s velocity accuracy. Focus on stable setups, good sealing, and repeatable technique before advanced corrections. [Elektroda, gag70, post #17764240]

Do I need pressure/temperature compensation for domestic checks?

Often not. For household ducts and common meters, the compensation error may be larger than the meter’s own accuracy. Prioritize consistent positioning and sealing. Add compensation only if your method is already repeatable. [Elektroda, gag70, post #17764240]
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