How to Calculate Duct Air Volume with UNI-T UT366A and Pitot Tube Malaysia
Air velocity and air volume are related, but they are not the same measurement.
The UNI-T UT366A Digital Manometer, when connected to a compatible Pitot tube, can measure air velocity inside a duct. To calculate air volume, the technician must also know the duct’s internal cross-sectional area.
The basic formula is:
Air volume = Average air velocity × Duct cross-sectional area
Air Velocity vs Air Volume
Air velocity describes how quickly air moves past a point.
Common units include:
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Metres per second: m/s
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Feet per minute: fpm
Air volume describes the quantity of air passing through the duct over time.
Common units include:
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Cubic metres per second: m³/s
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Cubic metres per hour: m³/h
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Cubic feet per minute: CFM
A high velocity in a small duct may produce less total airflow than a lower velocity in a large duct.
UT366A Air-Velocity Measurement
The UT366A is primarily a gauge-pressure and differential-pressure instrument.
When paired with the optional UNI-T Pitot tube, it can measure air velocity in:
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m/s
-
fpm
The compatible Pitot tube measures approximately:
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350 mm long
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8 mm in diameter
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Made from 304 stainless steel
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Supplied with separate black and white silicone tubes
The Pitot tube must face directly into the airflow and be connected correctly to the pressure ports.
Step 1: Measure the Internal Duct Dimensions
Measure the internal dimensions of the duct, not the external dimensions.
For a rectangular duct, measure:
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Internal width
-
Internal height
For a round duct, measure:
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Internal diameter
Use metres if the final result is required in m³/s or m³/h.
Step 2: Calculate the Duct Area
Rectangular Duct
Use:
Area = Width × Height
Example:
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Width: 0.60 m
-
Height: 0.40 m
Area = 0.60 × 0.40
Area = 0.24 m²
Round Duct
Use:
Area = π × Diameter² ÷ 4
Example:
-
Internal diameter: 0.50 m
Area = 3.1416 × 0.50² ÷ 4
Area ≈ 0.196 m²
Do not use the duct circumference as the cross-sectional area.
Step 3: Measure Air Velocity at Multiple Points
Air velocity is not normally uniform across the complete duct.
Velocity may be:
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Higher near the centre
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Lower near the duct wall
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Uneven after a bend
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Disturbed near a damper
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Turbulent after a fan
-
Different across rectangular duct sections
Take readings at several positions across the duct and calculate an average.
Using only one centre reading may overestimate the actual air volume.
Step 4: Calculate Average Air Velocity
Add all velocity readings and divide by the number of readings.
Example readings:
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4.6 m/s
-
4.9 m/s
-
5.1 m/s
-
5.3 m/s
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5.0 m/s
-
4.8 m/s
Total:
29.7 m/s
Average:
29.7 ÷ 6 = 4.95 m/s
Use the average value for the air-volume calculation.
Step 5: Calculate Air Volume in m³/s
Use:
Air volume in m³/s = Average velocity in m/s × Duct area in m²
Using the rectangular duct example:
-
Average velocity: 4.95 m/s
-
Duct area: 0.24 m²
Air volume = 4.95 × 0.24
Air volume = 1.188 m³/s
Rounded result:
Approximately 1.19 m³/s
Step 6: Convert m³/s to m³/h
One hour contains 3,600 seconds.
Use:
m³/h = m³/s × 3,600
Example:
1.188 × 3,600 = 4,276.8 m³/h
Rounded result:
Approximately 4,277 m³/h
Complete Calculation Example
A rectangular duct has:
-
Width: 600 mm
-
Height: 400 mm
-
Average air velocity: 5.0 m/s
Convert dimensions to metres:
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600 mm = 0.60 m
-
400 mm = 0.40 m
Calculate area:
0.60 × 0.40 = 0.24 m²
Calculate air volume:
5.0 × 0.24 = 1.20 m³/s
Convert to m³/h:
1.20 × 3,600 = 4,320 m³/h
Estimated air volume:
4,320 m³/h
Measuring a Round Duct
A round duct has:
-
Internal diameter: 400 mm
-
Average velocity: 6.0 m/s
Convert the diameter:
400 mm = 0.40 m
Calculate the area:
3.1416 × 0.40² ÷ 4 ≈ 0.126 m²
Calculate air volume:
6.0 × 0.126 = 0.756 m³/s
Convert to m³/h:
0.756 × 3,600 = 2,721.6 m³/h
Estimated air volume:
Approximately 2,722 m³/h
Why Multiple Measurement Points Matter
A Pitot tube measures velocity at its current position. It does not automatically represent the entire duct.
A proper traverse improves the estimate by sampling multiple points.
The number and location of points depend on:
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Duct shape
-
Duct size
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Required measurement method
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Available access
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Airflow condition
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Project procedure
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Applicable testing standard
For commissioning or acceptance work, follow the required traverse method rather than selecting points randomly.
Select a Suitable Test Location
Whenever possible, avoid measuring immediately:
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After a fan
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After a sharp bend
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After a damper
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After a branch connection
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Near an obstruction
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Where airflow is visibly turbulent
Select a straight duct section where the airflow profile is more stable.
If no ideal location is available, record the limitation in the test report.
Common Calculation Errors
Incorrect air-volume results may be caused by:
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Using external duct dimensions
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Forgetting to convert millimetres to metres
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Using one velocity reading
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Incorrect Pitot tube direction
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Reversed pressure connections
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Measuring in turbulent airflow
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Using the wrong area formula
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Forgetting to multiply m³/s by 3,600
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Mixing metric and imperial units
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Ignoring an internal liner or obstruction
Check every input before accepting the final result.
Air Leakage Affects the Result
Air volume measured at one duct section may not equal the airflow delivered at another point.
Differences may be caused by:
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Duct leakage
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Open branch ducts
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Damper positions
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Air bypass
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Multiple outlets
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Flexible-duct leakage
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Unsealed access panels
Record the measurement location clearly so the result is not mistaken for total system airflow.
Is the UT366A Suitable?
The UT366A with an optional Pitot tube is suitable when technicians need:
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Air velocity inside a duct
-
Access through a small test opening
-
Differential-pressure measurement
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Filter pressure-drop checking
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Duct-pressure measurement
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Air-volume calculation from average velocity
For quick airflow measurements directly at a grille or diffuser, a vane or hot-wire anemometer may be easier to position.
Enquire About Duct Airflow Measurement
For assistance selecting the UNI-T UT366A and Pitot tube for duct-velocity or air-volume calculations, WhatsApp MTM Precision with the duct dimensions, expected air velocity and measurement location.
MTM Precision Sdn Bhd
Showroom & Service Centre:
No. 29-1 & 29-2, Jalan Bandar 18,
Pusat Bandar Puchong,
47160 Puchong, Selangor, Malaysia
Tel: 03-8080 7172
WhatsApp: +6016-660 7346
Email: mtmpre@yahoo.com
Website: www.mtmpre.com.my
30 Sep 2026
