📊 Calculated Airflow
💨 Velocity
Result Breakdown
Full Calculation Table
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How to Use the Airflow Measurement Calculator
Choose pitot tube if you have a velocity pressure reading and duct area, flow hood if you captured airflow directly at a register, or temperature rise if you are estimating from furnace output and temperature split.
Fill in the fields specific to your chosen method. For pitot tube, that's velocity pressure and duct area. For flow hood, it's the direct reading and an optional correction factor.
For pitot tube readings at high elevation or unusual temperature, enter the site elevation and air temperature to correct velocity for actual air density.
Click Calculate Airflow to see the corrected airflow, velocity, and a breakdown of how the density correction affected your result.
Understanding Airflow Measurement Methods
Getting an accurate airflow number in the field usually comes down to picking the right method for the access you have. A duct with a clean straight run gives you a good pitot tube location. A finished register with no duct access is a flow hood job. A sealed system where nothing is exposed might only leave the temperature rise method available. Each has different strengths and different sources of error.
Pitot Tube Method
A pitot tube measures the difference between total pressure and static pressure at a point in the airstream, which is the velocity pressure. That pressure relates to velocity through air density, following the same Bernoulli relationship NASA Glenn uses to explain pitot tubes, so a single-point reading converts to velocity, and velocity multiplied by duct cross-sectional area gives airflow. For a full duct traverse rather than a single point, use the pitot tube traverse calculator, which averages multiple points across the duct for a more representative reading.
Flow Hood Method
A flow hood captures the entire airstream from a register or diffuser and reads airflow directly, without needing to know duct area or apply a velocity conversion. This makes it fast and simple for register-by-register readings, though results can be sensitive to how well the hood seals against the ceiling or wall and to backpressure the hood itself introduces on some diffuser types. Manufacturer correction factors for specific diffuser geometries help account for this.
Temperature Rise Method
When duct access is limited, the temperature rise method estimates airflow from a furnace's rated heat output and the measured temperature difference between supply and return air. More airflow moving past the same heat output produces a smaller temperature rise, so the math runs in reverse: a known heat input and a measured temperature rise back-calculate the airflow moving the heat away. The relationship underneath is sensible heat, which the sensible heat calculator lets you work through for any airflow and temperature difference. This method is convenient but depends heavily on accurate furnace output ratings and stable temperature readings taken away from duct fittings that could skew results.
Correcting for Air Density
Velocity pressure from a pitot tube is a function of both velocity and air density, and standard air density assumes sea level and about 21°C. Sites at higher elevation, like Calgary or Banff, have measurably thinner air, and very cold or very hot air also shifts density from standard. Skipping this correction on a mountain job site or in the middle of a Prairie winter can introduce a meaningful error into the reported airflow, so this calculator applies elevation and temperature corrections automatically for the pitot tube method. To see how far a particular site sits from standard conditions, the air density calculator reports density directly.
Choosing the Right Method for the Job
For commissioning and precise diagnostic work, a full pitot tube traverse at a straight duct section usually gives the most reliable number. Flow hoods are excellent for quickly checking whether individual registers are delivering design airflow, which pairs well with the test, adjust, and balance calculator. Formal balancing work, such as the procedures in the AABC National Standards for Total System Balance, records readings like these against design airflow. The temperature rise method is best treated as a sanity check or a fallback when the other two are not practical on a given system.
Frequently Asked Questions
A pitot tube measures velocity pressure at a point in a duct and converts it to velocity, then to airflow using duct area. A flow hood captures all the air from a register directly and reads airflow without needing duct area. The temperature rise method infers airflow from the heat added by a known-output furnace and the measured temperature rise across it, useful when direct duct access is not practical.
Velocity pressure from a pitot tube depends on air density, and colder or higher-altitude air is denser or less dense than standard conditions. Measurements taken at high elevation or extreme temperature need a density correction factor to report accurate standard airflow, which matters across Canada given the elevation range from coastal cities to mountain communities like Banff or Canmore.
The temperature rise method is a useful estimate but less precise than a direct pitot tube traverse or flow hood reading, since it depends on accurate furnace input and output ratings and a stable temperature rise measurement. It works best as a quick field check or when duct access for direct measurement is not available.
Most flow hoods read airflow directly rather than deriving it from velocity pressure, so they are generally less sensitive to elevation than pitot tube measurements. Some manufacturers still recommend a correction factor at very high elevation or extreme temperature, so check your specific instrument's documentation for guidance.
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