How to Measure Duct CFM

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Converting Velocity to CFM

US HVAC uses:

Area is the inside opening at the measurement plane, in square feet. Dimensions on a tape are usually inches:

• Round: A (ft²) = π × (d inches)² / (4 × 144)
• Rectangular: A (ft²) = (W × H) / 144, inside width and height

Subtract liner and internal insulation. A 10×8 lined with 1-inch liner is not 80 in² of air; it is closer to 8×6. The calculator will happily multiply the wrong rectangle if you feed it the outside stamp.

This is not duct sizing. Sizing from a known target CFM is the ductulator and the equal-friction guide (those pages also flag velocity maxima). This guide is how you measure what is actually moving.

Anemometer Placement

Pick a plane with reasonably developed flow: several diameters downstream of an elbow, takeoff, or transition. Measuring in the throat of a fitting is how you get a hero number that will not match the room.

1. Drill a test port sized for the probe (hot-wire wands like the Fieldpiece STA2 need a clear insertion; vane heads often will not fit a small round).
2. Insert to the marked depths for your traverse points. Keep the sensor axis with the flow — yaw error kills hot-wire accuracy.
3. Doors on, filter in, blower on the speed you are diagnosing. A missing blower door is not a CFM test.
4. Record each point. Average the velocities (equal-area / Log-Tchebycheff), then enter that average in the calculator — not the highest point and not the center.

Traverse Method — Multi-Point Average

Air in a duct is faster in the core and slower at the walls. ASHRAE / AMCA log-Tchebycheff layouts put more points near the walls for that reason. Field rule of thumb on small residential round:

• Round 6–10": at least six points on two diameters (three per diameter), not one centerline.
• Rectangular: a grid — often 5×5 or 6×6 on larger trunks. Never one hole in the middle of a 20×8.
• Skip a point that sits in a takeoff jet or against a hanging liner. Move the plane if half the grid is garbage.

Average the FPM values with equal weight if you used an equal-area layout. Do not average CFM-per-point unless each point represents equal area. Then multiply once by total area. The calculator is that last multiply.

How to Use the Result on the Job

Compare measured CFM to the design CFM for that trunk or room — Manual J/D / the drawing — not to a 400 CFM/ton guess pulled from the outdoor unit tonnage, and not to NEC service-load VA on the residential load calculator.

1. If you are 10%+ low: restriction or blower. Measure TESP with the ESP calculator and the ESP measurement guide. High static plus low CFM is a path problem. Low static plus low CFM is often leaks or a dying fan.
2. If you are high at the trunk but the room is starving: leakage between the plane and the register, or a closed damper downstream. Hood the grille.
3. If the duct itself is undersized for the CFM you actually need, size a replacement with the ductulator — velocity guidelines live there, not on a separate “FPM chart.”
4. Manometer vs anemometer as the first tool is already covered in Static Pressure vs Airflow (CFM). Do not re-litigate that here — this page is how to take the CFM once you have decided you need it.

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