Duct CFM calculator
This is airflow arithmetic — area times velocity, or volume times a change rate. It does not size a fan or a duct, judge static pressure, or account for the losses of a real run. Take it as a flow figure to aim at or to check, not an equipment specification.
Pick a method. The first turns a duct width, height and air velocity into a flow; the second turns a room volume and a target of air changes per hour into the flow that delivers them. Both read out in CFM, litres per second and cubic metres an hour.
Exemple détaillé
A 12 × 8 inch duct is (12 ÷ 12) × (8 ÷ 12) = 0.667 sq ft. At 900 feet per minute that carries 0.667 × 900 ≈ 600 CFM, about 283 litres per second. By the room method, a 2000 cubic foot room at 6 air changes an hour needs 2000 × 6 ÷ 60 = 200 CFM.
In metric: a 300 × 200 mm duct is 0.3 × 0.2 = 0.06 m². At 4.5 metres per second that carries 0.06 × 4.5 = 0.27 m³/s — 270 L/s, 972 m³/h, about 572 CFM. By the room method, a 50 m³ room at 6 air changes an hour needs 50 × 6 = 300 m³/h, which is 300 ÷ 3.6 = 83.3 L/s.
Sur le chantier
Round duct. A 10 in round is π × 5² = 78.5 in² = 0.545 ft². At 800 fpm that is 436 CFM. Put 10 in both the width and height fields and you get a 10 in square (0.694 ft²), which is 27% more duct than you have — for round, work the area by hand and use the velocity method with a made-up rectangle of the same area, or just multiply: 0.545 × 800.
Checking a fan against a room. A bathroom 8 × 10 with an 8 ft ceiling is 640 ft³. The fan on the shelf says 80 CFM: 80 × 60 ÷ 640 = 7.5 air changes an hour. Whether the job wants 7.5 is the designer's number; the arithmetic is the room method run backwards.
Reading an anemometer. The meter says 700 fpm in a 14 × 10 duct: 140 ÷ 144 = 0.972 ft² × 700 = 680 CFM. Take the reading in a traverse — several points across the duct — and average, because the middle of a duct runs faster than the edges.
Exercez-vous
Quick referencefor airflow
- CFM
- duct area (ft²) × velocity (fpm)
- Duct area
- rectangular: w × h ÷ 144 · round: 0.785 × d² ÷ 144
- Round duct, ft²
- 6 in 0,196 · 8 in 0,349 · 10 in 0,545 · 12 in 0,785 · 14 in 1,069 · 16 in 1,396
- Air changes
- ACH = CFM × 60 ÷ room ft³ · CFM = room ft³ × ACH ÷ 60
- Métrique
- L/s = duct area (m²) × velocity (m/s) × 1,000 · ACH = L/s × 3.6 ÷ room m³
- Round duct, m²
- 100 mm 0,0079 · 150 mm 0,0177 · 200 mm 0,0314 · 250 mm 0,0491 · 300 mm 0,0707
- Unités
- 1 CFM = 0.472 L/s = 1.7 m³/h · 1 fpm = 0.00508 m/s
Mistakes to catchbefore you trust the number
- Entering the outside (insulated) duct size — the airflow lives inside the liner, so type the inside dimensions.
- Reading a velocity in metres per second into a fpm field: 5 m/s is about 985 fpm, not 5.
- Taking the room volume from the floor area alone; a vaulted room has more air, and ACH depends on the true volume.
What an inspector looks forthe mechanical inspector, and the balancing contractor after them
On this part of the job
- Measured airflow against the design airflow for each space, taken at the grille with an instrument rather than by hand.
- Ventilation rates for the occupancy against what the mechanical code requires, and that any required continuous ventilation actually runs.
- Duct sizing and layout against the design, including the flexible duct — a long, kinked or compressed flex run destroys the flow that was designed.
- Sealing and insulation of ducts, and duct through unconditioned space done the way the energy requirements call for.
- Fire and smoke dampers where ducts cross a rated assembly, accessible and with their access doors labelled.
- The balancing report, where one is required as a condition of occupancy.
Have this ready
- The mechanical drawings with the design CFM marked per outlet.
- The balancing report, signed.
- Access to dampers and to the equipment's own data plate.
Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.
La formule
duct: CFM = area × velocity · room: CFM = volume × ACH ÷ 60
Flow is a volume passing per unit time. The duct method builds it from the face area and the speed of the air through it. The room method spreads a whole-room volume across the hour at the change rate you set, dividing by 60 to land in CFM.
Questions
- How do I work out CFM from duct size and velocity?
- Airflow is the duct face area times the air speed. Put the area in square feet and the velocity in feet per minute and the answer comes out in cubic feet per minute — CFM. A 12 by 8 inch duct is 0.667 sq ft; at 900 fpm that is about 600 CFM. This tool does the area for you from the width and height.
- How do air changes per hour relate to CFM?
- An air change replaces the whole room’s volume once. Multiply the room volume by the changes you want per hour and divide by 60 to get CFM. A 2000 cubic foot room at 6 air changes an hour needs 2000 × 6 ÷ 60 = 200 CFM. Switch this tool to the air-change method and enter the room and the rate.
- Does this size a fan or a duct for me?
- No — it is airflow arithmetic only. It will not select a fan, size a duct run for a given static pressure, or account for losses along the way. Use it to express a flow you already want or to check one you have measured, and leave the equipment selection to a mechanical designer.
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