★ CONSTRUCTION · HVAC · DUCTWORK
A 12 in round duct for 400 CFM. A 10 in duct carries only 382 CFM, so 400 CFM needs the next size up: 550 CFM of capacity, a 10×12 in rectangular equivalent, and air moving at 509 FPM. Enter your airflow to size your duct.
A duct is sized from the air it must carry: the capacity is the area times the design velocity, and the duct you pick is the smallest one whose capacity covers the airflow. This calculator works out the round diameter, the rectangular equivalent and the air velocity for any airflow, with the full CFM-to-size table at the 700 FPM residential design velocity.
A duct is sized from the airflow it must carry. The design basis here is 700 feet per minute (FPM), the usual residential design velocity for supply ducts. A round duct’s area is π times half the diameter squared, in square inches. Its CFM capacity is the area divided by 144 — to get square feet — times 700 FPM. The duct you pick is the smallest one whose capacity is at least the airflow you need. The rectangular equivalent is the listed standard size whose area is at least the round area, and the air velocity at your airflow is the CFM divided by the duct area in square feet.
At the default — 400 CFM — a 10 in duct carries only 382 CFM, so it is too small. A 12 in duct is 113.10 sq in, so it carries 550 CFM, which covers the 400 CFM with room to spare. The duct is a 12 in round duct, its rectangular equivalent is 10×12 in, and the air moves at 400 ÷ 0.7854 = 509 FPM.
| Round duct (in) | Area (sq in) | Max CFM at 700 FPM | Rectangular equivalent |
|---|---|---|---|
| 5 | 19.63 | 95 | 6×6 in |
| 6 | 28.27 | 137 | 6×8 in |
| 7 | 38.48 | 187 | 8×8 in |
| 8 | 50.27 | 244 | 8×8 in |
| 9 | 63.62 | 309 | 8×10 in |
| 10 | 78.54 | 382 | 10×10 in |
| 12 | 113.10 | 550 | 10×12 in |
| 14 | 153.94 | 748 | 12×14 in |
| 16 | 201.06 | 977 | 14×16 in |
| 18 | 254.47 | 1,237 | 16×18 in |
| 20 | 314.16 | 1,527 | 18×20 in |
| Case | Duct | Capacity | Rectangular | Velocity |
|---|---|---|---|---|
| Default (400 CFM) | 12 in | 550 CFM | 10×12 in | 509 FPM |
| 100 CFM | 6 in | 137 CFM | 6×8 in | 509 FPM |
| 200 CFM | 8 in | 244 CFM | 8×8 in | 573 FPM |
| 300 CFM | 9 in | 309 CFM | 8×10 in | 679 FPM |
| 600 CFM | 14 in | 748 CFM | 12×14 in | 561 FPM |
| 1,000 CFM | 18 in | 1,237 CFM | 16×18 in | 566 FPM |
| 1,500 CFM | 20 in | 1,527 CFM | 18×20 in | 688 FPM |
Setup: airflow 400 CFM, design velocity 700 FPM.
Line 1 — the 10 in duct. Area = π × 5² = 78.54 sq in; capacity = 78.54 ÷ 144 × 700 = 382 CFM — too small for 400 CFM.
Line 2 — the 12 in duct. Area = π × 6² = 113.10 sq in; capacity = 113.10 ÷ 144 × 700 = 550 CFM — the smallest size that covers 400 CFM.
Line 3 — the rectangular equivalent. 10 × 12 = 120 sq in, which is at least the round area of 113.10 sq in, so the equivalent is 10×12 in.
Line 4 — the air velocity. 400 ÷ (113.10 ÷ 144) = 400 ÷ 0.7854 = 509 FPM.
Reverse: a 10 in duct carries 382 CFM at 700 FPM; an 8 in duct carries 244 CFM; a 14 in duct carries 748 CFM; 250 CFM needs a 9 in duct (309 CFM capacity); 500 CFM needs a 12 in duct (550 CFM capacity).
100 CFM: a 5 in duct carries only 95 CFM, so the smallest size that covers it is a 6 in duct (137 CFM), rectangular 6×8 in, velocity 509 FPM.
200 CFM: a 7 in duct carries 187 CFM, so the smallest size that covers it is an 8 in duct (244 CFM), rectangular 8×8 in, velocity 573 FPM.
300 CFM: an 8 in duct carries 244 CFM, so the smallest size that covers it is a 9 in duct (309 CFM), rectangular 8×10 in, velocity 679 FPM — close to the 700 FPM design velocity.
600 CFM: a 12 in duct carries 550 CFM, so the smallest size that covers it is a 14 in duct (748 CFM), rectangular 12×14 in, velocity 561 FPM.
1,000 CFM: a 16 in duct carries 977 CFM, so the smallest size that covers it is an 18 in duct (1,237 CFM), rectangular 16×18 in, velocity 566 FPM.
1,500 CFM: an 18 in duct carries 1,237 CFM, so the smallest size that covers it is a 20 in duct (1,527 CFM), rectangular 18×20 in, velocity 688 FPM — the biggest size on the chart.
Reverse — the other way round: a 10 in duct carries 382 CFM; an 8 in duct carries 244 CFM; a 14 in duct carries 748 CFM; 250 CFM needs a 9 in duct; 500 CFM needs a 12 in duct.
The model assumes the airflow you enter and the 700 FPM residential design velocity, and it picks the smallest table diameter whose capacity covers the airflow. It assumes a single round duct for the whole airflow and the listed standard rectangular sizes, and it leaves out fittings, dampers, long runs and static pressure — a real design checks the whole system, not one duct. Airflows above 1,527 CFM are outside the chart and need an HVAC pro. Rounding convention: CFM capacities are rounded to the nearest whole CFM, velocities to the nearest whole FPM, and all figures are computed from unrounded values, rounded only for display.
| Output | Formula | What it means |
|---|---|---|
| Round duct | smallest diameter whose capacity ≥ CFM | 12 in. |
| Capacity at 700 FPM | round area ÷ 144 × 700 | 550 CFM. |
| Rectangular equivalent | listed size whose area ≥ round area | 10×12 in. |
| Air velocity | CFM ÷ duct area in sq ft | 509 FPM. |
| Max CFM of a 10 in duct | area ÷ 144 × 700 | 382 CFM. |
| Max CFM of an 8 in duct | area ÷ 144 × 700 | 244 CFM. |
| Max CFM of a 14 in duct | area ÷ 144 × 700 | 748 CFM. |
| Duct size for 250 CFM | smallest capacity ≥ 250 | 9 in (309 CFM capacity). |
| Duct size for 500 CFM | smallest capacity ≥ 500 | 12 in (550 CFM capacity). |
| 100 CFM | same model, 100 CFM | 6 in, 6×8 in, 509 FPM, capacity 137 CFM. |
| 200 CFM | same model, 200 CFM | 8 in, 8×8 in, 573 FPM, capacity 244 CFM. |
| 300 CFM | same model, 300 CFM | 9 in, 8×10 in, 679 FPM, capacity 309 CFM. |
| 600 CFM | same model, 600 CFM | 14 in, 12×14 in, 561 FPM, capacity 748 CFM. |
| 1,000 CFM | same model, 1,000 CFM | 18 in, 16×18 in, 566 FPM, capacity 1,237 CFM. |
| 1,500 CFM | same model, 1,500 CFM | 20 in, 18×20 in, 688 FPM, capacity 1,527 CFM. |
The math is exact for the numbers you enter: the capacity is the round area divided by 144 times 700 FPM, the duct is the smallest size whose capacity covers the airflow, and the velocity is the CFM divided by the duct area in square feet. What the model cannot do is design your system for you: check the airflow, because a 100 CFM branch and a 1,000 CFM trunk need very different ducts. It assumes one duct for the whole airflow at 700 FPM and leaves out fittings, dampers, long runs and static pressure. For the furnace behind the ducts, see furnace size calculator; for the filter the air passes through, see air filter size chart.
Bright Side Kit. "Duct Size Chart: CFM to Duct Diameter & Size Guide." https://brightoolbox.com/duct-size-chart/ Accessed October 25, 2026.A 12 in round duct. A 12 in duct carries 550 CFM at the 700 FPM residential design velocity, so it is the smallest size that covers 400 CFM. Its rectangular equivalent is 10×12 in, and the air moves at 509 FPM in it.
Divide the airflow by the design velocity, then pick the smallest duct that covers it. At 700 FPM, a duct’s CFM capacity is its area in square inches ÷ 144 × 700. A 10 in duct is 78.54 sq in, so it carries 382 CFM; a 12 in duct is 113.10 sq in, so it carries 550 CFM. Pick the smallest diameter whose capacity is at least the airflow.
A 6 in round duct. A 6 in duct carries 137 CFM at 700 FPM, so it covers 100 CFM with room to spare, and the air moves at 509 FPM. A 5 in duct carries only 95 CFM, so it is too small.
The rectangular size with at least the same area. A 12 in round duct is 113.10 sq in, so its equivalent is 10×12 in (120 sq in). A 10 in round duct is 78.54 sq in, so its equivalent is 10×10 in (100 sq in). The chart above gives the equivalent for every size.
The chart tops out at 20 in, which carries 1,527 CFM. Above that the calculator says so and points to an HVAC pro, instead of guessing a size. A duct that big needs a proper design, not a chart lookup.
700 feet per minute is the usual residential design velocity for supply ducts. Faster air is noisier and costs more fan power; slower air needs bigger ducts. The whole chart is built on 700 FPM, so every capacity here is the airflow at that velocity.
CFM divided by the duct area in square feet. At 400 CFM in a 12 in duct the velocity is 400 ÷ 0.7854 = 509 FPM. At 300 CFM in a 9 in duct it is 679 FPM, close to the 700 FPM design velocity.