Updated September 2026
Most duct sizing charts you'll find online are copied from a paper ductulator and only cover one friction rate, buried in a PDF you have to squint at. This page shows the same math the calculator runs, laid out as a table, at the standard residential friction rate of 0.1 in.wg per 100 ft — so you can scan straight to your airflow instead of typing it in, or use it to sanity-check a number the calculator gave you.
Each row solves the equal-friction equation FR = 0.109136 × Q1.9 / d5.02 for the given CFM, then rounds up to the nearest standard duct size and recomputes the actual velocity and friction rate for that size — exactly what the calculator does, just precomputed for common airflows.
| CFM | Round duct | Velocity (fpm) | Actual friction |
|---|---|---|---|
| 50 | 5" | 367 | 0.057 |
| 100 | 6" | 509 | 0.085 |
| 150 | 7" | 561 | 0.085 |
| 200 | 8" | 573 | 0.075 |
| 250 | 9" | 566 | 0.064 |
| 300 | 9" | 679 | 0.090 |
| 350 | 10" | 642 | 0.071 |
| 400 | 10" | 733 | 0.092 |
| 450 | 12" | 573 | 0.046 |
| 500 | 12" | 636 | 0.056 |
| 600 | 12" | 764 | 0.079 |
| 700 | 14" | 655 | 0.049 |
| 800 | 14" | 748 | 0.063 |
| 900 | 14" | 842 | 0.079 |
| 1000 | 14" | 935 | 0.096 |
| 1200 | 16" | 859 | 0.070 |
| 1400 | 16" | 1002 | 0.094 |
| 1600 | 18" | 905 | 0.067 |
| 1800 | 18" | 1018 | 0.083 |
| 2000 | 20" | 916 | 0.060 |
All values: galvanized round duct, equal-friction method targeting 0.1 in.wg/100 ft, rounded up to the nearest standard size. For an exact CFM not listed here, or a different friction rate, use the calculator directly.
Notice the "actual friction" column bounces around 0.05–0.10 rather than sitting exactly at 0.1 — that's expected, not an error. Because duct sizes only come in fixed increments, the exact diameter the friction equation solves for almost never lands on a stocked size. Rounding up to the next standard size means the duct is very slightly larger than strictly required, which pulls the actual friction rate (and velocity) down a bit below the 0.1 target. A CFM that lands just above a size break (like 400 CFM rounding up from 9.8" to a full 10") shows a friction rate close to target; one that lands just below a size break (like 450 CFM rounding up from 10.3" all the way to 12") shows a noticeably lower actual friction rate, because the jump to the next stocked size was a bigger step.
0.1 in.wg/100 ft is the common default for residential and light-commercial branch runs, but tighter mechanical spaces sometimes use a higher target (up to around 0.15–0.2) to keep duct sizes smaller, accepting more noise and static pressure in trade. A chart at a different friction rate would shift every row — smaller ducts at higher friction rates, larger at lower ones — which is exactly why a single static chart can only cover one scenario well. The calculator lets you set any friction rate and reflects the change immediately, including the velocity and rectangular-equivalent sizes for that specific target.
The CFM values here are inputs, not something this chart derives — they should come from a Manual J load calculation (whole-house or room-by-room) or the rated airflow of your equipment, not guessed. A rough field rule of thumb used for a quick trunk estimate is about 400 CFM per ton of cooling capacity — a 3-ton system works out to roughly 1,200 CFM, sizing to a 16" trunk in the table above.
Why is my exact CFM not in the table? The table covers common round-number airflows for quick reference. For an exact CFM, use the calculator — it solves the same formula for any value rather than the nearest table row.
Does this chart apply to flex duct? No — it's based on smooth galvanized sheet metal, the standard friction reference. Flex duct has meaningfully higher friction per foot (commonly 3–8× depending on installation) — size up one increment from these figures for a flex run.
What about rectangular duct sizes for these CFMs? See the round-to-rectangular conversion page — every round size in this table has an equivalent rectangular table there.
Is this the same as a "ductulator"? Functionally yes — a ductulator is a circular slide-rule tool built around this same friction equation. This table and the calculator solve the equation algebraically instead of reading it off a printed wheel, so the results should match a correctly-used ductulator for the same friction rate and material.