Duct friction chart — the ductulator chart, imperial and metric
Every ductulator wheel is a friction chart folded into a circle. This is the chart itself, drawn directly from the Darcy-Weisbach and Altshul-Tsal equations for galvanised steel at standard air — airflow on one axis, friction rate on the other, and the diameter and velocity lines that let you read a duct size from any two of the four quantities.
Basis: galvanised spiral steel (ε = 0.09 mm), standard air at 20 °C and sea level (ρ = 1.204 kg/m³), Darcy-Weisbach with the Altshul-Tsal friction factor. Solid lines are duct diameters; dashed lines are velocities.
Imperial — CFM and in.wg per 100 ft
Metric — L/s and Pa per metre
What the chart is actually showing
Four quantities describe the air in a straight round duct: how much air is moving, how big the duct is, how fast the air is going and how much pressure it loses per unit length. They are not independent. Fix any two and the other two follow, through the continuity equation for velocity and the Darcy-Weisbach equation for friction. A friction chart is simply all of those relationships drawn on one sheet of paper.
The logarithmic scales are what make it readable. On log axes, the relationship between airflow and friction rate for a fixed diameter is very nearly a straight line, so a whole family of duct sizes becomes a fan of near-parallel lines, and velocity — airflow divided by area — becomes a second family crossing them. The cardboard ductulator wheel engineers carried for decades is this same chart wrapped onto discs, with the diameter and velocity read through windows instead of along lines.
Both charts here are computed rather than traced. Every point on every line comes from the same equations the online ductulator uses, so the chart and the calculator can never disagree.
How to size a duct from the chart
- Start from the airflow. Find the design flow for the section on the vertical axis.
- Move across to your friction rate. Read along horizontally to the design friction rate on the bottom axis — 0.08 in.wg per 100 ft, for example.
- Take the next diameter up. The point usually falls between two diameter lines. The larger diameter is the one that meets the friction target.
- Check the velocity lines. Note which dashed velocity lines the point sits between. If the velocity is above the band for the application, go up a size.
- Read the real friction rate back. Slide along the chosen diameter line to your airflow and read the friction rate that size actually produces — that is the figure to multiply by the run length.
Example: 1000 CFM at 0.08 in.wg per 100 ft lands between the 14 and 16 inch lines, slightly past the 14. A 14 inch duct would run a little over the target; a 16 inch duct comes in under it at around 720 fpm. The exact required diameter is 14.28 in.
Airflow capacity of standard round duct
The same chart turned into numbers: the airflow each standard spiral diameter carries at four common friction rates, with the velocity that results. Read a row to see how much more air a duct takes if you accept a higher friction rate; read a column to size by capacity directly.
Imperial
| Diameter | 0.05 in.wg/100 ft | 0.08 in.wg/100 ft | 0.1 in.wg/100 ft | 0.15 in.wg/100 ft |
|---|---|---|---|---|
| 4 in | 25 CFM286 fpm | 32 CFM372 fpm | 37 CFM421 fpm | 46 CFM528 fpm |
| 5 in | 46 CFM334 fpm | 59 CFM435 fpm | 67 CFM492 fpm | 84 CFM617 fpm |
| 6 in | 75 CFM380 fpm | 97 CFM494 fpm | 110 CFM559 fpm | 137 CFM700 fpm |
| 7 in | 113 CFM423 fpm | 147 CFM550 fpm | 166 CFM622 fpm | 208 CFM778 fpm |
| 8 in | 162 CFM464 fpm | 210 CFM603 fpm | 238 CFM682 fpm | 298 CFM853 fpm |
| 9 in | 223 CFM504 fpm | 289 CFM654 fpm | 327 CFM740 fpm | 409 CFM925 fpm |
| 10 in | 296 CFM542 fpm | 384 CFM704 fpm | 434 CFM796 fpm | 542 CFM994 fpm |
| 12 in | 483 CFM615 fpm | 627 CFM798 fpm | 708 CFM902 fpm | 884 CFM1,126 fpm |
| 14 in | 732 CFM685 fpm | 948 CFM887 fpm | 1,072 CFM1,003 fpm | 1,333 CFM1,247 fpm |
| 16 in | 1,048 CFM751 fpm | 1,357 CFM972 fpm | 1,527 CFM1,094 fpm | 1,898 CFM1,359 fpm |
| 18 in | 1,439 CFM814 fpm | 1,851 CFM1,047 fpm | 2,086 CFM1,181 fpm | 2,592 CFM1,467 fpm |
| 20 in | 1,899 CFM870 fpm | 2,447 CFM1,121 fpm | 2,758 CFM1,264 fpm | 3,425 CFM1,570 fpm |
| 22 in | 2,445 CFM926 fpm | 3,149 CFM1,193 fpm | 3,549 CFM1,344 fpm | 4,405 CFM1,669 fpm |
| 24 in | 3,080 CFM980 fpm | 3,965 CFM1,262 fpm | 4,467 CFM1,422 fpm | 5,542 CFM1,764 fpm |
| 26 in | 3,808 CFM1,033 fpm | 4,900 CFM1,329 fpm | 5,519 CFM1,497 fpm | 6,846 CFM1,857 fpm |
| 28 in | 4,634 CFM1,084 fpm | 5,960 CFM1,394 fpm | 6,713 CFM1,570 fpm | 8,323 CFM1,946 fpm |
| 30 in | 5,563 CFM1,133 fpm | 7,152 CFM1,457 fpm | 8,053 CFM1,641 fpm | 9,982 CFM2,034 fpm |
| 32 in | 6,598 CFM1,181 fpm | 8,481 CFM1,519 fpm | 9,548 CFM1,710 fpm | 11,832 CFM2,118 fpm |
| 36 in | 9,009 CFM1,275 fpm | 11,573 CFM1,637 fpm | 13,025 CFM1,843 fpm | 16,132 CFM2,282 fpm |
| 40 in | 11,901 CFM1,364 fpm | 15,278 CFM1,751 fpm | 17,192 CFM1,970 fpm | 21,282 CFM2,439 fpm |
| 48 in | 19,254 CFM1,532 fpm | 24,695 CFM1,965 fpm | 27,776 CFM2,210 fpm | 34,359 CFM2,734 fpm |
Metric
| Diameter | 0.5 Pa/m | 0.8 Pa/m | 1 Pa/m | 1.5 Pa/m |
|---|---|---|---|---|
| 80 mm | 7 L/s1.4 m/s | 9 L/s1.8 m/s | 10 L/s2.0 m/s | 13 L/s2.5 m/s |
| 100 mm | 13 L/s1.6 m/s | 16 L/s2.1 m/s | 19 L/s2.4 m/s | 23 L/s3.0 m/s |
| 125 mm | 23 L/s1.9 m/s | 30 L/s2.4 m/s | 34 L/s2.8 m/s | 43 L/s3.5 m/s |
| 160 mm | 45 L/s2.2 m/s | 58 L/s2.9 m/s | 66 L/s3.3 m/s | 83 L/s4.1 m/s |
| 200 mm | 82 L/s2.6 m/s | 106 L/s3.4 m/s | 120 L/s3.8 m/s | 150 L/s4.8 m/s |
| 250 mm | 150 L/s3.0 m/s | 194 L/s4.0 m/s | 219 L/s4.5 m/s | 274 L/s5.6 m/s |
| 315 mm | 279 L/s3.6 m/s | 361 L/s4.6 m/s | 408 L/s5.2 m/s | 509 L/s6.5 m/s |
| 355 mm | 384 L/s3.9 m/s | 498 L/s5.0 m/s | 562 L/s5.7 m/s | 698 L/s7.0 m/s |
| 400 mm | 530 L/s4.2 m/s | 683 L/s5.4 m/s | 770 L/s6.1 m/s | 957 L/s7.6 m/s |
| 450 mm | 725 L/s4.6 m/s | 933 L/s5.9 m/s | 1,052 L/s6.6 m/s | 1,306 L/s8.2 m/s |
| 500 mm | 958 L/s4.9 m/s | 1,234 L/s6.3 m/s | 1,390 L/s7.1 m/s | 1,725 L/s8.8 m/s |
| 560 mm | 1,294 L/s5.3 m/s | 1,665 L/s6.8 m/s | 1,876 L/s7.6 m/s | 2,327 L/s9.4 m/s |
| 630 mm | 1,768 L/s5.7 m/s | 2,274 L/s7.3 m/s | 2,561 L/s8.2 m/s | 3,175 L/s10.2 m/s |
| 710 mm | 2,426 L/s6.1 m/s | 3,118 L/s7.9 m/s | 3,511 L/s8.9 m/s | 4,350 L/s11.0 m/s |
| 800 mm | 3,327 L/s6.6 m/s | 4,273 L/s8.5 m/s | 4,809 L/s9.6 m/s | 5,956 L/s11.8 m/s |
| 900 mm | 4,541 L/s7.1 m/s | 5,829 L/s9.2 m/s | 6,559 L/s10.3 m/s | 8,119 L/s12.8 m/s |
| 1000 mm | 5,997 L/s7.6 m/s | 7,694 L/s9.8 m/s | 8,656 L/s11.0 m/s | 10,709 L/s13.6 m/s |
| 1250 mm | 10,801 L/s8.8 m/s | 13,842 L/s11.3 m/s | 15,562 L/s12.7 m/s | 19,237 L/s15.7 m/s |
Where a friction chart stops being right
- It assumes one material. Flexible duct is about thirty times rougher than spiral steel even when pulled tight, and compressed flex is far worse. Use the flex duct calculator for it.
- It assumes one air. Density scales pressure loss directly. Hot exhaust, cold intake air and high-altitude sites all move away from the chart, while the velocity lines stay correct.
- It covers straight duct only. Elbows, tees and dampers add losses that can exceed the straight run. Add them with the fitting loss calculator.
- It is drawn for round duct. Rectangular sections are read through their equivalent diameter, and their real velocity is lower than the chart shows.
Frequently asked questions
What is a ductulator chart?
It is the duct friction chart that a ductulator wheel is built from: airflow plotted against friction rate on logarithmic scales, crossed by lines of constant duct diameter and constant velocity. Fix any two of airflow, friction rate, diameter and velocity, and the chart gives the other two. The wheel simply folds the same relationships onto rotating discs.
How do I read a duct friction chart?
Find your airflow on the vertical axis and move across to the friction rate you are designing to on the horizontal axis. The point where they meet sits between two diameter lines — take the larger one — and between two velocity lines, which tells you how fast the air will be moving. Then read back along the chosen diameter line to see the friction rate that size actually gives.
What material and air conditions is this chart drawn for?
Galvanised spiral steel with an absolute roughness of 0.09 mm, carrying standard air at 20 °C and sea level, density 1.204 kg/m³. That matches the conditions classic printed friction charts assume. For flexible duct, duct board, hot or cold air or high altitude, use the calculators instead, which take those inputs directly.
Can I use this chart for rectangular duct?
Yes, through the circular equivalent diameter. Read the round size from the chart, then convert it to a rectangular size with the same friction using the round to rectangular chart or the shape converter. Do not use the chart velocity for the rectangular duct — its real velocity is lower, because it has more area than the equivalent round duct.
Why does my old cardboard ductulator give a slightly different answer?
Mostly reading error. A logarithmic scale read by eye easily carries five to ten per cent uncertainty, and some older charts were based on slightly different roughness values and friction factor correlations. The lines here are computed point by point from the equations, so the remaining difference is the chart you are comparing against, not the maths.
What friction rate should I read the chart at?
For ordinary low-velocity commercial supply, around 0.08 to 0.10 in.wg per 100 ft, or 0.8 to 1.0 Pa per metre in metric practice. Note that those are two separate rules of thumb rather than conversions of each other. Lower rates suit long runs and quiet spaces; higher rates save sheet metal but cost fan energy for the life of the system.
Last updated: 24 October 2026