How to Use a Ductulator: Reading the Wheel and the Numbers

What a ductulator actually does

A ductulator is a duct friction chart turned into a calculator. The classic version is a cardboard wheel handed out by fan and duct manufacturers: two printed discs, a few windows, and a scale around the edge. Turn one disc to line up two numbers you know, and the windows show the numbers you need. Nothing about the device is clever — the cleverness is in the chart printed on it, which encodes the relationship between airflow, duct size, velocity and friction loss for one particular material and one particular air.

Every ductulator, wheel or online, answers the same question: for this much air, at this friction rate, what size duct do I need and how fast will the air move? Learn to drive it with that question in mind and the device makes sense; use it to answer a different question and it quietly gives wrong answers.

The two numbers you start with

Airflow is the volume of air carried by the section of duct you are sizing — in CFM or litres per second. It is the flow in that piece of duct, not the fan total. A main leaving the air handler carries everything; the duct after the first take-off carries less; the runout to a single diffuser carries only that diffuser's share.

Friction rate is the pressure loss you are willing to accept per unit length of straight duct — in inches of water per 100 feet, or pascals per metre. It is a design decision, not a property of the system. Ordinary low-velocity commercial supply is commonly sized somewhere around 0.08 to 0.10 in.wg per 100 ft, or 0.8 to 1.0 Pa/m in metric practice. Those two bands are separate rules of thumb, not conversions of one another: 0.08 in.wg per 100 ft is about 0.65 Pa/m.

A lower friction rate gives bigger ducts, lower velocities, quieter air and lower fan power for the life of the building. A higher one gives smaller, cheaper ducts that cost more to run and are harder to keep quiet. The ductulator does not choose for you.

Using a ductulator wheel, step by step

1. Find the airflow scale and the friction scale. On most wheels they sit next to each other in the main window, one on each disc.
2. Line up the airflow against the friction rate. Rotate the disc until your airflow sits directly opposite your design friction rate.
3. Read the round diameter. Without moving the disc, read the round duct diameter in the size window. It will usually fall between two sizes — take the larger.
4. Read the velocity. The velocity window, usually alongside, shows how fast the air moves at that airflow and size. Check it against the band for the application.
5. Read the rectangular equivalents. Most wheels have a rectangular window listing widths for each depth that match the round size on friction. Pick a depth that fits the ceiling void.
6. Re-read at the size you chose. If you rounded the diameter up, set the wheel to that size and your airflow and read the friction rate it actually produces. That is the figure to multiply by the run length.

A worked example

A branch carries 1000 CFM and the job is designed to 0.08 in.wg per 100 ft. Setting 1000 against 0.08 on a wheel points to a diameter just over 14 inches — the exact figure is 14.28 in. A 14 inch duct would run at 935 fpm and a friction rate slightly above target, about 0.088. Going up to 16 inches drops the velocity to roughly 715 fpm and the friction rate to about 0.046.

Which to choose depends on the run. On a short branch near the end of a system, the 14 inch duct is fine — the extra few hundredths of an inch over a short length are negligible. On a long run, or one feeding a quiet space, the 16 inch duct earns its extra metal. If the ceiling void will only take 12 inches of depth, the rectangular window shows the width that matches the round size at that depth, and the aspect ratio tells you whether the result is sensible.

The online ductulator on this site does exactly this, but solves the equations rather than reading a printed chart, and shows the working.

Three mistakes that oversize — or undersize — a duct

Sizing a branch on the main flow. The most common error in practice. Every section is sized on the air it carries. Sizing branches on the fan total produces a system of oversized ducts that cost more, fit worse and run so slowly that balancing becomes guesswork.

Using the steel chart for other materials. A wheel's chart assumes smooth galvanised steel. Fully extended flexible duct is about thirty times rougher, and flex that is bunched up on site loses several times more again. Duct board and internally lined duct are rougher too. For those, use a tool that takes the roughness — and for flex, the compression — as an input; the flex duct calculator exists for exactly that.

Quoting the round velocity for a rectangular duct. The rectangular equivalent matches the round duct on friction, not on area. The rectangular duct has more area, so its real velocity is lower than the wheel's velocity window shows. Use the real velocity — airflow divided by the actual rectangular area — for noise checks and for selecting grilles and diffusers.

When not to trust the answer

A ductulator is drawn for standard air at sea level, around 20 °C. Pressure loss is proportional to air density, so hot exhaust, very cold intake air and sites at significant altitude all lose less (or, for cold air, more) than the wheel says. The size it gives is usually still right, because velocity depends on volume flow, not density — but the pressure drop you hand to the fan selection should be corrected.

It also sizes straight duct only. Elbows, tees, take-offs, dampers and transitions carry their own losses, and on a congested route they can add up to more than all the straight duct combined. Size the duct with the ductulator, then add the fittings with a fitting loss calculator before choosing a fan.

Finally, the friction rate you start from fixes the character of the whole system. Equal friction sizing is fast and conventional, but it does not balance itself on long runs; the guide to equal friction versus static regain covers when that starts to matter.

Wheel or online?

The wheel is still a fine tool for a quick check in a site meeting, and reading one well is a useful skill. Its limits are reading precision — a logarithmic scale read by eye carries a few per cent of uncertainty — and the fixed assumptions printed on it. An online ductulator removes both: it solves the equations at the airflow and size you enter, lets you change the material and the air temperature, and prints the intermediate values so the answer can be checked. If you want the chart itself, the duct friction chart on this site is the wheel unfolded, drawn from the same equations.