DuctRule: HVAC Duct Sizing

The ductulator in your van is two cardboard discs and a rivet. This is that tool, on your phone, with the arithmetic done properly. Turn the outer ring to the airflow you have. Turn the blue wheel to the friction rate you are designing to. The answer sits in the middle, under the hairline: round diameter, air velocity, and the next stock size up. Up, never down, because a duct one size over costs a little sheet metal and a duct one size under costs the job. The velocity is not decoration. The ring around the answer runs green while the air stays inside the range for the run you picked, and turns amber the moment it leaves. Supply trunk, branch and return have different ranges, and you choose which one you are sizing. A duct that is slightly too small is quiet on paper and loud in the room. Under the dial are the rectangular sizes of equal friction, each with the velocity it produces, so you can trade depth for width when the ceiling is tight. Tap the one you are going to cut. Flat oval is there too, and so is the other direction: fix a height and the app solves the width. Save a run under the name you would write on the duct itself, like supply trunk A or branch to office 2, and the whole system becomes one list you can read back at the desk or send to the office. The friction comes from Colebrook-White, solved for every value rather than read off a power-law fit, with the roughness of galvanised steel and standard air. That is the same equation the printed chart was drawn from, which is why a printed chart and this app can differ a little in the middle of the range. The difference is in favour of the app, and the app says where its numbers come from. It works with no signal because it never asks for one. Nothing to sign into, nothing to subscribe to, no advertising, and nothing about you leaves the phone. there is no network code in it at all. Inches and CFM, or millimetres and litres per second. English, Spanish and Portuguese. Light and dark, and the instrument is drawn for both.