All the while staying within your local constituencies building codes, safety rules and regulations. Americans still use the feet and inches imperial measurement system , while up north in the colder climes of Canada both metric that is meters and cm and the older standard system of measurements feet and inches both prevail in the building and construction trades depending on the age of the building and real estate properties. Of course given the preference of the builder of geographical locality they can be made in either imperial feet or metric measurements and specifications.For diameters greater than eighteen inches increments vary from one to three inches. Yet you want to keep both building and maintenance costs to a minimum in these days of heady building material costs as well as all curtain fabrics wholesale the ever increasing extra governmental taxes paid on both building supplies and contractors labor costs and castings. In general for ducts up to eighteen inches in diameter, twenty-gauge galvanized steel is used.Air conditioning and heating ducts. One general principle which can be said to govern the whole process that is of the choice and choices of duct size and sizing is that the smaller the duct size for a given air capacity and set of capacities , then the greater the velocity of air inside the duct and the greater the frictional resistance within that system.Overall it can be said in the heating and air conditioning trades that duct sizes and diameters depend in both strict and simple senses to the air velocity and velocities needed in a duct or series of air circulation series of vents of ducts. For sizes between twelve inches and eighteen inches, only one inch incremental increases in diameter are usually found and made available at heating and air conditioning suppliers.Lastly ducts and ducting come in various thicknesses or gauges. In your home or office you want adequate and full not insufficient levels of air to flow fully to both heat, cool and vent the air and atmosphere flows.However, in the heating and air conditioning building and maintenance trades certain standard sizes are readily available and therefore readily available generally off hand less costly and expensive to buy and setup. Round ducts of four inches to twelve inches in diameter are made in one inch structural increments overall. Most elbows and transitions are generally made from sheet metal two gauges thicker than the duct. How do you properly and correctly assess and judge vent duct size, sizes and diameters correctly and fully according to professional heat and air-conditioning trades as well as state, federal and provincial building codes and standards. For between eighteen inches and thirty inches, eighteen inches is generally used. Most hoods are made from sixteen gauge galvanized or black steel.Sheet metal workers can construct ducts to practically any size in the universe of the heating and air conditioning world desired. Overall this basis and concept does not vary if the utilization of the duct is for heating in the fall winter and spring, air conditioning and cooling purposes in the summer or simple exhaust of gasses from a commercial establishment say for example a washer and dryer Laundromat or a pizzeria with a bank of ovens.