Check an existing home: measure, check, improve.
Is this existing duct system likely to be a problem, and what could help? Start near the equipment, where the supply plenum, return approach and filter carry the full system airflow. This is a quick check for an existing home. It does not replace a full load, duct or register design (Manual J, D or T).
CFM (cubic feet per minute) is how much air moves. FPM (feet per minute) is how fast it moves. Air speed = CFM ÷ clear area in square feet. At the same airflow, a larger clear opening slows the air. Sharp turns, crushed flex, filters and leaks still matter.
In Equipment, use 350 CFM per ton or enter required CFM. A restriction can lower the measured airflow. Do not size the ducts for that lower airflow.
Enter outside dimensions for the default 1½-inch ductboard; the tool subtracts 3 inches from each dimension. Change material when needed. Use inside diameter for flex. Check the tightest clear return opening, too. Tap ? for measurement sketches.
Example: 3 tons = 1,050 CFM. A 21 × 27-in. outside ductboard box is 18 × 24 in. clear inside: 3 ft², giving 350 FPM.
Supply green is 350–550 FPM; return green is 350 FPM or less. Yellow needs review; red flags a concern. The colors do not tell you fitting pressure drop or prove the system meets the 0.50-in. w.c. pressure limit. in. w.c. means inches of water column, a pressure unit. Filter face area is a starting check. Use filter pressure drop to judge resistance. Use one cabinet or at most two external filters.
Enter room CFM, or estimate from system CFM, house area, room area and load character. The area method gives each room a rough share of system airflow. It does not calculate the room’s heat load. Choose 1–3 supplies and throw distance. Branches get at least one common size above the minimum at 550 FPM. Put dampers in the branches. Keep that margin and use a gradual reducer into a smaller confirmed boot collar; its air speed is shown separately. Slower air in a large branch can be useful.
Room returns and jump ducts follow the total intended supply CFM into the room. A jump duct needs enough open grille area at both ends. Suggested sizes do not guarantee room pressure: verify −3 to +3 Pa with the door closed. Pa means pascals, a pressure unit.
Use Fittings & sizing to review tight openings, abrupt turns and flex routing. Check the throat (inside turn) as closely as the heel (outside turn). Check boot and collar fit, how far the register throws, and how much air reaches the room. Then balance the airflow. Open Field summary to copy the findings.