Duct replacement calculator: airflow, static pressure & sizing

Duct size comes from two numbers: the airflow each run must carry (CFM) and the friction rate your system can afford. Residential equipment needs roughly 400 CFM per ton of capacity, and the friction rate comes from the Manual D worksheet: the blower's rated static pressure, minus filter and register losses, spread over the longest duct path. A 3-ton system moving 1,200 CFM typically needs a 14–16 inch round metal trunk and 6–8 inch branch runs — but every home's friction rate is different, which is why sizing from a chart alone produces the noisy, starved systems we get called to fix.

Static pressure is the resistance your blower pushes against, measured in inches of water column. Residential air handlers are typically rated for 0.50 in. w.c. total external static — and everything eats into it: the filter (0.10–0.25), supply registers, return grilles, dampers, and finally the ducts themselves. When ducts are too small, static rises past the rating, airflow falls, coils freeze or overheat, and the blower motor runs hot and dies young. Measuring total external static pressure with a manometer is the duct system's blood-pressure test — our techs do it at every commissioning.

Manufacturers require installation to their published specifications, and airflow is the specification: most require roughly 350–450 CFM per ton and operation within the blower's rated external static pressure. A compressor or heat-exchanger failure traced to chronic low airflow from undersized ducts is a textbook basis for a denied claim — the parts warranty covers defects, not equipment run outside spec. The protection is documentation: a Manual D-sized duct system, an AHRI-matched equipment combination, and commissioning readings (measured static, verified airflow) kept with your paperwork.

Sheet metal is smooth inside, so air moves with less friction; flex duct's spiral core adds resistance even when perfectly stretched — and most flex isn't perfectly stretched. The published field rule our calculator uses: a flex run needs to be one size larger than the sheet-metal answer, and it must be pulled tight and supported per the Air Diffusion Council guidance. Flex has legitimate uses (short branch runs, tight retrofits, sound damping); a flex trunk line sagging through a crawl space is how a right-sized system delivers wrong-sized airflow.

Technician mode is the Manual D equal-friction worksheet: enter the equipment's design airflow and rated external static, itemize the component pressure drops, measure the total effective length of the longest supply and return path (including fitting equivalent lengths), and add the room-by-room airflows from the Manual J load. The calculator returns the friction rate, per-run metal and flex sizes, velocity flags against the Manual D residential limits, and a commissioning checklist to document static and airflow for the warranty file.

No — it runs the real equal-friction method with published values, so it's a genuinely useful planning and sanity-check tool, but a bindable duct design starts with a room-by-room Manual J load calculation and accounts for your actual floor plan, fitting selections, and equipment blower tables. That full Manual J + Manual D design is exactly what a proper Eco ductwork replacement quote includes, and permits and inspections apply to duct replacement in Washington.