Duct Design Calculator
The planner does not invent any new math. Each room's branch duct is sized by the equal-friction method — the same ASHRAE friction equation the Duct Size Calculator uses — and each room's supply register is sized by face velocity, exactly as the Return Air Grille Size Calculator does. Flexible duct is sized one step larger than sheet metal, and every branch is held under about 700 feet per minute (the trunk under 900) so the system stays quiet. Change the material or the friction rate once at the top and it applies to every run, so the whole system rests on a single, honest assumption.
The system rollup is what makes this more than running two calculators twice. Once every room is entered, the airflows are summed to size the supply trunk and the return duct, and that total also sizes the main return grille — as a filter grille (held slow, around 400 feet per minute) or a plain return, split across more than one grille if a single one would be too big. Undersized returns are the single most common duct problem: a restricted return starves the blower and raises the static pressure the whole system fights, cutting airflow at every register, so the planner leans generous on the return.
One honest limit, stated plainly: this is a screening estimate, not a code duct design. The friction rate is assumed unless you measure your blower's available static pressure, grille free areas vary by model, and a real system needs room-by-room loads to begin with. A full ACCA Manual D — driven by Manual J room loads and your measured static pressure — is required for a design you can balance and permit. Not sure of a room's airflow? Size it from that room's load on the Manual J calculator first. Free, no signup — part of the HVAC and ventilation section.
Not sure of each room's airflow? Start with the Manual J load calculator →
Duct Design Calculator
Plan a whole supply-and-return system in one pass. Enter each room's airflow once and get its supply branch duct and supply register side by side, then the supply trunk, return duct, and main return grille sized from the total — no carrying CFM between calculators by hand. A screening planner that runs the same standalone Duct Size and Return Air Grille calculators; not a substitute for ACCA Manual D. Free, no signup.
System setup
Applied to every branch and the trunk. Duct material and friction rate drive the duct sizes; the return setting picks the face-velocity limit for the main return.
Friction rate is an assumption. The honest rate is your blower's available static pressure ÷ the longest run's total effective length — measure it for a real design.
Rooms
Enter each room's design airflow once. Not sure of a room's CFM? Size it from the room's heating/cooling load on the Manual J calculator first.
How the planner sizes a whole system
This planner doesn’t invent a new method — it runs the same standalone Duct Size and Return Air Grille calculators for every room, then totals the system. These engineering diagrams show how one CFM per room becomes two sizes, how the branch duct and the register are each sized, and why an undersized return undoes the rest.
The rollup diagram is the whole idea in one picture: each room’s airflow, entered once, is sized two ways — its supply branch duct and its supply register — and then every room’s CFM is added up to size the supply trunk, the return duct, and the main return grille. Enter the room once; the planner produces the branch, the register, and the system total together.
Each room’s branch duct is sized by the equal-friction method — one friction rate held across the whole system, solved for the diameter that carries the flow. That shared rate is what keeps the branches balanced against each other, and it’s exactly what the standalone Duct Size Calculator does.
The supply register is sized differently — by face velocity, not friction — because the air has to pass through the grille’s open area slowly enough to stay quiet. The same CFM therefore gives one answer for the duct and another for the register, and both come straight from the Return Air Grille Size Calculator.
Finally, the reason the planner leans generous on the return: an undersized return is the single most common duct fault. The same airflow forced through a smaller opening speeds up, whistles, and raises the static pressure the whole system fights — starving the blower so every supply register delivers less air. The fix is more return grille area, not a bigger blower.
Related Calculators
Duct Size Calculator
Free duct size calculator: turn a run's CFM and friction rate into a round duct diameter, rectangular equivalents, and velocity. No signup.
Return Air Grille Size Calculator
Free return air grille size calculator: turn CFM into the grille or supply register size by face velocity, with standard sizes that fit. No signup.
Manual J Calculator
Free Manual J calculator: estimate your home's heating and cooling load in BTU and tons from size, climate, insulation, and windows. No signup.
AC Size Calculator
Free AC size calculator: the cooling load and the right air conditioner tonnage for your home by size, climate, insulation, and windows. No signup.
How to Use This Calculator
- Set the system defaults once: duct material (sheet metal or flex), a friction rate, and whether the main return has a filter. These apply to every run.
- Add a row for each room and enter that room's design airflow in CFM. Not sure of the number? Size it from the room's load on the Manual J calculator first.
- Use "Add room" for as many rooms as the system serves, and remove any you don't need. Give each a name so the results are easy to read.
- If you want to split the main return across more than one grille, set the number of return grilles.
- Click Calculate. Each room shows its supply branch duct size and supply register size with the resulting velocity; a velocity over the quiet limit is flagged in red.
- Read the system rollup for the supply trunk, return duct, and main return grille sized from the total airflow, then copy, save, or email the plan.
Why size branches, registers, trunk, and return together
A duct system is one connected airflow problem, but it is usually sized one piece at a time with a calculator per piece — which means entering the same CFM into a duct-size tool and then a grille tool for every room, and separately totaling everything for the trunk and return. This planner collapses that into one entry per room. The branch duct uses the equal-friction method (a friction rate in inches of water column per hundred feet drives the diameter), while the register uses face velocity (the air must pass through the grille slowly enough to stay quiet), so the same CFM produces two different answers that both matter. Summing the rooms gives the trunk and return airflow; the trunk is sized at a higher velocity limit than branches because it is a main, and the return grille is held slowest of all when it carries a filter. The reason to see it all at once is that the pieces trade off: a tight return shows up as a starved blower that undoes careful branch sizing, and an oversized trunk wastes material and space. Keeping material and friction rate shared across every run keeps the design internally consistent. It remains a screening tool — a full ACCA Manual D from Manual J loads and measured blower static pressure is required for a real design, balancing, and permit — but it turns the room-by-room busywork into a single, traceable pass where every number links back to the standalone calculator that produced it.
Frequently Asked Questions
What does the duct design calculator do?
It plans a whole supply-and-return system in one pass. You enter each room's design airflow in CFM once, and it sizes that room's supply branch duct (by the equal-friction method) and its supply register (by face velocity) side by side. It then sums every room's airflow to size the supply trunk, the return duct, and the main return grille. It is a planner over two standalone calculators — the Duct Size Calculator and the Return Air Grille Size Calculator — so you never carry a CFM value between tools by hand. It is a screening estimate, not a substitute for a full ACCA Manual D.
How do I know each room's CFM?
Room airflow comes from the room's heating and cooling load — roughly, CFM is the room's sensible load divided by the system's temperature rise, which is why a proper design starts with a room-by-room ACCA Manual J. If you don't have those numbers, size each room's load on the Manual J calculator first and bring the CFM here. A rough rule of thumb is about 400 CFM per ton of cooling spread across the conditioned rooms, but that is a sanity check, not a design — unequal room loads need the real per-room numbers.
Does this replace ACCA Manual D?
No. Manual D is the full residential duct design standard: it uses room-by-room Manual J loads, your measured available static pressure, and the total effective length of the longest run to size and balance every duct. This calculator is a screening planner — it assumes a friction rate unless you've measured your blower, and it sizes pieces independently rather than balancing the whole system. Use it to plan and sanity-check sizes; a licensed design and a permit still require Manual D with measured static pressure.
Why are the branch duct and the register different sizes for the same room?
They're sized by different rules. A branch duct is sized by the equal-friction method — a target pressure loss per hundred feet sets the diameter — and is a round or rectangular duct. A register is sized by face velocity — the air has to pass through the grille's open area slowly enough to stay quiet — and is a rectangular face with only about 75% free area. The same CFM therefore gives one answer for the duct and another for the register, and both matter: the duct carries the air, the register delivers it into the room.
Why does the calculator size the return so generously?
Because undersized returns are the single most common and most damaging duct problem. A return that's too small forces the air through too fast — which is noisy — and, worse, it starves the blower, raising the static pressure the whole system fights and cutting the airflow at every supply register. Filter grilles are held especially slow (around 400 feet per minute) so the filter doesn't add noise or pressure. The planner leans toward a larger return and will suggest splitting it across two grilles when a single one would be too big.
Does it handle flexible duct versus sheet metal?
Yes. Flexible duct has more friction than smooth sheet metal, so it's sized at half the friction rate — which usually works out to one size larger for the same airflow. Pick the material once at the top and it applies to every branch and the trunk. One caveat the calculator can't enforce: flex only performs at its rated size when it's pulled drum-tight. Even 15% sag can double the friction and choke the airflow, so support it well and keep it straight.
What isn't included?
No pricing, no labor, and no fitting-by-fitting pressure-drop accounting. It sizes supply branches, supply registers, the supply trunk, the return duct, and the main return grille from airflow and a friction-rate assumption. It does not design the trunk takeoffs, balancing dampers, transfer grilles for closed rooms, or the equipment itself, and it doesn't verify that your blower can actually move the total airflow against the system's static pressure. Those belong to a full Manual D and a measured static-pressure check.