Ductwork Sizing Calculator

Find the right duct size for any room from CFM, friction rate, or velocity

Project Details
Enter your measurements and requirements
Results
Calculated estimates for your project
Round duct8 in for 200 CFM (exact 7.9 in). 7 in metal would starve this run; use 8 in.
Rectangular equivalents16 x 4 in, 10 x 6 in, 8 x 8 in
Velocity573 FPM (quiet)
Friction rate0.075 in. w.c. per 100 ft
Total effective length105 ft including fittings
Pressure drop for the run0.079 in. w.c.

Turn these numbers into a build plan.

  • 200 CFM airflow
  • 8" round duct size
  • 573 FPM velocity
trunktakeoff8 in branch, 200 CFMboot

Running a line set too? See the refrigerant line calculator. Wrap ducts in unconditioned space with the insulation calculator, and check the permit and inspection cheat sheet before changing a system.

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Project plan · built from your numbers

Supply duct run, 8" round

A 8" round supply run sized for 200 CFM, which moves air at about 573 FPM (quiet). Planned for a typical 20 ft run from the trunk to one register; anything touching the furnace, refrigerant or gas belongs to a licensed HVAC tech.

Materials
$194
Time
1 weekend
Level
Intermediate
8 steps 8 tools 8 materials
  1. 1

    Shut off the system and check scope

    Turn the thermostat off and kill power to the air handler at its switch. This plan covers duct between the trunk and the register only. Furnace, coil, refrigerant, gas and electrical work belong to a licensed HVAC tech, and many areas need a mechanical permit for new runs.

    • System off at thermostat and service switch
    • Check if your area requires a mechanical permit
    • Confirm the trunk has capacity for the added run
  2. 2

    Plan the route

    Map the shortest path with the fewest turns from the trunk to the register. Each elbow adds resistance, so 8" round sized for 200 CFM only works if the run stays short and smooth.

    • Route measured end to end
    • Two elbows or fewer
    • Clear of joists, wiring and plumbing
  3. 3

    Cut in the takeoff

    Trace the takeoff on the trunk, drill a starter hole, cut with snips, then bend the tabs over and screw it in place. Set the damper fully open for now.

    • Opening traced and cut clean
    • Takeoff screwed every 3 to 4 inches
    • Damper open
01 · Your numbers
Airflow
200 CFM
Duct size
8" round
Velocity
573 FPM
Est. cost
$194
02 · The plan

8 steps · 8 tools · 8 materials

5 more steps, the full tool list and a shopping list you can check off as you go.

03 · Finished

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Supply duct run, 8" round

Finished in 1 weekend · $194 DIY

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CFM by duct size

Max CFM at 0.08 in. w.c. per 100 ft, capped at 700 FPM
Round sizeMetalFlexRectangular equivalent
4 in33264 x 4, 6 x 6, 8 x 8
5 in59486 x 4, 6 x 6, 8 x 8
6 in96788 x 4, 6 x 6, 8 x 8
7 in14511712 x 4, 8 x 6, 8 x 8
8 in20616616 x 4, 10 x 6, 8 x 8
9 in28122720 x 4, 12 x 6, 10 x 8
10 in37230026 x 4, 16 x 6, 12 x 8
12 in54948740 x 4, 24 x 6, 16 x 8
14 in74873260 x 4, 34 x 6, 22 x 8

How to size a duct

  1. Find the airflow. Use the room's CFM from a Manual J load, or about 1 CFM per sq ft, or the system's 400 CFM per ton split by room area.
  2. Pick a friction rate. 0.08 in. w.c. per 100 ft is the common residential design rate; 0.10 for short runs, 0.06 for long or noisy ones.
  3. Read the round size. Solve D = (0.109 x CFM^1.9 / friction)^(1/5.02) and round up to a stock size. 200 CFM at 0.08 is 7.9 in, so 8 in.
  4. Check velocity and the run. Keep branches under about 700 FPM. Add fittings as equivalent length and keep the run's drop under about 0.1 in.
  5. Convert if needed. For joist or stud bays, convert to a rectangle with the same equivalent diameter: 8 in round is about 16 x 4 or 10 x 6.
Equivalent length of common fittings (ballpark, ACCA Manual D)
FittingEquivalent length
Smooth 90° elbow15 ft
Trunk takeoff35 ft
Register boot35 ft

Worked examples

RunAirflowSizeVelocityRun pressure drop
Bedroom, 20 ft metal, 1 elbow120 CFM7 in449 FPM0.059 in
Bonus room addition, 35 ft flex, 2 elbows300 CFM10 in550 FPM0.107 in
Bath exhaust fan, 15 ft flex80 CFM7 in299 FPM0.012 in

Bath fans are rated at 0.1 in or 0.25 in of static pressure; size the duct so the run stays under the fan's rating.

When to call a pro

One added branch is a reasonable DIY job. Replacing a system, adding a zone, or finishing a basement with several new runs needs a Manual J load and a Manual D duct design so every room gets its air and the blower is not starved. Check the permit and inspection cheat sheet, pick filters with the MERV filter cheat sheet, and see the refrigerant line calculator for split systems.

Duct sizing FAQ

What size duct for 100 CFM?

7 in round metal at 0.08 friction (6 in is just short, at 6.1 in exact), or 7 in flex. 200 CFM needs 8 in metal and 400 CFM needs 12 in.

What is a ductulator?

A slide-rule style calculator HVAC techs use to size ducts from airflow and friction rate. This page does the same math, and adds rectangular equivalents and the pressure drop for the whole run.

What friction rate should I use?

0.08 in. w.c. per 100 ft is the usual residential starting point. The right rate comes from Manual D: available static pressure divided by the longest run's total effective length.

Is flex duct sized differently?

Yes. Even pulled tight, flex has roughly 1.5 times the friction of smooth metal, so it usually needs one size larger. Sagging or compressed flex can double that.

How many CFM per ton?

About 400 CFM per ton of cooling, so a 3 ton system moves 1,200 CFM. Humid climates often run 350 per ton for better dehumidification.

How do I convert round duct to rectangular?

Use the equivalent diameter, De = 1.30 x (a x b)^0.625 / (a + b)^0.25. An 8 in round equals about 16 x 4, 10 x 6 or 8 x 8 in. Keep the aspect ratio under 4 to 1 where you can.

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