DUCT • ROUND SIZING

Duct sizing (round)

Size round duct using a velocity target. This is an estimate and does not model friction losses or fittings—use it as a quick starting point and confirm with full duct design practices.

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Calculator
v1
Inputs
Velocity method
Enter airflow in CFM.
A common compromise for branches; adjust for noise and comfort constraints.
Enter velocity in feet per minute (fpm).
Missing/invalid inputs:
  • Enter airflow (CFM).
Results
Required area
— ft²
Computed round diameter
—"
Formula: Area = CFM ÷ fpm, then Diameter = 2·sqrt(Area/π).
Standard size suggestions tap
Enter CFM and a target velocity to see suggestions.
Recommendation logic: picks the smallest shown standard size that does not exceed your target velocity (when possible). Always confirm against noise and static pressure constraints.
What this does (and does not) include tap
  • Includes: velocity-based sizing for a single round duct run (area/diameter relationships).
  • Does not include: fittings, equivalent length, friction rate, material roughness, coil/return restrictions, or full static pressure calculations.
  • Use this as a starting point. For real duct design, you typically validate pressure loss and noise constraints end-to-end.
Safety note: This tool provides guidance and estimates. Always follow OEM procedures, local codes, and safe work practices.
Saved calculations
Duct sizingtap
Saved calculations (tap to expand).
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About this calculator
REFERENCE

This calculator recommends duct sizes based on airflow targets and common sizing tradeoffs (velocity, friction, and practical constraints). It’s designed for fast field use and quick iteration.

Who it’s for

  • Service techs doing quick sizing sanity checks.
  • Installers planning a run and needing a starting size.
  • Anyone comparing tradeoffs (noise vs static vs available space).

When to use it

  • You need a rough round duct size for a given CFM target.
  • You’re comparing options (noise vs pressure drop vs available space).
  • You’re doing a quick sanity check on an existing duct run.

Required inputs

Target airflow (CFM).
Required to compute results.
Friction rate (or your chosen sizing rule/target).
Required to compute results.
Duct type/shape selection (round vs rectangular, if supported in the UI).
Required to compute results.

Outputs

Recommended duct size(s)
Suggested size(s) based on your airflow target and sizing assumptions.
Velocity / friction indicators
Quick indicators to help you judge tradeoffs (noise/static). Availability depends on the calculator’s current version.
Copy-ready summary
A short text block you can paste into job notes (inputs + outputs) for repeatable documentation.

How to use (field notes)

  1. Start with your target CFM for the branch/trunk you’re sizing.
  2. Pick a friction/velocity target that matches your design approach and constraints.
  3. Use the result as a starting point, then validate against fittings, available space, and total external static.
Assumptions & limitations tap
  • Real systems include fittings, flex, transitions, and losses not captured by a simple straight-duct estimate.
  • Noise and comfort constraints can require lower velocities than a 'minimum size' result.
  • Rectangular ducts need aspect ratio considerations; round equivalents may not translate 1:1.
Safety note: Results are estimates for informational use. Always follow OEM charging procedures, safety practices, and local codes.
FAQs
DIRECT ANSWERS
Why does the calculator recommend a bigger duct than what’s installed? tap

Installed ducts are often undersized due to space and cost constraints. A larger recommendation usually reflects lower velocity and pressure drop targets. If you can’t increase size, you may need to adjust expectations (noise/static) or address restrictions elsewhere.

Is higher velocity always bad? tap

Not always, but it often increases noise and pressure drop. Higher velocity can be acceptable in short runs or certain trunks, but it’s a tradeoff. Use the velocity/friction indicators to decide what’s reasonable for the application.

What friction rate should I use? tap

Use the target that matches your design approach and standards. If you don’t have one, start with a conservative default and sanity-check against total external static and noise constraints. Document the assumption in your notes.

Does this replace Manual D? tap

No. It’s a fast field helper. Manual D accounts for fittings, equivalent lengths, room-by-room flows, and system constraints. Use this tool for quick estimates and iteration, then verify with proper design methods when required.