MasterTool

Manual D Duct Sizing Calculator

Estimate a round duct diameter from airflow, velocity and friction targets.

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What this tool does

Estimate a round duct diameter from airflow, velocity and friction targets. Friction equation is an empirical smooth round metal duct estimate; fittings, total effective length and fan static pressure are not modeled. Not a complete ACCA Manual D design.

How to use the Manual D Duct Sizing Calculator

  1. Enter or select airflow (cfm).
  2. Enter or select maximum air velocity (ft/min).
  3. Enter or select friction rate (in water/100 ft).
  4. Read the calculated result; change any measurement to compare alternatives.

Formula

velocity diameter(in)=12×sqrt(4×CFM/(π×velocity)); friction diameter(in)=(0.109136×CFM^1.9/friction)^(1/5.02); use larger diameter
cfm
Airflow (CFM)
velocity
Maximum air velocity (ft/min)
friction
Friction rate (in water/100 ft)

Friction equation is an empirical smooth round metal duct estimate; fittings, total effective length and fan static pressure are not modeled. Not a complete ACCA Manual D design.

Worked example

For manual d duct sizing calculator, the following measurements illustrate the exact method: Airflow (CFM): 400; Maximum air velocity (ft/min): 700; Friction rate (in water/100 ft): 0.08.

Inputs

  • Airflow (CFM)400
  • Maximum air velocity (ft/min)700
  • Friction rate (in water/100 ft)0.08

Result

  • Round duct diameter10.27 in
  • Velocity-based diameter10.24 in
  • Friction-based diameter10.27 in

Results explained

Round duct diameter
Round duct diameter from the formula above, expressed in in before metric conversion. Friction equation is an empirical smooth round metal duct estimate; fittings, total effective length and fan static pressure are not modeled. Not a complete ACCA Manual D design.
Velocity-based diameter
Velocity-based diameter from the formula above, expressed in in before metric conversion. Friction equation is an empirical smooth round metal duct estimate; fittings, total effective length and fan static pressure are not modeled. Not a complete ACCA Manual D design.
Friction-based diameter
Friction-based diameter from the formula above, expressed in in before metric conversion. Friction equation is an empirical smooth round metal duct estimate; fittings, total effective length and fan static pressure are not modeled. Not a complete ACCA Manual D design.

Frequently asked questions

velocity diameter(in)=12×sqrt(4×CFM/(π×velocity)); friction diameter(in)=(0.109136×CFM^1.9/friction)^(1/5.02); use larger diameter

Friction equation is an empirical smooth round metal duct estimate; fittings, total effective length and fan static pressure are not modeled. Not a complete ACCA Manual D design.

No. Electrical and HVAC results are estimates. Installation must comply with local codes and be checked by a qualified licensed professional.

Nameplate values, conductor temperature, power factor, efficiency, weather and actual installation conditions may differ from your entered assumptions.

Confirm units, manufacturer data and the formula's assumptions, then verify sizing with the equipment instructions and applicable local standards.