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Calculation Machine

Manual D Friction Rate Calculator

Design friction rate for duct sizing.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
Design friction rate
0.167 in wc / 100 ft
Assessment
Typical residential range

Quick answer

The Manual D design friction rate is the available static pressure divided by the total equivalent length of the longest duct run, expressed per 100 feet.

The formula

Friction rate = (available SP / total equivalent length) x 100

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Available static pressure0.25 in wc
  • Total equivalent length150 ft

Result

  • Design friction rate0.167 in wc / 100 ft
  • AssessmentTypical residential range

How to use the Manual D Friction Rate Calculator

  1. 1Take the available static pressure left after equipment component losses.
  2. 2Divide by the total equivalent length of the system's longest duct run.
  3. 3Multiply by 100 to express the friction rate per 100 feet, then use it on a duct friction chart to size every run.

Frequently asked questions

What's a typical friction rate?

Residential systems commonly land between 0.08 and 0.18 in wc per 100 ft.

Does every duct in the house use this rate?

Yes — the same design friction rate is applied throughout so branches and trunks are sized consistently.

About this calculator