Manual D Friction Rate Calculator
Design friction rate for duct sizing.
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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.
Friction rate = (available SP / total equivalent length) x 100
How to use the Manual D Friction Rate Calculator
- 1Take the available static pressure left after equipment component losses.
- 2Divide by the total equivalent length of the system's longest duct run.
- 3Multiply by 100 to express the friction rate per 100 feet, then use it on a duct friction chart to size every run.
Worked example
Using the values the calculator loads with:
Inputs
- Available static pressure0.25 in wc
- Total equivalent length150 ft
Results
- Design friction rate0.167 in wc / 100 ft
- AssessmentTypical residential range
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
- Built and reviewed by James Whitfield · How we build and test calculators
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Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.