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Calculation Machine

Duct Friction Loss Calculator

Pressure drop along a duct run.

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

Your values

ⓘ Live results

Results
Total friction loss
0.197 in w.c.
Friction rate
0.281 in w.c. per 100 ft
Total equivalent length
70 ft
Velocity
1,146 FPM

Working

  1. 10.281 per 100 ft x 70 ft = 0.197 in w.c.

Quick answer

Friction loss rises roughly with the square of velocity and falls sharply as duct diameter grows.

The formula

ΔP ≈ 0.109136 x L x Q^1.9 / d^5.02 (in w.c. per 100 ft basis)

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Airflow400 CFM
  • Duct diameter8 in
  • Duct length40 ft
  • Equivalent length of fittings30 ft

Result

  • Total friction loss0.197 in w.c.
  • Friction rate0.281 in w.c. per 100 ft
  • Total equivalent length70 ft
  • Velocity1,146 FPM

How to use the Duct Friction Loss Calculator

  1. 1Enter airflow and the duct diameter.
  2. 2Add the straight length plus the equivalent length of elbows and fittings.
  3. 3Aim for a friction rate near 0.08–0.10 in w.c. per 100 ft on residential systems.

Frequently asked questions

What equivalent length is an elbow?

A smooth 90° elbow is roughly 15 ft of equivalent length on 8 inch duct; a sharp one is far worse.

Does flex duct differ?

Yes — compressed flex can triple the loss, so use fully stretched flex or rigid pipe.

About this calculator