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Duct Static Pressure Calculator

Work out duct static pressure instantly with clear inputs, formula shown and shareable results.

About Duct Static Pressure Calculator

External static pressure is the friction the fan must overcome. Straight duct and fittings are combined into a total effective length, multiplied by the friction rate per 100 ft, then added to the fixed losses of coil, filter and terminals. On typical residential systems fittings contribute more than the straight duct does.

Formulas

Total effective length

TEL = straight length + sum of fitting equivalent lengths

Static pressure

ESP = friction rate x TEL / 100 + component losses

Worked example

With the default values below, Duct Static Pressure Calculator returns Total external static pressure of 0.470. Change any input above and every figure here is recalculated in your browser.

Inputs used

Longest straight duct runminimum 1 ft
60 ft
Fitting equivalent lengthminimum 0 ft
90 ft
Friction rateminimum 0.01 in.wg/100 ft
0.08 in.wg/100 ft
Coil, filter and grille lossesminimum 0 in.wg
0.35 in.wg

Results

Total external static pressure
0.470 in.wg
Total effective length
150 ft
Duct friction loss
0.120 in.wg
Share from fittings
60.0% %

Step by step

  1. 1Values used
  2. 2Total effective length
  3. 3Static pressure
  4. 4Total external static pressure= 0.470 in.wg
  5. 5Total effective length= 150 ft
  6. 6Duct friction loss= 0.120 in.wg
  7. 7Share from fittings= 60.0 %

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Duct Static Pressure figure worked out?

TEL = straight length + sum of fitting equivalent lengths. External static pressure is the friction the fan must overcome.

What is a normal target?

Most residential air handlers are rated at 0.5 in.wg total external static. Exceeding the rated value cuts airflow below design and can freeze the coil.

What values do I need to enter?

This calculator takes 4 inputs: Longest straight duct run, Fitting equivalent length, Friction rate, Coil, filter and grille losses. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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