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Static Pressure Budget Calculator

Available pressure → TEL → friction rate

Quick answer: available duct pressure = blower external-static allowance − external component losses. Friction rate = available duct pressure × 100 ÷ total equivalent length (IP) or available duct pressure ÷ TEL (SI).

Project inputs

Read the equipment fan table at the design airflow. Confirm which components are already inside its rating boundary.
External component losses (in. w.g.)
Add only losses outside the blower-table boundary, such as a silencer or VAV box.
Controlling circuit length (ft)
Straight duct plus fitting equivalent length.
Use the same accounting method as supply.

Formula reference

Available pressure = blower allowance − Σ external losses

Friction rate = available pressure × 100 ÷ TEL (IP); available pressure ÷ TEL (SI)

Use the result as a design input

This worksheet turns the equipment and route data you provide into one pressure budget for the controlling supply/return circuit. It does not supply manufacturer coil, filter, grille, terminal, or fitting losses; enter the values at the design airflow and maintenance condition.

  1. Read the blower's external-static allowance and identify its test boundary.
  2. Subtract only external component losses that are outside that boundary.
  3. Add straight and fitting equivalent lengths for the critical supply and return circuit.
  4. Use the resulting friction rate to select each duct section, then recompute actual losses and verify velocity, noise, balancing, and fan selection.

Boundaries

A zero or negative available pressure means the stated components consume the entire allowance. Review the boundary, component selection, route, or equipment before sizing duct. Parallel branches are not added to the controlling TEL.

FAQ

What does this static pressure calculator calculate?

It subtracts external component losses from the blower's stated external-static allowance, adds supply and return equivalent lengths, and divides the remaining pressure by the complete circuit length to show a friction rate.

What is total equivalent length (TEL)?

TEL is the controlling supply and return route length after straight duct and fitting equivalent lengths are accounted for. Do not add parallel branches to the critical circuit.

Can this calculator select a fan or duct size?

No. It is a pressure-budget worksheet. Use the equipment fan table and manufacturer component curves, then verify each duct section, fitting, velocity, balancing condition, and the final fan selection.

Sources and next steps

The numeric example values are editable teaching inputs. Equipment and fitting manufacturer data control the final design.