HVAC Duct Calculators

Pressure loss and static pressure

Available Static Pressure and Friction Budget

Subtract component losses from available pressure and use route length to find the friction budget.

Calculation inputs

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Use compatible pressure definitions and one stated operating airflow.
Use measured or manufacturer performance at the stated operating point.
Airflow associated with TESP and component drops.
Use pressure drop at this operating airflow and selected replacement condition.
Only subtract if outside the chosen equipment pressure-tap boundary.
Exclude losses already included by the pressure-tap boundary.

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Formula and source basis

Concepts behind this calculation

Calculate a pressure budget at a stated equipment operating point, compare path margin, or derive friction rate from ASP and total effective length.

Available static pressure is the pressure remaining for the selected duct boundary after compatible nonduct component losses are deducted. Components already inside the equipment rating boundary must not be deducted again.

A design friction budget spreads available pressure over effective length. It allocates pressure; it does not calculate the actual resistance of a proposed duct or establish delivered airflow.

How to use this calculator

  1. Choose TESP to available static pressure and enter total external static pressure, operating airflow and only the filter, coil and other nonduct component drops outside the matching equipment boundary.
  2. For available static pressure to design friction rate enter the resulting available static pressure with the compatible route total effective length.
  3. For the pressure-margin comparison enter available static pressure and required compatible path loss on the same boundary and airflow basis; investigate a deficit before changing the allocation.

Calculation method and applicability

ASP = TESP − nonduct component drops; design friction = ASP/TEL×100; margin = ASP−path loss

ASP = available static pressure; TESP = total external static pressure, both in. w.g. TEL is total effective length in feet. Multiplication by 100 expresses design friction in in. w.g./100 ft.

Input definitions and units

Pressure budget calculation
Use compatible pressure definitions and one stated operating airflow. Choices: TESP to available static pressure; Available static pressure to design friction rate; Compare available pressure and required path loss.
Total external static pressure (in. w.g.)
Use measured or manufacturer performance at the stated operating point. Applies to Pressure budget calculation: TESP to available static pressure.
Operating airflow (CFM)
Airflow associated with TESP and component drops. Applies to Pressure budget calculation: TESP to available static pressure.
Filter drop (in. w.g.)
Use pressure drop at this operating airflow and selected replacement condition. Applies to Pressure budget calculation: TESP to available static pressure.
Coil drop (in. w.g.)
Only subtract if outside the chosen equipment pressure-tap boundary. Applies to Pressure budget calculation: TESP to available static pressure.
Other nonduct component drops (in. w.g.)
Exclude losses already included by the pressure-tap boundary. Applies to Pressure budget calculation: TESP to available static pressure.
Available static pressure (in. w.g.)
ASP for duct and fittings at a compatible pressure basis. Applies to Pressure budget calculation: Available static pressure to design friction rate.
Total effective length (ft)
Source-based index-path length. Applies to Pressure budget calculation: Available static pressure to design friction rate.
Available static pressure (in. w.g.)
Available at the same airflow/pressure reference as required loss. Applies to Pressure budget calculation: Compare available pressure and required path loss.
Required compatible path loss (in. w.g.)
Sum only compatible path losses and pressure references. Applies to Pressure budget calculation: Compare available pressure and required path loss.

How to interpret the result

Keep ASP, allocated friction rate and path margin distinct. Positive margin is an arithmetic budget result at the stated airflow, not proof of a selected fan or duct network.

Worked examples

TESP to available static pressure

Inputs: TESP 0.50 in. w.g. at 1,200 CFM; filter drop 0.10; coil drop 0.15; other 0.

Calculation: Subtract only nonduct losses outside the equipment pressure boundary.

Result: ASP 0.250 in. w.g. at 1,200 CFM.

Available static pressure to design friction rate

Inputs: ASP 0.30 in. w.g.; TEL 150 ft.

Calculation: 0.30/150×100.

Result: 0.200 in. w.g. per 100 ft.

Compare available pressure and required path loss

Inputs: Available 0.50 in. w.g.; required compatible loss 0.45.

Calculation: Subtract required loss from available pressure.

Result: 0.050 in. w.g. margin.

Assumptions and limits

  • TESP, fan static pressure and fan total pressure are not interchangeable.
  • Subtract only components outside the selected equipment pressure-tap boundary.
  • ASP is not design friction rate; design friction also depends on total effective length.
  • Use pressure-drop data at the stated airflow; no filter/coil curve is included.

FAQs

Can I subtract a clean-filter drop at a different CFM?

Use losses at the same operating airflow and condition as the TESP point. A clean or replacement filter state should be identified explicitly.

What does a negative pressure margin mean?

The compatible entered path requirement exceeds the available pressure. It does not predict the new operating CFM; that needs fan and system performance.

Sources

Related calculators

Duct Total Effective Length Calculator Combine straight lengths and documented fitting allowances to total a duct route’s effective length. Fan and System Curve Calculator Intersect entered fan data with a system resistance model to estimate the operating airflow and pressure.

Related reading

Available static pressure, TEL, and design friction rate Build a compatible pressure allowance for ducts and fittings, then divide it by a matching total effective length. Duct design workflow: from airflow to a pressure check Connect an established airflow requirement to clear duct geometry, constructible routes, compatible losses, and an equipment pressure boundary in a traceable sequence. Duct leakage testing: CFM25, leakage class, and operating leakage Compare a leakage test with a supplied class or allocation criterion, fit a same-specimen pressure curve when appropriate, and keep test leakage separate from operating airflow. Add system losses and identify the critical path Build compatible complete paths through a duct system, preserve shared segments and endpoint pressure requirements, then identify the route that governs a single-fan duty. Use matched filter and damper performance curves Match product, size, condition, curve basis, and pressure boundary before using a filter or damper pressure result in a system budget. How to read a duct calculator result A useful result includes the number, its unit, its method, its assumptions, and the next check the number still needs. Static, velocity, and total pressure: where TESP fits Keep fan-plane pressure, equipment TESP, velocity pressure, and total pressure tied to their correct definitions and measurement locations. Target velocity vs. equal-friction duct sizing Choose a target-velocity screen or a friction-based straight-duct size by identifying the governing input, air properties, geometry method, and next pressure check.