HVAC Duct Calculators

Fans and energy

Fan Static and Total Pressure Calculator

Use signed fan-plane pressures, velocities, and densities to calculate fan static and total pressure rise.

Calculation inputs

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An example is loaded automatically. Replace it with your project values; the result updates as you edit.

Record the measurement source, actual fan inlet/outlet pressure planes, common ambient gauge reference and average-velocity method. Pressure taps in remote duct sections need independently justified corrections to the fan planes.
Worked-example data and reference

EngineerZ peer reproduction: inlet/outlet pressures referenced to common ambient; planes at fan connections; 0 FPM open inlet plenum, 1,574.803 FPM outlet; both plane densities 0.074913553 lbm/ft³ converted from 1.2 kg/m³.

These example values are for the worked calculation. Replace them with the source and conditions for your own case.

Confirm both static readings use the same ambient gauge reference, velocities represent the fan inlet/outlet planes, and inlet/outlet densities are established on the selected rating basis.
Use the signed static pressure at the fan inlet plane relative to the same ambient reference as the outlet reading. Negative suction pressure is valid.
Use the signed static pressure at the fan outlet plane on the same gauge datum as the inlet reading.
Use average velocity at the fan inlet plane. Enter 0 only for a large open inlet/plenum where inlet velocity is negligible; negative velocity is not supported.
Use average velocity at the fan outlet plane, based on the actual discharge area or a representative traverse. Remote-duct velocity is not automatically the fan-plane velocity.
Use established air density at the fan inlet measurement plane. No standard-air default, altitude lookup or psychrometric correction is applied.
Use established air density at the fan outlet measurement plane. Fan outlet velocity pressure uses outlet density per the AMCA fan-rating definition.

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Formula and source basis

Concepts behind this calculation

Convert fan inlet/outlet static pressures and average velocities into inlet/outlet total pressures, fan total pressure rise and fan static pressure.

At each fan plane, total pressure includes static pressure and velocity pressure. Velocity pressure follows local average speed and density, so the inlet and outlet can have different values. Fan total pressure is the rise from inlet total to outlet total pressure. Fan velocity pressure uses the outlet condition, and subtracting it from fan total pressure gives the fan static pressure used by this model.

The difference between two static taps generally exceeds fan static pressure by the inlet velocity pressure. That distinction disappears only when inlet velocity is negligible. Both static readings need one gauge datum and representative fan-plane locations; remote duct taps can include intervening losses or area changes. Correcting those measurements to the fan planes is a separate task that this calculator does not perform.

How to use this calculator

  1. Complete Fan test reference and pressure planes, then establish Fan test plane and pressure reference basis.
  2. Enter signed Fan inlet static pressure and Fan outlet static pressure on the common gauge datum.
  3. Enter Fan inlet and outlet average velocities and their respective air densities. Review plane total pressures, fan total rise, outlet velocity pressure and fan static pressure separately.

Calculation method and applicability

VPi = ρin × (Vi × 0.00508)² × 16.01846337396 / (2 × 249.08891); VPo = ρout × (Vo × 0.00508)² × 16.01846337396 / (2 × 249.08891). Pti = PSi + VPi; Pto = PSo + VPo; FTP = Pto − Pti; FVP = VPo; FSP = FTP − FVP = PSo − PSi − VPi. Pressures are in. w.g., velocities FPM, density lbm/ft³.

PSi/PSo = signed inlet/outlet static pressures; VPi/VPo = inlet/outlet velocity pressures; Pti/Pto = total pressures. FTP = fan total-pressure rise; FSP = fan static pressure. All pressure outputs are in. w.g.

Input definitions and units

Fan test reference and pressure planes
Record the measurement source, actual fan inlet/outlet pressure planes, common ambient gauge reference and average-velocity method. Pressure taps in remote duct sections need independently justified corrections to the fan planes.
Fan test plane and pressure reference basis
Confirm both static readings use the same ambient gauge reference, velocities represent the fan inlet/outlet planes, and inlet/outlet densities are established on the selected rating basis. Choices: Not established; Planes, common reference and density basis documented.
Fan inlet static pressure (in. w.g. gauge)
Use the signed static pressure at the fan inlet plane relative to the same ambient reference as the outlet reading. Negative suction pressure is valid.
Fan outlet static pressure (in. w.g. gauge)
Use the signed static pressure at the fan outlet plane on the same gauge datum as the inlet reading.
Fan inlet average velocity (FPM)
Use average velocity at the fan inlet plane. Enter 0 only for a large open inlet/plenum where inlet velocity is negligible; negative velocity is not supported.
Fan outlet average velocity (FPM)
Use average velocity at the fan outlet plane, based on the actual discharge area or a representative traverse. Remote-duct velocity is not automatically the fan-plane velocity.
Fan inlet air density (lbm/ft³)
Use established air density at the fan inlet measurement plane. No standard-air default, altitude lookup or psychrometric correction is applied.
Fan outlet air density (lbm/ft³)
Use established air density at the fan outlet measurement plane. Fan outlet velocity pressure uses outlet density per the AMCA fan-rating definition.

How to interpret the result

Select fan total or fan static pressure to match the downstream curve or efficiency definition. Retain both plane densities and the measurement reference; the simple static-tap difference is shown for comparison and is not generally interchangeable with fan static pressure.

Worked examples

Fan Static and Total Pressure Calculator

Inputs: Peer reproduction in customary units: inlet static −0.602194614 in. w.g., outlet static 1.204389228 in. w.g., inlet velocity 0 FPM (open plenum), outlet velocity 1,574.803 FPM, and inlet/outlet densities 0.074913553 lbm/ft³ converted from 1.2 kg/m³.

Calculation: Inlet total pressure = −0.602195 + 0 = −0.602195 in. w.g. Outlet velocity pressure = 0.154162 in. w.g.; outlet total pressure = 1.358551 in. w.g. Fan total pressure rise = 1.960746 in. w.g. Fan static pressure = 1.960746 − 0.154162 = 1.806584 in. w.g. (450.0 Pa). The peer case has equal inlet/outlet densities.

Result: 1.807 in. w.g. fan static pressure; 1.961 in. w.g. fan total pressure rise. The static tap difference is 1.807 in. w.g. for this open-inlet example only; it is not the general FSP formula.

Assumptions and limits

  • Use static readings from the fan inlet/outlet rating planes, or separately correct remote test taps to those planes. The tool does not estimate duct losses or fan/system installation effects between taps and the fan.
  • Inlet velocity pressure uses inlet density. Fan outlet velocity pressure uses outlet density per the AMCA fan-rating definition. Neither density is inferred from temperature, altitude, weather, humidity or a standard-air default.
  • Uses area-average plane velocities and low-speed, incompressible fan-rating relations. It does not correct nonuniform swirl, traverse quality, compressibility, leakage or system effects.
  • Fan static pressure is fan total pressure rise minus outlet fan velocity pressure. The outlet-minus-inlet static reading difference alone omits inlet velocity pressure and is not generally FSP.
  • This calculator derives pressure quantities from user-supplied readings; it does not validate a fan curve, select equipment, determine efficiency or establish code/field-test certification.

FAQs

Why is fan static pressure lower than the static-tap difference?

The fan static-pressure definition subtracts inlet velocity pressure from the outlet-minus-inlet static difference. A moving inlet stream already carries that kinetic pressure contribution.

When is zero inlet average velocity appropriate?

Use zero only when the documented inlet arrangement has genuinely negligible average velocity, such as an appropriate large open plenum. It is not a substitute for a missing inlet velocity measurement.

Sources

  • ANSI/AMCA Standard 99-25, Definitions, fan total/static/velocity pressureThe public definitions summary specifies fan total-pressure rise from fan outlet total to inlet total, fan static pressure as fan total pressure less fan velocity pressure, and fan velocity pressure using fan outlet average velocity at fan outlet density. The downloadable full standard PDF returned HTTP 403 in this session.
  • ASHRAE Handbook—Systems and Equipment 2024, IP Chapter 21: FansFan pressure and efficiency definitions distinguish static pressure, velocity pressure and total pressure. The selected pressure basis must match the efficiency/curve basis used downstream.
  • NISTIR 4758, Fan System Design Criteria and Specifications, Chapter 5Government fan-system guide explains fan static pressure as fan total pressure minus outlet velocity pressure and warns that it is not ordinarily the simple difference of outlet and inlet static readings. Historical guide; not represented as a current code.
  • EngineerZ Corner: Fan Static & Total Pressure CalculatorHTTP 200; directly executed 4/8 m/s, 1.2 kg/m³, −150/300 Pa case. Peer displayed 450.0 Pa FSP, 488.4 Pa FTP, 9.6 Pa inlet VP and 38.4 Pa outlet VP. Independent relation check; peer output is rounded and not an authority.
  • AIHA: Fan Static Pressure CalculatorHTTP 200; direct task fields SPout, SPin and VPin; printed FSP = SPout − SPin − VPin. Executed with 1.0, −0.5 and 0.062335 in. w.g.; default precision rounded the displayed result, so no higher-precision peer output is claimed.
  • SVE Group engineering calculator directorySearch index lists separate Fan Static Pressure and Fan Total Pressure calculators. Direct browser visit returned HTTP 403 Cloudflare challenge; individual calculator forms or results were not inspected.
  • PlantCalcKit: Fan and Blower Performance CalculatorHTTP 200; fan performance controls include total/static pressure and corresponding efficiency-basis selectors. Adjacent fan-duty evidence only; not a direct measured-static-to-FSP output reproduction.

Related calculators

Fan Power Calculator Use airflow, pressure, and efficiencies to estimate fan shaft power and electrical input. Duct Traverse Airflow Calculator Combine traverse readings with duct area and air properties to estimate average velocity and airflow. Fan and System Curve Calculator Intersect entered fan data with a system resistance model to estimate the operating airflow and pressure.

Related reading

Fan laws and density: when operating-point comparisons apply Scale a documented fan operating point only when geometry, density, pressure basis, and similarity assumptions match the calculation. Fan power: match pressure basis with efficiency Convert a fan duty point from air power to shaft and electrical power only after matching total or static pressure with the corresponding efficiencies. 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.