About this calculator

This calculator estimates the static pressure loss of a duct run entered one section at a time from the blower outlet to the discharge outlet along the path of highest expected resistance. A section is a length of straight duct with its own size and airflow, ending in a fitting or diffuser.

  • Two fittings back-to-back are simply two sections, the second with 0 ft of straight duct.
  • Straight duct is charged at the friction loss rate per 100 ft for the section's airflow and size (rectangular duct is converted to an equivalent round diameter for friction only).
  • Flexible duct sections are charged at the flex-duct friction rate, which runs two to three times the galvanized rate at comparable size and airflow, even when fully stretched. This rate is increased if compression is modeled — the friction factor doubles for every 15% of compression, and since fully stretched flexible duct is rare in the field, the calculator assumes 15% compression (~2x the friction rate of fully extended) until a different value is entered. Compression and factor are stated.
  • Each fitting is charged by the SMACNA loss-coefficient method: its coefficient C comes from the published table for that fitting (smooth-radius and mitered elbows, elbows with turning vanes, conical contractions, and round or rectangular tees) using the relevant inputs, and is multiplied by the velocity pressure of the section feeding it (for a contraction, the smaller section leaving it).
    • A contraction and a tee read the following section, which is the smaller duct or branch the run follows, so they only resolve once that section is entered.
    • Values between the table's grid points are interpolated; a combination the table leaves blank, which SMACNA never tested, is reported rather than guessed.
  • The last section can end at a diffuser (4-way ceiling diffuser, linear slot diffuser, concentric diffuser, or perforated plenum diffuser) calculated using typical diffuser performance data.
    • The 4-way diffuser by its neck size and neck velocity, and the perforated diffuser by its collar size and airflow. The section's diameter is the neck, and only necks within each diffuser's range are accepted (6–15 in for the 4-way, 8–12 in for the perforated); sizes between the tabulated ones are interpolated.
    • The slot diffuser by its slot count, length, and airflow.
    • The concentric diffuser by its face size and airflow on the supply side.
  • A fitting value, such as a velocity or area ratio, that runs off the edge of its SMACNA table uses the table's edge value and is flagged with a warning.
  • The run's static pressure is the sum of every section's straight and fitting losses, in inches of water column. Give the run an optional name; on the web it can be printed as a report that includes a link to reopen the run. Use it when sizing a fan or air handling unit to confirm the external static pressure it must overcome, when checking whether an existing run can carry a higher airflow, or when comparing duct sizes and fittings for the same run.
Use when Calculating the pressure drop of a duct run (e.g. the external static pressure a fan or unit must overcome), including runs that step down in size and airflow after each branch.
Formula Static Pressure = ∑ Straight Duct Loss + ∑ (C × Vp) + Diffuser Loss, where Vp = (V ÷ 4005)²
Variables
Duct Dimensions Each section's round duct diameter in inches, or width and height for rectangular duct; a flex section is round and charged at the higher flexible-duct friction rateCompression For a flex section, how much shorter it is installed than fully stretched, in percent; the friction factor doubles every 15% (2× at 15%, 4× at 30%), and 15% is assumed unless changedLength Straight duct in the section, in feet — 0 for a fitting with no straight duct before itAirflow Airflow through the section in CFM — a new section starts with the previous section's size and airflowEnds In The fitting at the end of the section: none, an elbow, a contraction into the next section, a tee the run follows into the branch, or the diffuser the run ends at (4-way ceiling, linear slot, concentric, or perforated)Fitting Details What the fitting's table needs: radius ratio and angle for a smooth elbow, angle for a mitered elbow, vane thickness and geometry for turning vanes, taper length for a contraction, branch angle for a round tee, slot count and length for a linear slot diffuser, face size for a concentric diffuser, damper for a perforated diffuserC Loss coefficient — from SMACNA for elbows, contractions, and tees, and from diffuser performance data for the 4-way and perforated diffusers — taken from the table using the section's size, velocity, and the next section after the fittingVp Velocity pressure in inches of water column at standard air, (velocity in FPM ÷ 4005)²
Name
Optional — printed on the report
Duct Run
Section 1Incomplete
Duct Shape
Diameter (in)
Length (ft)
Straight duct in this section
Airflow (CFM)
Air carried by this section
Ends In
The fitting at the end of this section
Complete the current section to add another
Static Pressure (in. w.c.) —
Straight Duct Loss (in. w.c.) —
Fitting Loss (in. w.c.) —
Total Straight Duct (ft) —
Static Pressure Calculator
November 12, 2026
# Duct Length Airflow Velocity Ends In C Friction Rate Straight Loss Fitting Loss Section Loss
1 — in round — — — no fitting — — — — —
Total Straight Duct— ft
Straight Duct Loss— in. w.c.
Fitting Loss— in. w.c.
Static Pressure— in. w.c.
Length in feet, airflow in CFM, velocity in FPM, friction rate in inches of water column per 100 ft, and every loss in inches of water column, all at standard air. Straight duct is charged at the friction loss rate for the section's airflow and size (rectangular duct by its equivalent round diameter; flexible duct at the flexible-duct rate raised for its compression). Each fitting is its SMACNA loss coefficient C times the velocity pressure of the section feeding it (for a contraction, the smaller section leaving it), Vp = (V ÷ 4005)²; a diffuser is charged from its performance data.

What is duct static pressure?

Static pressure in a duct system is the resistance to airflow that a fan must overcome, expressed in inches of water column (in. w.c.). It comes from friction between the moving air and the duct walls plus the turbulence losses at fittings such as elbows, contractions, and tees. Friction loss depends on airflow, duct size, and duct roughness: for a given airflow, a smaller duct means higher velocity and a sharply higher loss rate, roughly with the square of velocity. Fitting losses are expressed as a loss coefficient C, a dimensionless number published by SMACNA (the Sheet Metal and Air Conditioning Contractors' National Association) for each fitting geometry, multiplied by the velocity pressure at the fitting (for a contraction, the velocity pressure of the smaller outlet): a gentle smooth-radius elbow may have a C near 0.15 while a sharp mitered elbow is over 1.0, so the same airflow loses several times more pressure through the sharper turn. Velocity pressure at standard air is (V ÷ 4005)², with V in feet per minute. Rectangular duct is handled by converting its width and height to an equivalent round diameter for friction, while the fitting tables use the real width and height. The resulting static pressure feeds fan selection: the fan must deliver the design CFM against the total static pressure of the system, including coils, filters, and terminal devices as well as the ductwork itself.

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