Skip to main content
Calcimator

Ductwork Pressure Loss Calculator

Calculate total static pressure loss through ductwork including straight runs, fittings, and equipment for blower selection.

Inputs

ft
"WC/100ft

SMACNA/ACCA Manual D: residential 0.08–0.10" WC/100 ft; commercial 0.08–0.15" WC/100 ft

CFM
"WC
"WC

Results

Total Static Pressure

0.66 "WC

External Static Pressure

0.36 "WC

Duct Friction Loss0.12 "WC
Fitting Loss0.21 "WC
Equipment Loss (filter+coil+grille)0.33 "WC
How to Use This Calculator
  1. Enter Total Equivalent Length, Friction Rate, and System CFM.
  2. Set Number of Elbows, Loss per Elbow, and Number of Tees/Transitions.
  3. Adjust Loss per Tee as needed.
  4. Review Total Static Pressure () and External Static Pressure ().
  5. Use Duct Friction Loss () and Fitting Loss () to inform your decision.

How the result changes with Total Equivalent Length

Total Equivalent LengthTotal Static PressureExternal Static Pressure
2090.71 "WC0.41 "WC
7071.11 "WC0.81 "WC
1,3041.58 "WC1.28 "WC
1,8011.98 "WC1.68 "WC

What each input means

Total Equivalent Length
Total equivalent length of straight duct in the longest run (supply + return).
Friction Rate
Design friction rate per 100 feet of duct per SMACNA HVAC Duct Construction Standards and ACCA Manual D. Residential: 0.08–0.10" WC/100 ft; commercial low-velocity: 0.08–0.10" WC/100 ft; high-velocity: 0.20–0.50" WC/100 ft.
System CFM
Total system airflow for reference.
Number of Elbows
Count of elbows (90° turns) in the longest duct run.
Loss per Elbow
Pressure drop per elbow. Typical: 0.01–0.03" WC depending on type and radius.
Number of Tees/Transitions
Count of tee junctions and transitions in the longest run.
Loss per Tee
Pressure drop per tee junction. Branch tees are typically 0.02–0.05" WC.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Total Equivalent Length = 150, Friction Rate = 0.08, System CFM = 1200, Number of Elbows = 6 = 7 input(s) provided
  2. Calculate Total Static Pressure
    Total Static Pressure
    0.66 = 0.66
  3. Calculate External Static Pressure
    External Static Pressure
    0.36 = 0.36
  4. Calculate Duct Friction Loss
    Duct Friction Loss
    0.12 = 0.12
  5. Calculate Fitting Loss
    Fitting Loss
    0.21 = 0.21

Engine last updated . Checked against 1 independently-derived test — how we verify calculators.

Related Calculators

The questions that sit next to this one — chosen by subject, including calculators filed under a different category.

HVAC

Duct Sizing Calculator

Size round or rectangular ductwork using the equal friction method based on CFM and desired friction rate.

HVAC

Air Balance Calculator

Calculate building air balance, pressurization status, and required adjustments from supply, return, exhaust, and outside air flows.

Mechanical Engineering

Fan Selection Calculator

Select an HVAC or industrial fan by calculating brake horsepower, motor size, estimated tip speed, noise level, and annual energy consumption.

Insulation

Duct Insulation Calculator

Insulation R-value and material for HVAC ductwork.

HVAC

BTU Calculator

Calculate heating and cooling BTU requirements for your room or home. Determine the right AC tonnage and furnace size based on room dimensions, insulation, and climate.

More in Construction & Building Trades.