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HVAC

Static Pressure Calculator

Work out available static and friction rate from component losses, or compare a measured total external static to the blower rating to see if airflow is choked.

Mode
in wc

From the blower table — often 0.5 in wc.

in wc

Dry coil ~0.15, wet ~0.25.

in wc

Clean 1 in. ~0.10; MERV 13 or loaded ~0.25.

Registers, grille & duct length
in wc
in wc
in wc

Balancing dampers, UV, humidifier, etc.

ft

Longest supply run + longest return, plus fitting equivalents. Default 250 ft.

Friction rate
Low friction rate — duct can be smaller
0.036/100 ft
Available SP
0.09in wc
Friction rate
0.036/100ft
Losses
0.41in wc
ƒShow the math
  1. 1.Component pressure drops

    Wet evaporator coil 0.25, Air filter 0.1, Supply registers 0.03, Return grille 0.03

    = Σ = 0.41 in wc
  2. 2.Available static pressure
    ASP = rated ESP − Σ component drops
    0.5 − 0.41
    = 0.09 in wc
  3. 3.Friction rateper external-static friction-rate method
    FR = ASP × 100 / total effective length
    0.09 × 100 / 250
    = 0.036 in wc / 100 ft
Before you rely on this
  • Friction rate under ~0.06 means generous duct for this blower. Fine, though you could downsize runs on a new install.
  • Use this friction rate to size trunks and branches in the duct calculator. Confirm the real number by measuring static once the system runs.

Two ways to use it

A blower is rated to move design airflow only up to a certain external static pressure. Everything the air passes through eats into that budget; whatever is left is what pushes air through the duct.

  • Estimate from parts — subtract the coil, filter, register, and grille drops from the rated static to get available static, then divide by total effective length for a friction rate to size duct.
  • I measured static — add your supply and return readings for total external static and compare it to the blower rating. Over rating means the system is delivering less than design airflow.

A magnehelic or manometer reading in the field always beats an estimate. Use the estimate to size new duct, and the measurement to diagnose an existing system.

Frequently asked

What is total external static pressure (TESP)?
TESP is the total pressure the blower works against outside the cabinet, measured as the supply-side static plus the return-side static (as magnitudes). If you read 0.5 in wc on the supply and 0.4 on the return, TESP is 0.9 in wc. Compare it to the blower's rated external static — often 0.5 in wc.
What is a good static pressure reading?
At or below the blower's rated external static, commonly 0.5 in wc for residential equipment. A measured 0.9 in wc on a 0.5-rated blower means airflow is badly choked. High static is one of the most common causes of low airflow, frozen coils, and comfort complaints.
How do you calculate friction rate?
Friction rate = available static pressure × 100 ÷ total effective length. Available static is the blower's rated external static minus the pressure drops of the coil, filter, registers, and grille. If 0.09 in wc is available over 250 ft of effective length, the friction rate is 0.036 in wc per 100 ft.
What friction rate should I design duct to?
Residential duct is usually designed near 0.06 to 0.10 in wc per 100 ft. Much lower and the duct is larger than it needs to be; much higher and the runs get small, fast, and noisy with starved airflow. Use the friction rate from this worksheet to size trunks and branches.
What is total effective length?
It is the longest supply run plus the longest return run, plus equivalent lengths added for fittings like elbows, boots, and takeoffs. It represents the worst-case path the air travels. The default of 250 ft is a common starting estimate; measure and add fitting equivalents for accuracy.

Related tools

HVAC
Duct Size Calculator

Size trunks and branches to the friction rate from this worksheet.

HVAC
CFM per Ton Calculator

The airflow the blower has to deliver against this static.

HVAC
Target ΔT Calculator

High static shows up as a wide temperature split.

Sources & references

Estimates for planning purposes only. Verify all results against the code edition adopted in your jurisdiction and with your Authority Having Jurisdiction (AHJ). This tool is not a substitute for a licensed HVAC contractor. See our methodology, sources, and code editions.