Duct Static Regain Calculator
Static pressure regain at a duct expansion.
Enter your values
Enter the upstream velocity in FPM.
Enter the downstream velocity in FPM.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Upstream velocity pressure
- 0.0898 in wc
- Downstream velocity pressure
- 0.0399 in wc
- Static pressure regained
- 0.0374 in wc
Quick answer
Static regain occurs when duct velocity decreases at an expansion, converting some velocity pressure back into static pressure.
Regain = 0.75 x (VP1 - VP2), where VP = (FPM/4005)^2
At a glance
| What it does | Static pressure regain at a duct expansion. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Upstream velocity, Downstream velocity |
| Main output | Upstream velocity pressure |
| Formula | Regain = 0.75 x (VP1 - VP2), where VP = (FPM/4005)^2 |
| Cost | Free — no sign-up, no download |
How to use the Duct Static Regain Calculator
- 1Enter the duct velocity just before an expansion or branch takeoff.
- 2Enter the lower velocity just after the expansion.
- 3The 0.75 regain factor converts most of the recovered velocity pressure back into usable static pressure for downstream duct.
Inputs explained
- Upstream velocity(FPM)
- Enter the upstream velocity in FPM.
- Downstream velocity(FPM)
- Enter the downstream velocity in FPM.
Worked example
Using the values the calculator loads with:
Inputs
- Upstream velocity1200 FPM
- Downstream velocity800 FPM
Results
- Upstream velocity pressure0.0898 in wc
- Downstream velocity pressure0.0399 in wc
- Static pressure regained0.0374 in wc
Frequently asked questions
What is static regain design used for?
Large commercial systems use it to size each trunk segment so the recovered pressure offsets friction loss in the next segment, keeping static pressure nearly constant at every outlet.
Is this relevant to small residential ducts?
It's mostly a commercial design method; residential systems typically use the simpler equal-friction method instead.
What do I need to enter into the Duct Static Regain Calculator?
Just 2 values: upstream velocity, downstream velocity. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Duct Static Regain Calculator work out the answer?
Static regain occurs when duct velocity decreases at an expansion, converting some velocity pressure back into static pressure. It applies the formula Regain = 0.75 x (VP1 - VP2), where VP = (FPM/4005)^2 and shows the working so you can check each step by hand.
Is the Duct Static Regain Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.