Duct pressure drop calculator
Enter airflow, duct diameter (or width × height), length and material and get velocity, Reynolds number, friction rate and total pressure loss for a single round or rectangular duct run — the quick, single-duct version of the tools below, for when you just need one number fast.
How to use this calculator
- Enter the airflow.
- Pick round or rectangular, and enter the diameter or width × height.
- Enter the duct length and material.
- Set temperature and altitude if they differ from standard air.
- Read the velocity, Reynolds number, friction rate and total pressure loss.
What this calculates
One duct run — round or rectangular — from airflow, size, length and surface roughness to velocity and pressure loss. It reuses the exact same Darcy-Weisbach + Colebrook-White engine already implemented and verified elsewhere on this site (against Moody-chart reference points: Re=1×10⁵, ε/D=1×10⁻⁴ → f=0.0180; Re=1×10⁶, ε/D=1×10⁻³ → f=0.0195, both matched to chart-reading precision) — the same calculation the duct network configurator runs for every straight section of a full branching system. This page is that one calculation on its own, without building a network, for when you just need velocity and pressure drop for a single duct fast.
Re = ρ·v·Dh / μ Reynolds number
1/√f = −2·log₁₀( (ε/Dh)/3.7 + 2.51/(Re·√f) ) Colebrook-White (implicit, solved iteratively)
Δp = f · (L/Dh) · (ρ·v²/2) Darcy-Weisbach
Dh = D round duct
Dh = 2ab/(a+b) = 4·Area/Perimeter rectangular duct
Colebrook-White has no closed-form solution — f appears on both sides — so it's solved by fixed-point iteration, seeded with the explicit Swamee-Jain approximation for a fast first guess. Below Re=2300 (laminar flow, unusual in real ductwork but included for completeness) the exact f=64/Re is used instead. Absolute roughness ε by material is the same published table (ASHRAE Fundamentals / Crane TP-410 / Idelchik) used throughout this site's duct tools, with galvanized steel set to ASHRAE's own 0.09 mm duct-friction-chart basis for spiral/longitudinal-seam duct.
When to use the other duct tools instead
This page only answers the forward question — given this duct, what happens. Two related tools on this site solve different problems with the same underlying physics: the duct sizing calculator solves it in reverse (given a target velocity or friction rate, what size duct do I need), and the duct network configurator extends this same duct calculation across a full branching system with transitions, elbows, tees, dampers and louvers, balancing flow across parallel paths. Reach for those when the question is "what size" or "what does the whole system add up to" rather than "what does this one duct do".