Drawing No. EH–TH–023 // Thermal Engineering & HVAC
HVAC Duct Size & Pressure Drop Calculator
Reviewed August 2026
Six focused modes covering everything from sizing a new duct run to checking an existing one: solve diameter, solve pressure loss, rectangular ↔ equivalent diameter, airflow/velocity, friction rate, and fitting losses — all sharing the same verified duct physics underneath.
What problem does this solve?
Duct sizing questions come in different shapes depending on what you already know and what you're trying to find — sometimes you have an airflow and need a diameter, sometimes you have a duct and need its pressure drop, sometimes you just need to convert a rectangular duct to its round equivalent. Rather than forcing every question through one rigid form, this tool offers six focused modes sharing the same verified duct-airflow physics, so you can jump straight to the specific calculation you need, whether you're a mechanical engineer checking a system design or a homeowner sizing a bathroom fan duct.
Inputs
Results
Background
ΔP = f × (L/D) × (ρV²/2), the same relationship used for any internal pipe or duct flow. f is the Darcy friction factor, L and D are duct length and diameter, ρ is air density (≈1.2 kg/m³), and V is air velocity. Every mode in this tool ultimately traces back to this one equation, applied differently depending on what you're solving for.
For turbulent flow (Reynolds number above ~2,300, which covers essentially all real duct airflow), f is found from an explicit approximation to the Colebrook equation: f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re⁰⋅⁏)]², where ε is duct roughness. Below Re=2,300 (laminar flow, essentially never encountered in real ducts), f = 64/Re instead.
De = 1.30 × (ab)⁰⋅⁾²⁵ / (a+b)⁰⋅²⁵, the standard ASHRAE/SMACNA (Huebscher) formula, where a and b are the rectangular duct's two side dimensions. This is the diameter of the round duct that would produce the same friction loss per unit length at the same airflow as the rectangular duct — not the diameter of a round duct with the same cross-sectional area, which is a common point of confusion. A very flat, high-aspect-ratio duct (like 600×100mm) has notably more friction than a squarer duct of the same area, because it has more wetted perimeter relative to its flow area.
For the same cross-sectional area, a circle has less perimeter than any rectangle — and friction scales with the wetted perimeter in contact with the airstream. This is exactly why round ducts generally have lower friction loss (and are quieter) than a rectangular duct of the same flow area, which is why round is usually preferred where space allows, with rectangular reserved for situations where duct height is constrained.
The equal friction method sizes every duct section in a system for the same friction rate (Pa/m, or in.wg per 100 ft in traditional U.S. units), which tends to produce quieter, well-balanced systems. The velocity method instead caps velocity directly, which is simpler but can leave a system poorly balanced if applied naively across many branches. Typical target ranges: residential systems are often designed around 0.8–1.0 Pa/m (roughly 0.1 in.wg/100ft) and 4–8 m/s branch velocity; commercial mains often run somewhat higher on both.
ΔPfitting = C × Vp, where C is the fitting's dimensionless loss coefficient and Vp = ρV²/2 is the velocity pressure. C values come from measured or computed data (this tool uses verified SMACNA coefficients for common round elbows) and vary hugely by fitting geometry — a smooth, large-radius elbow can have a coefficient 5–6× lower than a tight, mitered one, which is exactly why elbow radius matters so much for both pressure loss and noise.
A smooth 90° elbow's loss coefficient drops steeply as its centerline-radius-to-diameter ratio (R/D) increases: from C≈0.71 at R/D=0.5 (a tight elbow) down to C≈0.12 at R/D=2.5 (a gentle, sweeping bend) — nearly a 6× difference for the same 90° turn. This is why space-constrained tight elbows carry a real, quantifiable pressure and energy penalty compared to a more gradual bend, beyond just looking worse.
Frequently asked questions
Practical questions about inputs, assumptions and interpretation.
Friction loss depends on wetted perimeter relative to flow area (captured by the equivalent diameter formula), not on cross-sectional area alone. A flat duct has proportionally more surface area in contact with the airstream for the same flow area, so its equivalent diameter — and therefore its friction performance — is worse than a squarer duct carrying the same airflow, even though both have identical cross-sectional area.
Both are used in practice — velocity limits are simple and directly control noise (which is why bedrooms and quiet spaces often get lower velocity targets than mechanical rooms), while the equal-friction method tends to produce a naturally balanced system across many branches without needing as much manual damper adjustment. Many designers use friction rate as the primary sizing method and then check velocity as a secondary noise/erosion limit.
Because fan power scales with pressure drop, and pressure drop from fittings compounds with every bend in a run — a system with several tight elbows can accumulate a fitting pressure loss comparable to or exceeding the straight-duct friction loss itself. Using the largest practical elbow radius throughout a design is one of the cheapest ways to reduce a system's total fan energy and noise.
For preliminary sizing and checking, yes — the underlying Darcy-Weisbach and equivalent-diameter formulas are the same ones used in professional duct design software. But this tool's fitting library covers only a handful of common elbow types out of the 220+ fittings in the full ASHRAE Duct Fitting Database (tees, transitions, dampers, diffusers, and many elbow variants aren't included), so a complete system design should use the full database or dedicated duct design software.
"Solve Diameter" answers "what size duct do I need to hit a target?" — a sizing/design question, working forward from a target velocity or friction rate. "Friction Rate" answers "what is this specific existing duct actually doing?" — a checking/verification question, working forward from a diameter you already have (or are considering) to see its resulting friction rate and whether it falls in a sensible range.