Reynolds number calculator

Because the density is already inside the kinematic viscosity. The group can be written two ways — ρvD/μ with the dynamic viscosity, or vD/ν with the kinematic one — and ν is defined as μ/ρ, so the density cancels. Asking for it separately would be asking for the same information twice and would let a user enter an inconsistent pair. The only reason a density field appears on this page is to convert a centipoise figure off a data sheet into centistokes.

All three are in print and the disagreement does not matter, because nothing is designed to operate there. Reynolds' own experiments give a critical value near 2300 for the transition from laminar flow; Crane TP-410, which this page follows, puts the laminar limit at 2000 and the fully turbulent threshold at 4000 and calls the gap the critical zone. What is common to every version is the message: inside that band the friction factor is not predictable, and a design point that lands in it should be moved.

No, that is ordinary, and the interesting thing is how hard it is to get a water system to be anything else. One foot per second in a 1/2 in Schedule 40 pipe is already Re 4,300, and 1 in Schedule 40 at 60 °F water goes turbulent above 1.5 gpm. Laminar water in a building is a symptom, not a design: it usually means a dead leg, a closed balancing valve, or a flow figure entered in the wrong unit. Laminar flow is normal in oil, syrup and lubrication lines, where the viscosity is one to four orders of magnitude higher.

The hydraulic diameter, four times the cross-sectional area divided by the wetted perimeter — so a 12 in by 8 in duct gives 4(96)/(40) = 9.6 in, not 12, not 8, and not the 14.4 in diagonal. For a round pipe running full that definition reduces to the bore, which is why it never gets mentioned. For a pipe running partly full, as a gravity drain does, the wetted perimeter changes with depth and the whole open-channel treatment applies instead.

Because it is not a property of your flow — it is a property of your pipe, and it is the number Crane's valve and fitting resistances are defined against at any Reynolds number. Every L/D in a fitting table is converted to a resistance coefficient by multiplying it by f_T, so the value is needed to price an elbow whether the pipe is laminar or turbulent. It is Colebrook with the Reynolds term deleted, which is the far right of the Moody chart where the curves go flat.

No, and confusing the two is the most common misreading of the Moody chart. Roughness has no place in the group at all — Re is only inertia against viscosity, and a glass tube and a rusted steel tube of the same bore at the same velocity have identical Reynolds numbers. Roughness enters afterwards, as the second axis of the chart, and decides what friction factor that Reynolds number maps to. Below Re 2000 it does not even do that: laminar friction is 64/Re regardless of the wall.

For the Reynolds number itself, yes — the group is fluid-agnostic and takes any kinematic viscosity you can supply, which is why this page asks for a viscosity rather than for the name of a fluid. This site does not carry air or gas viscosities, so take yours from a gas-property table at your own temperature and pressure rather than from anything here; air's kinematic viscosity is an order of magnitude above water's, so a duct velocity that sounds fast sits at a far lower Reynolds number than the same velocity in a pipe. What does not carry over at all is the pressure loss afterwards: a gas is compressible, and the incompressible loss equations hold only while the total drop stays small against the absolute inlet pressure.