Pipe flow is laminar below Re ≈ 2,300 and reliably turbulent above ~4,000.
The Reynolds number compares inertia to viscosity. Between the two thresholds lies an unpredictable transition zone — avoid designing in it.
The transition is not a switch. Between about 2,300 and 4,000 the flow is genuinely intermittent, and a rough or disturbed inlet can trip turbulence below 2,300. In very careful laboratory conditions laminar flow has been held past Re 10,000, so the number describes practice, not physics.
Estimate with the rule, then check it against the calculator that models it properly.
Open Pipe Flow & Pressure Drop Calculator →Pipe flow is laminar below Re ≈ 2,300 and reliably turbulent above ~4,000. The Reynolds number compares inertia to viscosity.