Reynolds Number Calculator
Find the Reynolds number for any liquid or gas and instantly see whether the flow is laminar, transitional or turbulent. Pick a fluid preset and a flow geometry, or enter density and viscosity directly.
What Is the Reynolds Number?
The Reynolds number is a dimensionless ratio that compares inertial forces to viscous forces in a moving fluid. That ratio predicts how the fluid will behave: at low Reynolds numbers viscosity keeps the flow smooth and layered (laminar), and at high Reynolds numbers inertia takes over and the flow becomes chaotic (turbulent). It is the first quantity you check before choosing a friction factor, sizing a pump, or rating a heat exchanger.
where ρ is density, V is velocity, D is the hydraulic diameter and μ is dynamic viscosity. Many engineers work with kinematic viscosity ν = μ / ρ instead, so the identical relationship is also written:
This calculator accepts either path: pick a fluid preset and it supplies density and viscosity, or switch to Advanced and enter the properties yourself.
Flow Regime Thresholds
| Reynolds number | Flow type |
|---|---|
| Re < 2,300 | Laminar |
| 2,300 to 4,000 | Transitional |
| Re > 4,000 | Turbulent |
These are the standard ranges for flow in a round pipe. The calculator colour-codes the result — green for laminar, yellow for transitional, blue for turbulent — and drops a marker on the visual scale so you can see at a glance where your flow sits.
Laminar vs Turbulent Flow
| Flow type | Characteristics |
|---|---|
| Laminar | Smooth, orderly flow in parallel layers; low mixing; predictable pressure loss |
| Transitional | Mixed behaviour that flickers between smooth and chaotic; hard to predict |
| Turbulent | Chaotic, well-mixed flow with eddies; higher friction but better heat transfer |
Applications
- Pipe flow — sets the pressure-loss method for water, fuel and process lines.
- HVAC ducts — airflow in rectangular and round ducting, using hydraulic diameter.
- Pump systems — the regime feeds the friction factor and the head a pump must overcome.
- Heat exchangers — turbulence boosts heat transfer at the cost of pressure drop.
- Hydraulic systems — thick oils often stay laminar even at speed, which changes losses.
- Aircraft aerodynamics — boundary-layer behaviour over wings and bodies depends on Reynolds number.
Frequently Asked Questions
Is a higher Reynolds number better?
Not inherently. Turbulent flow mixes well and transfers heat better, but it also causes more friction and pressure loss. The right regime depends on the job.
What is hydraulic diameter?
For a round pipe it equals the inside diameter. For ducts and channels it is D = 4A / P — four times the cross-sectional area divided by the wetted perimeter — so the same formula applies. This calculator computes it for you from rectangular-duct and open-channel dimensions.
Why does temperature matter?
Viscosity changes sharply with temperature. Hot water flows far more freely than cold, and oil even more so. The fluid presets set typical properties at their stated temperature, and you can override the temperature to recompute.
Why is my flow turbulent at low speed?
Large passages and low-viscosity fluids reach turbulence easily, while thick oils can stay laminar even at high velocity. Reynolds number captures all of those effects in one value.
Related calculators
- Pipe Flow Calculator — flow, velocity, pressure drop and pump HP in one tool.
- Pressure Drop Calculator — Darcy-Weisbach losses, built into Pipe Flow.
- Pump Horsepower Calculator — hydraulic and brake HP, built into Pipe Flow.
- Hydraulic Diameter Calculator — non-circular passages (coming to the Engineering hub).
- Friction Factor Calculator — Moody / Colebrook (coming to the Engineering hub).