Reynolds Number Calculator for Pipe Flow

Last updated September 23, 2025

Reynolds Number Calculator — Pipe Flow

Enter pipe size and either velocity or flow. Provide viscosity (dynamic or kinematic) and, if needed, density/SG. Calculates velocity, Reynolds number, and flow regime. Export to Excel — changing inputs there updates outputs automatically.

Velocity input

By Velocity By Volumetric Flow

Viscosity input

Dynamic μ Kinematic ν

Pipe Diameter

Positive number. Typical range: 5–3000 mm.

Velocity

Use this OR enter Flow (then velocity is computed).

Dynamic Viscosity μ

If dynamic μ is given, density is required (to compute ν = μ/ρ).

Fluid Density (optional if ν given)

If SG selected, density = SG × 1000 kg/m³ (approx.).

Hydraulic Diameter D (m)

–

Cross-sectional Area A (m²)

–

Velocity V (m/s)

–

Dynamic Viscosity μ (Pa·s)

–

Kinematic Viscosity ν (m²/s)

–

Density ρ (kg/m³)

–

Reynolds Number Re

–

Flow Regime

–

Formulas & assumptions

Area: A = π D² / 4
Velocity from flow: V = Q / A
Unit conversions: mm→m: ×1e−3; inch→m: ×0.0254; cP→Pa·s: ×1e−3; cSt→m²/s: ×1e−6; L/s→m³/s: ×1e−3; m³/h→m³/s: ÷3600; gpm→m³/s: ×6.309e−5; ft/s→m/s: ×0.3048
Density from SG: ρ ≈ SG × 1000 kg/m³
Reynolds number (pipe): Re = ρ V D / μ = V D / ν
Regime thresholds: Laminar if Re < 2300; Transitional 2300–4000; Turbulent > 4000
Calculator is for single-phase, incompressible flow in a circular pipe.