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Reynolds number calculator

Reynolds number and flow regime for pipe flow, with water properties at any temperature built in.

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ft/s
in
°F

IAPWS-95 properties. The PE Civil Handbook's water table prints ν = 1.217 × 10⁻⁵ ft²/s at 60 °F; this gives 1.2085 × 10⁻⁵ (0.7% lower).

Result

Reynolds number
82,750
Flow regime
turbulent (Re > 4,000)
  1. 1.Water at 60 °F (IAPWS-95): ν = 1.208 × 10⁻⁵ ft²/s, ρ = 62.37 lbm/ft³
  2. 2.D = 4 in ÷ 12 = 0.3333 ft
  3. 3.Re = VD/ν = 3 × 0.3333 / 1.208 × 10⁻⁵ = 82,750

How it's tested on the exam

Shows up on: PE Mechanical (Thermal & Fluid Systems) · PE Mechanical (HVAC & Refrigeration) · PE Civil (Water Resources & Environmental)

  • Kinematic vs dynamic viscosity. Re = VD/ν uses kinematic viscosity (ft²/s); Re = ρVD/μ uses dynamic. Mixing them is the most common unit error.
  • Diameter in inches. The diameter must be in the same length unit as the velocity and viscosity — convert inches to feet first.
  • Nominal vs inside diameter. Use the pipe's inside diameter from the schedule, not the nominal size.
  • Laminar is below about 2,000. Only there is f = 64/Re; above about 4,000 the flow is turbulent and f depends on roughness too.

Worked example

Water at 60 °F flows at 3 ft/s in a pipe with a 4-inch inside diameter. Is the flow laminar or turbulent?

  1. 1.ν of water at 60 °F = 1.2085 × 10⁻⁵ ft²/s
  2. 2.D = 4 / 12 = 0.3333 ft
  3. 3.Re = 3 × 0.3333 / 1.2085 × 10⁻⁵ = 82,700

Re ≈ 82,700 — turbulent.

Where it is in the Handbook

  • Hydraulics, Fluids, and Pipe Flow — Reynolds Number — PE Mechanical Reference Handbook
  • Hydraulics — Reynolds Number—Circular Pipes — PE Civil Reference Handbook

Section names as printed in the current NCEES PE Reference Handbooks (Mechanical 2.1, Civil 2.2); numbering can shift between versions. On exam day you'll use your own approved calculator — here's which models NCEES allows.