Reynolds number pipe flow calculator

Reynolds number sketches show up when you are checking whether a pipe flow example is laminar, transitional, or turbulent before you pick a friction-factor method. For internal pipe flow the teaching form is Re = V D / ν, with mean velocity V, internal diameter D, and kinematic viscosity ν in consistent units (for example ft/s, ft, and ft²/s). Many textbooks treat Re < ~2,300 as laminar, roughly 2,300–4,000 as transitional, and Re > ~4,000 as turbulent for smooth circular pipes—bands that are teaching conventions, not a code deliverable. This page explains that sketch and gives a free browser calculator. It is for learning and quick checks—not a substitute for the criteria adopted by your authority having jurisdiction, manufacturer data, a sealed hydraulic calculation, or review by a licensed design professional.

How Reynolds number is used

1. Enter mean velocity V (ft/s) or enter flow Q (gpm) and diameter so the page can sketch V = Q / (2.448 × d_in²).
2. Enter internal diameter D (in); the calculator converts to feet for Re = V D / ν.
3. Enter kinematic viscosity ν in ft²/s, or pick the freshwater ~68 °F preset (≈ 1.08×10⁻⁵ ft²/s) for a classroom water sketch.
4. Compute Re and label a laminar / transitional / turbulent teaching band.
5. Use the regime only as a hint for which friction-factor discussion applies; Hazen-Williams fire-protection methods have their own adopted rules and do not require you to publish Re on every sheet.

Free calculator

Educational approximation only. Formula: Re = V D / ν with D in feet. Nothing you enter leaves your browser. For Darcy-Weisbach head loss see the Darcy-Weisbach calculator. For Hazen-Williams psi/ft see the friction loss calculator. Multi-segment HW paths live in the Sprinkler Pipe Friction Loss workbook.

Worked example

Water at ~68 °F (ν ≈ 1.08×10⁻⁵ ft²/s), D = 2.067 in (0.1723 ft), V = 10 ft/s: Re = 10 × 0.1723 / 1.08e-5 ≈ 1.6×10⁵, a turbulent teaching-band result. Always confirm viscosity for your temperature, the published internal diameter, and check friction-factor assumptions against the adopted edition, manufacturer data, and AHJ—not this page alone.

Next steps: estimate Darcy head loss with the free Darcy-Weisbach calculator, compare Hazen-Williams with the friction loss calculator, check pipe speed with the sprinkler pipe velocity calculator, and build multi-segment HW paths in the Sprinkler Pipe Friction Loss workbook ($29).