Reynolds Number Calculator
Professional Reynolds number calculator for engineers and students. Compute Reynolds number using fluid density and dynamic viscosity or kinematic viscosity, with unit conversions, fluid presets (air, water, oil), and flow regime classification.
Reynolds Number Calculator
This professional-grade Reynolds number calculator helps engineers, researchers, and students quickly predict flow regime for internal and external flows. Enter velocity and characteristic length, then choose to use density and dynamic viscosity or kinematic viscosity. The tool offers authoritative unit handling, trusted fluid presets, and clear, accessible results.
Results
Authoritative Data Source and Methodology
Primary reference for definitions and usage of the Reynolds number: Frank M. White, Fluid Mechanics, 8th ed., McGraw‑Hill, 2016.
Fluid property presets and correlations:
- Water viscosity via Andrade’s correlation: μ = 2.414×10⁻⁵ × 10^{247.8/(T+133.15)} Pa·s (T in °C).
- Air viscosity via Sutherland’s law with C = 110.4 K; air density via ideal gas at 1 atm.
- Representative values cross-checked with NIST Chemistry WebBook (https://webbook.nist.gov/chemistry/).
Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.
The Formula Explained
$$ \mathrm{Re} = \frac{\rho \, v \, L}{\mu} $$
For kinematic viscosity ν known:
$$ \mathrm{Re} = \frac{v \, L}{\nu} $$
Typical internal pipe flow classification: laminar for Re < 2300, transitional for 2300 ≤ Re ≤ 4000, turbulent for Re > 4000.
Glossary of Variables
- v — flow velocity (m/s, ft/s, km/h, mph)
- L — characteristic length (m, mm, cm, in, ft)
- ρ — density (kg/m³, lb/ft³)
- μ — dynamic viscosity (Pa·s, mPa·s, cP)
- ν — kinematic viscosity (m²/s, cSt)
- Re — Reynolds number (dimensionless)
How It Works: A Step-by-Step Example
Suppose water flows in a 5 cm diameter pipe at v = 1.5 m/s. At 20 °C, water density ρ ≈ 998.2 kg/m³ and μ ≈ 0.001002 Pa·s. Using Re = (ρ v L) / μ with L = 0.05 m:
Since Re ≈ 74,800 is well above 4,000, the flow is turbulent for internal pipes.
Frequently Asked Questions (FAQ)
Do I choose diameter or another length?
For internal pipe flow, use pipe diameter (or hydraulic diameter for non-circular ducts). For external flow over bodies, use the appropriate characteristic length (e.g., chord for airfoils).
How accurate are the fluid presets?
They are based on standard correlations and representative values at atmospheric pressure. For critical design, consult detailed property tables for your exact temperature and pressure.
What if I only know kinematic viscosity?
Select “Kinematic viscosity” mode and input ν directly. The calculator will use Re = vL/ν.
Can this calculator handle gas compressibility effects?
Reynolds number itself does not capture compressibility. For high Mach numbers or significant density variation, additional non-dimensional parameters are required.
Why is my Re extremely large or small?
Check units. Viscosity often causes mistakes: 1 cP = 1 mPa·s = 0.001 Pa·s. Ensure length and velocity units are consistent.
What thresholds apply to external flow?
Thresholds differ. For example, flow over a flat plate transitions around Re_x ≈ 5×10⁵ based on distance from the leading edge; consult appropriate references.
Does surface roughness affect Re?
Surface roughness does not change Re (a fluid-property/flow/geometry ratio), but it affects friction factors and transition behavior in turbulent flow.
Formula (LaTeX) + variables + units
','
\mathrm{Re} = \frac{\rho \, v \, L}{\mu}
\mathrm{Re} = \frac{v \, L}{\nu}
\mathrm{Re} = \frac{998.2 \times 1.5 \times 0.05}{0.001002} \approx 7.48 \times 10^{4}
For density ρ and dynamic viscosity μ known: $ \mathrm{Re} = \frac{\rho \, v \, L}{\mu} $ For kinematic viscosity ν known: $ \mathrm{Re} = \frac{v \, L}{\nu} $
$ \mathrm{Re} = \frac{998.2 \times 1.5 \times 0.05}{0.001002} \approx 7.48 \times 10^{4} $
- No variables provided in audit spec.
- NIST — Weights and measures — nist.gov · Accessed 2026-01-19
https://www.nist.gov/pml/weights-and-measures - FTC — Consumer advice — consumer.ftc.gov · Accessed 2026-01-19
https://consumer.ftc.gov/
Last code update: 2026-01-19
- Initial audit spec draft generated from HTML extraction (review required).
- Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
- Confirm sources are authoritative and relevant to the calculator methodology.
Full original guide (expanded)
Skip to main contentSkip to main contentReynolds Number Calculator
This professional-grade Reynolds number calculator helps engineers, researchers, and students quickly predict flow regime for internal and external flows. Enter velocity and characteristic length, then choose to use density and dynamic viscosity or kinematic viscosity. The tool offers authoritative unit handling, trusted fluid presets, and clear, accessible results.
Results
Authoritative Data Source and Methodology
Primary reference for definitions and usage of the Reynolds number: Frank M. White, Fluid Mechanics, 8th ed., McGraw‑Hill, 2016.
Fluid property presets and correlations:
- Water viscosity via Andrade’s correlation: μ = 2.414×10⁻⁵ × 10^{247.8/(T+133.15)} Pa·s (T in °C).
- Air viscosity via Sutherland’s law with C = 110.4 K; air density via ideal gas at 1 atm.
- Representative values cross-checked with NIST Chemistry WebBook (https://webbook.nist.gov/chemistry/).
Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.
The Formula Explained
$$ \mathrm{Re} = \frac{\rho \, v \, L}{\mu} $$
For kinematic viscosity ν known:
$$ \mathrm{Re} = \frac{v \, L}{\nu} $$
Typical internal pipe flow classification: laminar for Re < 2300, transitional for 2300 ≤ Re ≤ 4000, turbulent for Re > 4000.
Glossary of Variables
- v — flow velocity (m/s, ft/s, km/h, mph)
- L — characteristic length (m, mm, cm, in, ft)
- ρ — density (kg/m³, lb/ft³)
- μ — dynamic viscosity (Pa·s, mPa·s, cP)
- ν — kinematic viscosity (m²/s, cSt)
- Re — Reynolds number (dimensionless)
How It Works: A Step-by-Step Example
Suppose water flows in a 5 cm diameter pipe at v = 1.5 m/s. At 20 °C, water density ρ ≈ 998.2 kg/m³ and μ ≈ 0.001002 Pa·s. Using Re = (ρ v L) / μ with L = 0.05 m:
Since Re ≈ 74,800 is well above 4,000, the flow is turbulent for internal pipes.
Frequently Asked Questions (FAQ)
Do I choose diameter or another length?
For internal pipe flow, use pipe diameter (or hydraulic diameter for non-circular ducts). For external flow over bodies, use the appropriate characteristic length (e.g., chord for airfoils).
How accurate are the fluid presets?
They are based on standard correlations and representative values at atmospheric pressure. For critical design, consult detailed property tables for your exact temperature and pressure.
What if I only know kinematic viscosity?
Select “Kinematic viscosity” mode and input ν directly. The calculator will use Re = vL/ν.
Can this calculator handle gas compressibility effects?
Reynolds number itself does not capture compressibility. For high Mach numbers or significant density variation, additional non-dimensional parameters are required.
Why is my Re extremely large or small?
Check units. Viscosity often causes mistakes: 1 cP = 1 mPa·s = 0.001 Pa·s. Ensure length and velocity units are consistent.
What thresholds apply to external flow?
Thresholds differ. For example, flow over a flat plate transitions around Re_x ≈ 5×10⁵ based on distance from the leading edge; consult appropriate references.
Does surface roughness affect Re?
Surface roughness does not change Re (a fluid-property/flow/geometry ratio), but it affects friction factors and transition behavior in turbulent flow.
Formula (LaTeX) + variables + units
','
\mathrm{Re} = \frac{\rho \, v \, L}{\mu}
\mathrm{Re} = \frac{v \, L}{\nu}
\mathrm{Re} = \frac{998.2 \times 1.5 \times 0.05}{0.001002} \approx 7.48 \times 10^{4}
For density ρ and dynamic viscosity μ known: $ \mathrm{Re} = \frac{\rho \, v \, L}{\mu} $ For kinematic viscosity ν known: $ \mathrm{Re} = \frac{v \, L}{\nu} $
$ \mathrm{Re} = \frac{998.2 \times 1.5 \times 0.05}{0.001002} \approx 7.48 \times 10^{4} $
- No variables provided in audit spec.
- NIST — Weights and measures — nist.gov · Accessed 2026-01-19
https://www.nist.gov/pml/weights-and-measures - FTC — Consumer advice — consumer.ftc.gov · Accessed 2026-01-19
https://consumer.ftc.gov/
Last code update: 2026-01-19
- Initial audit spec draft generated from HTML extraction (review required).
- Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
- Confirm sources are authoritative and relevant to the calculator methodology.
Reynolds Number Calculator
This professional-grade Reynolds number calculator helps engineers, researchers, and students quickly predict flow regime for internal and external flows. Enter velocity and characteristic length, then choose to use density and dynamic viscosity or kinematic viscosity. The tool offers authoritative unit handling, trusted fluid presets, and clear, accessible results.
Results
Authoritative Data Source and Methodology
Primary reference for definitions and usage of the Reynolds number: Frank M. White, Fluid Mechanics, 8th ed., McGraw‑Hill, 2016.
Fluid property presets and correlations:
- Water viscosity via Andrade’s correlation: μ = 2.414×10⁻⁵ × 10^{247.8/(T+133.15)} Pa·s (T in °C).
- Air viscosity via Sutherland’s law with C = 110.4 K; air density via ideal gas at 1 atm.
- Representative values cross-checked with NIST Chemistry WebBook (https://webbook.nist.gov/chemistry/).
Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.
The Formula Explained
$$ \mathrm{Re} = \frac{\rho \, v \, L}{\mu} $$
For kinematic viscosity ν known:
$$ \mathrm{Re} = \frac{v \, L}{\nu} $$
Typical internal pipe flow classification: laminar for Re < 2300, transitional for 2300 ≤ Re ≤ 4000, turbulent for Re > 4000.
Glossary of Variables
- v — flow velocity (m/s, ft/s, km/h, mph)
- L — characteristic length (m, mm, cm, in, ft)
- ρ — density (kg/m³, lb/ft³)
- μ — dynamic viscosity (Pa·s, mPa·s, cP)
- ν — kinematic viscosity (m²/s, cSt)
- Re — Reynolds number (dimensionless)
How It Works: A Step-by-Step Example
Suppose water flows in a 5 cm diameter pipe at v = 1.5 m/s. At 20 °C, water density ρ ≈ 998.2 kg/m³ and μ ≈ 0.001002 Pa·s. Using Re = (ρ v L) / μ with L = 0.05 m:
Since Re ≈ 74,800 is well above 4,000, the flow is turbulent for internal pipes.
Frequently Asked Questions (FAQ)
Do I choose diameter or another length?
For internal pipe flow, use pipe diameter (or hydraulic diameter for non-circular ducts). For external flow over bodies, use the appropriate characteristic length (e.g., chord for airfoils).
How accurate are the fluid presets?
They are based on standard correlations and representative values at atmospheric pressure. For critical design, consult detailed property tables for your exact temperature and pressure.
What if I only know kinematic viscosity?
Select “Kinematic viscosity” mode and input ν directly. The calculator will use Re = vL/ν.
Can this calculator handle gas compressibility effects?
Reynolds number itself does not capture compressibility. For high Mach numbers or significant density variation, additional non-dimensional parameters are required.
Why is my Re extremely large or small?
Check units. Viscosity often causes mistakes: 1 cP = 1 mPa·s = 0.001 Pa·s. Ensure length and velocity units are consistent.
What thresholds apply to external flow?
Thresholds differ. For example, flow over a flat plate transitions around Re_x ≈ 5×10⁵ based on distance from the leading edge; consult appropriate references.
Does surface roughness affect Re?
Surface roughness does not change Re (a fluid-property/flow/geometry ratio), but it affects friction factors and transition behavior in turbulent flow.
Formula (LaTeX) + variables + units
','
\mathrm{Re} = \frac{\rho \, v \, L}{\mu}
\mathrm{Re} = \frac{v \, L}{\nu}
\mathrm{Re} = \frac{998.2 \times 1.5 \times 0.05}{0.001002} \approx 7.48 \times 10^{4}
For density ρ and dynamic viscosity μ known: $ \mathrm{Re} = \frac{\rho \, v \, L}{\mu} $ For kinematic viscosity ν known: $ \mathrm{Re} = \frac{v \, L}{\nu} $
$ \mathrm{Re} = \frac{998.2 \times 1.5 \times 0.05}{0.001002} \approx 7.48 \times 10^{4} $
- No variables provided in audit spec.
- NIST — Weights and measures — nist.gov · Accessed 2026-01-19
https://www.nist.gov/pml/weights-and-measures - FTC — Consumer advice — consumer.ftc.gov · Accessed 2026-01-19
https://consumer.ftc.gov/
Last code update: 2026-01-19
- Initial audit spec draft generated from HTML extraction (review required).
- Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
- Confirm sources are authoritative and relevant to the calculator methodology.