CFD Fluid Properties Calculator - Air & Water Properties vs Temperature (Fluent & OpenFOAM)

Every CFD simulation is only as accurate as the fluid properties you feed it — and those properties change with temperature. Run a hot-air case with room-temperature density and your Reynolds number, heat transfer and results can all be off. Yet digging property values out of textbooks for every temperature is tedious. This free CFD Fluid Properties Calculator gives you accurate, temperature-dependent density, viscosity, thermal conductivity, specific heat and Prandtl number for air and water — with copy-ready output formatted for ANSYS Fluent and OpenFOAM incompressible simulations.

Figure 1 CFD Fluid Properties Calcualtor 

Table of Contents

1. The Fluid Properties Calculator
2. Why Properties Matter in CFD
3. Air Properties & Temperature
4. Water Properties & Temperature
5. Properties for Incompressible Flow
6. Using in ANSYS Fluent
7. Using in OpenFOAM
8. Quick Reference Tables
9. Common Mistakes
10. FAQ

The Fluid Properties Calculator

Pick air or water, enter your simulation temperature (and pressure for air), and get all the properties a CFD solver needs — plus ready-to-paste snippets for Fluent and OpenFOAM.

離 CFD Fluid Properties Calculator

Air & water vs temperature · density, μ, ν, k, cₚ, Pr · Fluent & OpenFOAM ready

 Air

 Water

Conditions

Temperature (°C)

Pressure (Pa, for air density)

—

Density ρ

kg/m³

—

Dyn. visc. μ

Pa·s

—

Kin. visc. ν

m²/s

—

Thermal cond. k

W/m·K

—

Specific heat cₚ

J/kg·K

—

Prandtl Pr

–

ANSYS Fluent (material inputs)

—

OpenFOAM (transportProperties)

—

Air: density from the ideal-gas law ρ = P/(RT), R = 287.06 J/kg·K; viscosity & conductivity from Sutherland's law; cₚ ≈ 1005–1055, Pr ≈ 0.71. Water: validated engineering-table correlations (0–100 °C, 1 atm). ν = μ/ρ, Pr = μcₚ/k. Accurate to ~1–2% for near-atmospheric CFD. For high pressure, phase change or real-gas effects, use dedicated property software.

Validation note: the correlations match standard engineering data. At 20 °C the calculator gives air ρ ≈ 1.204 kg/m³, μ ≈ 1.81×10⁻⁵ Pa·s, k ≈ 0.0257 W/m·K, Pr ≈ 0.71; and water ρ ≈ 998.2 kg/m³, μ ≈ 1.00×10⁻³ Pa·s, k ≈ 0.60 W/m·K, Pr ≈ 7.0 — all within ~1–2% of published values. Water density correctly peaks near 4 °C.

Why Fluid Properties Matter in CFD

A CFD solver doesn't "know" your fluid — it computes everything from the property values you supply. Those properties drive the core dimensionless groups that govern the whole simulation:

• Density & viscosity set the Reynolds number — laminar vs turbulent, the single most important flow parameter.
• Thermal conductivity, specific heat & viscosity set the Prandtl number and heat-transfer behaviour, feeding correlations like those in our Nusselt number calculator.
• Density governs momentum, pressure and buoyancy.

The classic mistake: using 20 °C properties for a hot or cold flow. Air viscosity rises ~35% from 20 °C to 300 °C, and water viscosity halves from 20 °C to 50 °C. Get the temperature right, or your Reynolds number — and results — will be wrong.

How Air Properties Vary with Temperature

For gases, temperature changes properties strongly and predictably:

• Density follows the ideal-gas law, ρ = P/(RT) — it falls as temperature rises.
• Viscosity follows Sutherland's law — it rises with temperature (gas molecules move faster, transferring more momentum).
• Thermal conductivity also rises with temperature.
• Prandtl number stays remarkably constant at ~0.71 from −50 to 1000 °C.

μ = μₕₑₘ (T/Tₕₑₘ)^1.5 (Tₕₑₘ+S)/(T+S)

with μ_ref = 1.716×10⁻⁵ Pa·s, T_ref = 273.15 K, S = 110.4 K. This is the exact law tabulated by Engineering ToolBox and built into every major solver.

How Water Properties Vary with Temperature

Liquid water behaves differently from a gas:

• Density is nearly constant but peaks at 4 °C (~1000 kg/m³) and falls slightly as it warms.
• Viscosity drops sharply with temperature — roughly halving from 20 to 50 °C.
• Thermal conductivity rises with temperature up to ~130 °C.
• Prandtl number falls steeply (from ~13 at 0 °C to ~1.7 at 100 °C).

These match the standard water property tables derived from the IAPWS formulations used worldwide.

Properties for Incompressible Simulations

For an incompressible CFD run (the default for liquids and low-speed gas flows, Mach < 0.3), you treat properties as constant at the operating temperature:

Simulation type	Properties needed
Isothermal flow only	ρ and μ (or just ν = μ/ρ in OpenFOAM)
Flow + heat transfer	ρ, μ, k, cp (Pr derived)

Key point for OpenFOAM: incompressible solvers (simpleFoam, pimpleFoam) use kinematic viscosity ν, not dynamic μ. The calculator gives ν directly — copy it straight into transportProperties.

Using These Properties in ANSYS Fluent

In Fluent, open Materials → Create/Edit, and enter the calculator's values:

• Density (kg/m³) — constant for incompressible, or select "ideal-gas" for compressible.
• Viscosity (kg/m·s = Pa·s) — enter the dynamic viscosity μ.
• Thermal conductivity (W/m·K) and c_p (J/kg·K) if solving energy.

For temperature-varying runs, Fluent also offers a built-in Sutherland viscosity model — the same law this calculator uses. See the ANSYS Fluent materials documentation for the dialog.

Using These Properties in OpenFOAM

For incompressible OpenFOAM cases, edit constant/transportProperties:

nu nu [0 2 -1 0 0 0 0] 1.5e-05;

Paste the calculator's kinematic viscosity as the value. For heat transfer, add the Prandtl number (and thermal properties) where your solver expects them. The OpenFOAM documentation details each solver's dictionary — and our OpenFOAM 2026 guide walks through the full case setup.

Quick Reference Tables

Air at 1 atm

T (°C)	ρ (kg/m³)	μ (×10−5 Pa·s)	k (W/m·K)	Pr
0	1.292	1.72	0.0241	0.72
20	1.204	1.81	0.0257	0.71
100	0.946	2.17	0.0317	0.69
300	0.616	2.93	0.0450	0.68
500	0.457	3.55	0.0554	0.66

Water at 1 atm

T (°C)	ρ (kg/m³)	μ (×10−3 Pa·s)	k (W/m·K)	Pr
0	999.9	1.75	0.564	13.0
20	998.2	1.00	0.600	7.0
50	988.1	0.544	0.641	3.6
80	971.8	0.354	0.670	2.2
100	958.1	0.279	0.677	1.7

Common Mistakes

• Using room-temperature properties for hot/cold flows. The biggest error — always match properties to your operating temperature.
• Confusing dynamic (μ) and kinematic (ν) viscosity. OpenFOAM wants ν; Fluent wants μ. Mixing them is a factor-of-ρ error.
• Forgetting density in ν. ν = μ/ρ — use the density at the same temperature.
• Wrong units. Keep SI: Pa·s, kg/m³, W/m·K, J/kg·K.
• Using incompressible air at high speed. Above Mach ~0.3, density variation matters — switch to compressible.
• Ignoring property variation in large ΔT cases. For big temperature ranges, use temperature-dependent property models, not a single constant.

Authoritative external references: Engineering ToolBox — Air properties, Engineering ToolBox — Water properties, IAPWS (water/steam standard), ANSYS Fluent, and the OpenFOAM documentation.

Related calculators & guides on Free CFD Tutorial: check your flow regime with the Reynolds Number Calculator, get heat-transfer coefficients with the Nusselt Number Calculator, set up your solver with the OpenFOAM 2026 guide, and size the near-wall mesh with the Wall y+ First-Cell-Height Calculator.

Frequently Asked Questions

Why do fluid properties matter in CFD?

They control the physics the solver computes — density and viscosity set the Reynolds number and flow regime, while conductivity, c_p and Pr govern heat transfer. Wrong-temperature properties give wrong results.

How does temperature affect air and water properties?

Air: density falls, viscosity and conductivity rise (Sutherland), Pr ≈ 0.71 constant. Water: density falls slightly, viscosity drops sharply, conductivity rises, Pr falls fast.

What properties do I need for an incompressible CFD simulation?

Density and viscosity (or just ν in OpenFOAM) for isothermal flow; add k and c_p for heat transfer. Incompressible is valid below about Mach 0.3.

What is Sutherland's law?

The standard correlation for gas viscosity vs temperature: μ = μ_ref(T/T_ref)^1.5(T_ref+S)/(T+S), with S = 110.4 K for air. Built into Fluent and OpenFOAM.

How do I use kinematic viscosity in OpenFOAM?

Incompressible solvers use ν in transportProperties: nu nu [0 2 -1 0 0 0 0] value;. The calculator gives ν directly to paste in.

Are these properties accurate enough for CFD?

Yes — within ~1–2% of standard reference data at near-atmospheric pressure, well inside typical CFD uncertainty. For high pressure or phase change, use dedicated property tools.

Conclusion

Accurate, temperature-correct fluid properties are the quiet foundation of every trustworthy CFD simulation. Air follows the ideal-gas law and Sutherland's law; water follows well-established engineering correlations — and both change enough with temperature to seriously affect your Reynolds number, heat transfer and results if you ignore them. Use the calculator above to grab the right ρ, μ, ν, k, c_p and Pr for your operating temperature, and paste them straight into Fluent or OpenFOAM.

Set your properties right, and the rest of your simulation stands on solid ground.

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