Y+ & First Layer Thickness Calculator

Estimate the height of the first inflation (boundary) layer needed to hit your target y+ for CFD meshing in ANSYS Fluent, CFX, or OpenFOAM.

Which y+ should I use?

y+ ≈ 1 — required to resolve the viscous sublayer with low-Re turbulence models (SST k-ω, Spalart-Allmaras with near-wall resolution).
y+ 30–300 — suitable for wall-function based models (standard k-ε, RNG k-ε).

Flow inputs

Fluid

Velocity U (m/s)

Length L (m)

Density ρ (kg/m³)

Viscosity μ (Pa·s)

Target y+

Results

Reynolds number, Re-

Skin friction coeff., Cf-

Wall shear stress, τw-

Friction velocity, uτ-

- mm first layer height (- µm)

Heads up: with these inputs, Re is below ~5×10⁵, which suggests the flow may be laminar rather than turbulent. This calculator assumes turbulent flow (the common case for most CFD simulations), so the result above may not be accurate for a laminar case — increase velocity/length or double-check your regime.

How many inflation layers do I need?

Starting from the first layer height calculated above, each next layer is made thicker by the growth rate (e.g. 1.2 = 20% thicker than the previous one). This estimates how many layers it takes to reach your target total boundary layer thickness — use these two numbers (growth rate and layer count) directly in ANSYS Meshing / Fluent Inflation settings.

Growth rate

Target boundary layer thickness (mm)

Layers needed: -

Equations used

This calculator uses the standard turbulent flat-plate approach (Schlichting skin-friction correlation), the same method used by most CFD y+ calculators.

Reynolds numberRe = ρ U L / μ

Skin friction coefficientC_f = [2 log₁₀(Re) − 0.65]^−2.3

Wall shear stressτ_w = C_f × ½ ρ U²

Friction velocityu_τ = √(τ_w / ρ)

First layer heighty₁ = y+ × μ / (ρ × u_τ)

Layers needed (growth rate r, target thickness δ)n = ⌈ log(1 + δ(r−1)/y₁) / log(r) ⌉

Where ρ = density, U = velocity, L = characteristic length, μ = dynamic viscosity. Valid for turbulent flow, roughly Re < 10⁹.

Need help setting up inflation layers in ANSYS Meshing?

Browse our meshing tutorials

Free Download: CFD Meshing Quick Reference

A PDF reference sheet with y+ targets by turbulence model, inflation layer growth rates, and mesh quality thresholds — all in one place.

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