# How many cells/elements do I need?

I'm relatively new in the CFD modelling and I'm making a VOF model of a rectangular box ($$l \times w \times h = 120\times 80 \times8$$ m) with an inlet ($$w\times h = 10\times 8$$) at the long side.

The pressure outlet is the upper face of the box ($$l\times w=120\times 80$$m). The box is initially filled with $$5$$m water. Water is flowing in through the inlet, velocities at inlet: around $$0.1$$ to $$0.01$$ m/s

How many cells/elements do I need to get a good indication for the flow pattern in the box?

Concerning the flow simulation, an upper bound is given by the relation $N\geq Re^{9/4}$, where $N$ is the number of mesh nodes and $Re$ is the Reynolds number, derived for direct numerical simulation .
In your case with $L=120m$, $u=0.1m/s$ and $\nu = 10^{-6}m^2/s$, you have $Re \approx 1.2 \cdot 10^5$ what - in theory - means that you will need about $2.7\cdot 10^{11}$ nodes in your mesh to resolve all scales of the flow dynamics.