How to calculate flowrate of water through a crosssection of pipe?

There's Torricelli's equation for hydrodynamic calculations purpose and for calculating the velocity of perfect fluid flow at the $'open' end$ could be presented as: $$V = sqrt{2gh}$$ $V $ is the velocity, $m/s$

$g $ is the acceleration due to gravity, $m/s^2$

$h $ is the liquid column height (above the 'open' end), $m$

The same conclusion could be drawn from the Bernoulli's incompressible flow equation

Sorry, I don't have enough reputation to commenton Nikolay Jolshin's answer. Regardless of the pressure at the exit, if you know the difference in pressure you can use Torricelli's equation.

I just want to add that, depending on the application, you might also need to consider loses due to speed. For example adding a speed coeficient to Torricelli's eq. Many manufacturers provide such coeficient, which from my experience normally dances around 0.98