# Does the size of the tank matter with water pressure?

I remember hearing someone say this. Let’s say you have a bath tub with a one-inch hole near the bottom. Now, let’s say you have the same bath tub but it’s proportions have increased to the point that it’s a square mile in surface area. The water pressure for the one-inch spurt is the same.

True? A botched misstating of something that is true? In either case, what determines the “water pressure?” Does the height matter?

This is true. The pressure at any point in the water column (such as at the one-inch hole in your example) does not depend on the area of the free surface, only on the vertical height from the hole to the free surface.

The expression for pressure is rho x g x h (pascals)

where
rho is the density of the water (typically around 1000 kg / m[sup]3[/sup], slightly dependent on water temperature)
g is the acceleration due to gravity (typically 9.8 m/s[sup]2[/sup], slightly variable over the earth’s surface)
and
h is the height of the water column in metres.

In most applications you can take rho and g as constant, and say the pressure depends only on the height of water.

Conversely, you can express pressure as inches or millimetres of water.

Thanks a bunch!