Why the Width of a Straw Doesn't Matter

Assuming that you already understand how a straw works, you know that a straw uses the difference between the pressure inside the straw and the pressure outside the straw.

To understand how this relates to the width of a straw, let's look at the pressure down exerted by the water in the straw.

We know (from physics) that

The force in this case is the force of gravity acting on the water. The force of gravity is

where m is the mass of the water and g is the gravitational constant (how hard the Earth is pulling on the water).

We know that

and therefore

Substituting in, we get

The water in the straw is in the shape of a cylinder. The volume of that cylinder (or any cylinder) is

Substituting that formula in gives us

Since area appears on both the top and the bottom of the equation, they cancel and, thus, don't affect the pressure. The width of the straw (which determines the area) is irrelevant.