How does opening size affect pour rates in bottles? Does doubling the opening size double the pour rate? What if there is a separate, smaller, opening to let air in?

In general doubling the area of the opening should cause flow rate to double. The reason is because flow rate equals the velocity of the liguid flow times the area of the opening. For low viscosity fluids (like water) and a reasonably-sized opening in the container the velocity of the water flow is dependent only on the pressure (head) of water in the container minus the effect of the vacuum caused by decreased air pressure as the water pours out, so is the same regardless of size of opening. That's the theory. But in practice your mileage may vary, especially if there is no air vent, as the chaotic flow due to air forcing its way upstream into the container (causing the "glub glub" effect) as the water flow alternately starts and stops may have a significant impact. My guess is that the larger hole makes it easier for air to get in, so that effect should be reduced as the hole size increaes. The result is that doubling the size of the opening may more than double the flow rate.

As for providing an air vent - obviously that reduces the vacuum that restricts the flow and the flow rate increases.

In general doubling the area of the opening should cause flow rate to double. The reason is because flow rate equals the velocity of the liguid flow times the area of the opening. For low viscosity fluids (like water) and a reasonably-sized opening in the container the velocity of the water flow is dependent only on the pressure (head) of water in the container minus the effect of the vacuum caused by decreased air pressure as the water pours out, so is the same regardless of size of opening. That's the theory. But in practice your mileage may vary, especially if there is no air vent, as the chaotic flow due to air forcing its way upstream into the container (causing the "glub glub" effect) as the water flow alternately starts and stops may have a significant impact. My guess is that the larger hole makes it easier for air to get in, so that effect should be reduced as the hole size increaes. The result is that doubling the size of the opening may more than double the flow rate.

As for providing an air vent - obviously that reduces the vacuum that restricts the flow and the flow rate increases.


Thank you, this was very helpful.