@Sunflower - I'd say it's about one dime sized hole where you want the top of the water in the pan to reach(inch or two from the bottom), I just used a drill for it and only did one hole cause I have to cover it with my finger when I fill the bottle. It's simple to fill, just uncap the rubber cap on top, plug hole, filler up, replace cap, and once the water in the pan reaches the point where the hole is it will stop on its own.