These tubes work well for kittens. We use size 5 for the first week then go up to 8 through the next week. If you make a mark for the stomach by measuring mouth to last rib, you can usually be sure that if it goes all the way in, you are in the right place. The bronchial "Y" stops the tube at halfway to the mark, so you know to back out and reinsert. We recently had a size 5 slip easily past the "Y" in an older kitten and drown the kitten! When we switched to an 8, we could tell that would not happen again. We will switch up for safety from now on, the experience was horrible. Also, be sure when the kittens get teeth to use a guard to prevent them biting and swallowing the tube., which is also deadly. A cut off piece of a reusable Starbucks straw works very well for us. It has a knob on it that is easy to hold, and is bite proof. If all of this makes tube feeding sound worrisome, it is still SO much safer and simpler to save kittens tube feeding than with bottles or syringes in our experience.