The usable height of a 15" drawer is 13.25, 10" is 8.25, and 5" is 3 3/8. Sorry, I have no 20" drawers at the moment - I should have one after my next trip.

Thank you!
Probably can surmise the 20" from your other figures.
You just saved me a long drive.

I must say, I just installed an Ikea kitchen, and, unless I did something wrong, which I doubt as we followed the instructions carefully, I find the 5" top drawer almost useless. There is only 3" of storage in the drawers which means it's only useful for flatware which is carefully spread out. I had intended to store my cooking utensils there and will now have to reach down into my 10" drawer. Sadly I purchased three of these cabinets.