How to tell what size my lens is?

So, I want to buy a fisheye lens for my Nikon P90.
It says 4.6-110.4mm 1:2.8-5.0 on the lens.
How do I figure out what size of fisheye to buy for it?

All dslr lenses have that information printed on the front lens element around the ring. It will read
perhaps 52mm, or 58mm. That's your thread size - true with all dslr lenses.

Add-on fisheye "lenses" are neither unless they are specifically designed by Nikon for your specific camera. There are none

Unless there's a list of accessories under the P90 specification on NikonUSA.com, there's NO such lens. You will have to buy a Nikon dSLR and Nikkor 10.5 mm fisheye lens for it