My ring has 5 numbers then 18 ci plat written in it what does this mean ?

I'm guessing it's 18 carat gold plated... does it look gold plated?

Cap, could be 18 carat PLATINUM, normally they wouldn't mark it up if it was just plated.
Well that's certainly true for UK and Eurpoe and as this person is from Ireland same applies ;)

I thought carats only measured gold.

Nope it's a measure of purity as well as weight for gemstones:
Carat (purity) - Wikipedia

Does that mean I have taught YOU something scientific!!

Ah yes it does say platinum, I knew it was purity and weight I just thought it was only purity of gold.

I must admit I thought it was only Gold as well until we started looking for wedding rings ;)

The ring is probably 18 kt and platinum and the CI is more than likely the vendor stamp.

So what does it mean if my ring only says 18ci.