I want to buy a lens filter but am unsure on what size to get?

I have a nikon d40 with the standard 18-55mm lens. I want to buy some filters for it but am unsure what size to get. I've read that I should get a 52mm but after measuring across the lens it comes out as 55, is that the size I should get or is there something else I'm missing?

According to the Nikon site your 18-55mm f3.5-5.6 zoom lens requires 52mm diameter filters. So go forth and buy the 52mm diameter filters you desire.

If you go to a reputable camera shop, they should be able to help you but you will probably find that it is a standard fit (when asked for Nikon). I own a Nikon D60 as well as an FM2. Both have interchangeable lens filters which fit both. (The FM2 came well before the D60).

1- What filter are you trying to buy?
2- What other lenses do you have / are planning in buying? Need to keep that in mind so you don't end up with a bunch of lenses all with different diameters and copies of the same filter in different sizes just to accommodate.

- If what you are buying is a UV filter to use as a lens protection, then, ok, go ahead and buy one in 52mm (diameter should be stated in the front of your lens)

- If you are buying a polarizer and have other lenses/planning on having other lenses, then buy a bigger one and buy an step-up adapter ring. That way you can fit that filter to any lens you might have in the future

- If you are buying a neutral grad, then don't. At least don't buy the threaded one. Get a square one (cokin / Lee / Singh-Ray).It gives you more flexibility.

You could also consider systems like cokin (for an amateur) as a starting set (in case, again, that you are looking for something else than just UV filters to use as a lens protection)

Look on your lens cap and see what size you need (lens cap, not lens; although your lens might have it, too)