What lens cap would fit a 18-55mm lens?

I got bought a nikon 40 with the 18-55mm lens. But it didn't come with a lens cap. What size would it be?
or how do figure it out?

If you looked on the side of the lens, you will have noticed that the filter thread size is 52 mm.

Fhotoace's got it, but you can go to best buy and the camera section and they have tons of lens caps for 18-55 mm lenses since that's the standard kit size.