Is your 6 month old currently wearing?
Or when LO was 6 months old what size were they in?
DS has been in size 4 Pampers Cruisers since 5 months old...and I just bought another box of size 4's. I think it's starting to slow down. I was buying the next size every time I would buy another value box. Ugh!

Re: What size diapers?
DS is only 18 lbs. We switched to size 4 around 15 lbs. I never go by the lbs on the box....they never match for us.