Funerals are sad and somber occasions that usually require formal clothing. For funerals, the more formal side of business casual clothing is appropriate while a full-on suit is also typical. Depending on the location, type of year and formality of the deceased's family, either might be appropriate.
Only dark clothing should be worn to a funeral - never light clothing. This can include a suit for men or a nice looking business casual ensemble including a sport coat. This includes a traditional long sleeved shirt, dark wool pants, black dress shoes, a dark-colored sport coat (or even suit jacket) and matching belt and socks
Never wear light-colored clothing to a funeral (unless it's stated this is appropriate in advance)
If wearing a business causal outfit, be sure to include a sport coat
Funerals are usually outdoors, so wear an appropriate fabric for the season