One of the key references for Ham Radio are the Maidenhead Grid Locators. It is named after a place in England where the system originates. It is a system of grids across the Earth which are designated by two letters, two numbers and two more letters. The last two letters are often too specific to be of much interest, so most grid squares are designated by just the first two letters and the two numbers.
Grids are two degrees wide and one degree from top to bottom. In Hawaii, the important lines are 156 West Longitude and 158 West Longitude in one direction and 19, 20, 21 and 22 degrees North Latitude in the other direction.
The two longitude lines fall on Oahu and the Big Island. The first, 156 West Longitude just cuts off the Western tip of the Big Island. The second slices down through Oahu basically running through Pearl City and the tip of Kahuku Point.
North of 20 degrees North Latitude, Hawaii grids start with BL. South of that line the grids start with BK. Land masses in Hawaii are contained in grids BL02, BL01, BL11, BL10, BL20, BK19, BK29 and BK28. Eight grids on major land areas. This is important since many VHF contests and awards are based on working a certain number of grid squares. Unlike many Mainland locations, some of our promising grid squares like BL00, BL12 and BL21 are just ocean.
The University of Hawaii Press publishes a set of maps, one for each island, which include latitude and longitude lines making it easy to find your exact location of your home QTH. Maps with such latitude and longitude references are rare for Hawaii. They are: Reference Maps of Hawaii, Cartography by James A. Bier.
The Big Island, including Hilo, is mostly grid square BK29. The BK29 grid takes in about 80% of the island.
It has a southern point, called strangely enough "South Point", that extends below 19 degrees North Latitude and falls in BK28.
It has a western section that crosses 156 West Longitude at the Keahole Kona Airport area and falls in grid BK19.
It has a northern tip which crosses 20 degrees North Latitude and falls in grid BL20.
This makes the Big Island the king of the islands in terms of grid locators, with four major grids on its land mass. True there is an eastern tip that extends across the 155 West Longitude line, but remember that grids are two degrees wide, so this is still grid BK29.
Close to the south edge of the Big Island, but well offshore is the grid BK18, but it misses landfall.
All of Molokai is in grid BL11 which it shares with the entire Eastern half of Oahu. Honolulu, Kailua, Kaneohe, Hawaii Kai, all areas like this are in BL11.
The Western half of Oahu, starting about at Waikele Shopping Center, is in grid BL01. Pearl City High School is still in BL11, so people near that area would need to check a map looking to see which side of 158 West Longitude they really are on. Oahu shares BL01 with the southern half of the Island of Kauai. The northern half of Kauai is grid square BL02.
Maui is mostly in grid BL10. However, there is a very tiny part of the far Eastern tip of Maui the slips over the line and into grid BL20 which is shares with a rather large chunk of the far north end of the Big Island. On Maui's far Northwest tip, a thin strip is north, across the east/west line of 21 North Latitude and into the very popular grid BL11.
So, BL11 which includes a tiny bit of Northwestern Maui, all of Molokai, and the eastern half of Oahu is the most populous grid in Hawaii. However, BK29 may score highest in terms of most land area (non-ocean area) in a Hawaii grid.
One of the possible VHF/UHF awards in Hawaii is the VUCC award. At the higher frequencies only five grids are required. There are places on the Big Island where five grids can be contacted from a single site.