Emergency kits of some style or function are necessary for most hikes. If you are just going for the neighborhood hike, it might just be a bottle of water. For longer hikes on groomed safe trails, you may just need to add a snack. The longer and more arduous the hike the more involved your emergency kit should be. There are many different types of kits to purchase or you can make your own. Your hike determines the complexity of your kit.
Emergency kits can include everything from water and snacks to flares and tourniquets. These kits can cost anywhere from twenty to hundreds of dollars. The kits can be purchased all-inclusive with the carrying pack or items purchased individually and put together into a pack. The all-inclusive packs are nice if you want compact all together type convenience; but you pay for convenience and like variety packs of anything, you might not like everything in it. If you choose to build your own you can pick how much of each item you want and pick the carrying case, but it might not be as nicely packed.
There are some necessities of a good emergency pack and you can decide which apply to your situation. Water with a reusable container and a filter or something to make water pure is non-negotiable unless you are going for a very short hike. Snacks ranging from nuts and fruit to protein bars or ready to eat meals might be necessary if you are going on a hike of long duration but might be more necessary if your hike is extended beyond your control. Navigation devices are a good idea if you are going somewhere you are not familiar with. A first aid kit is important and can be scalable depending upon your hike duration and location. Sunblock or other sun protection and rain gear or cold weather gear might be necessary to protect you from the elements. Fire starting tools and lighting devices are important as just in case items.
The longer and more involved your hike is will determine if you want to have the nice to have items. If you are going, on a hike that definitely involves an overnight stay or you could be delayed longer than expected a shelter would be nice. Cooking items like heat tabs and cooking utensils are not necessary for a day hike but sure are nice for that meal after a long hike. Extravagant medical supplies are only necessary if you are doing some vigorous hiking.
The first thing to consider for your emergency kit are where you are going and how long you are going to be gone. Next, determine how much weight and how cumbersome you want your pack to be. Then determine the amount of each of the necessities and nice to have items you want to pack. Once you determine all these things you can either purchase a pre-made pack or build your own from scratch.
Think about your hike before you head out and decide what you are going to bring.