On one YouTube video I watched, about an ultralight hiker’s gear, the first aid kit was a few items in a sandwich bag. An ounce (30 grams), perhaps. And good for a skinned knee, I guess, but not for anything worse. My own first aid kit is built around a serious accident* leading to multiple injuries: large and small cuts (including with persistent bleeding), scrapes over large areas, a sprain or broken bone, and splinters or thorns driven well into the skin. If you’re prepared for a worst-case scenario you may never face it, but better to carry a little extra weight than to bandage your hiking partner with toilet paper and duct tape.
Non-injury medical problems can be just as urgent. You need to stop diarrhea to keep dehydration from killing you, if it's a hot day with limited water. One full-strength aspirin, chewed, may save the life of someone having a heart attack. You also need to treat minor issues such as backache and heartburn. And you may need to do other things unexpectedly: start a fire, for example, or repair clothing, and I like to carry items to do both in my first aid kit.
These days the crush-proof case I use is almost an anachronism; most cases are cloth. As I describe on a different page, I pack my first aid tape separately. My separate items also include an elastic bandage, a 36 inch Sam Splint, and a bandanna large enough to support an injured arm. The items listed below are the ones actually stored in the kit.
For cuts and scrapes: six 3 by 3 inch gauze pads (three at the top), two 4 by 3 inch adhesive bandages, three 4 by 2 inch adhesive bandages, six “owie size” cloth Band-Aids, two knuckle bandages, three butterfly bandages, and a 2 inch by 4.1 yard roll of gauze. Also three packets of triple antibiotic ointment and an iodine swab.
Over-the-counter tablets: six aspirin, six ibuprofen, six acetominophen, six Benadryl, four Pepto-Bismol, four Imodium, four electrolyte, and four antacid. (If you’re on prescribed medications, don’t forget those.)
To keep blisters from developing on feet: nail clippers and moleskin. Other items: a pair of disposable examination gloves (at the top), scissors, tweezers, a tiny sewing kit (needles, thread, safety pins), and a Tick Key tick remover. To start fires, a box of small kitchen matches and a small “tea light” candle, both wrapped in foil. And a lightweight mirror, for self-examination of facial injuries or signaling.
The next photo shows the kit when it’s closed and secured with a mini-bungee cord. Including the cord, it weighs 11.5 ounces (330 grams). I used duct tape to seal the little breathing holes from
the inside, so there’s tinder besides the tea light. The case used to belong to my college hiking partner but after I admired a little too openly, he gave it to me. Great guy.
If you're not interested in building a first aid kit from scratch, take a look at Adventure Medical Kits' Ultralight/Watertight series. The ".5" weighs just under 4 ounces (110 grams)and is best for taking care of yourself. The ".7" weights 8 ounces (230 grams) and its contents are much like those in my own kit.
Last but definitely not least: you need to know how to use the first aid items in your pack. If you haven't already taken a first aid course, make that a priority.