Beginners Guide to Bike Bags
For as long as I can remember, I've been putting bags on bikes. Back when I started riding in the mid-late 90's as a child it was a triangular frame bag (funny how those came back into fashion), or a saddle bag for my inner tube, tyre levers and multi tool.
Over the years carrying things on a bike has progressed from "touring" setups of panniers and a Carradice saddle bag (shout out to the OG's who have been doing this since 1932), to aerodynamic bags made from the latest and greatest materials designed for multi-day races such as the Transcontinental.
At some point the term touring gave way to bikepacking (although both exist quite happily). Bikepacking.com says "Simply put, bikepacking is the synthesis of all-terrain cycling and self-supported backpacking". and that basically sums it up. Go anywhere on your bike while carry everything you might need.
If you're new to carrying things on your bike, I would suggest reading the Bikepacking 101 as well as Advice for New Bikepackers, in fact the entire site is essentially an encyclopedia of bikepacking (hence the name) and includes more information than I could even begin to describe.
That being said, I am going to give my - admittedly somewhat biased - opinion on what bags work best for the kind of rides you're going to be doing.
Personally, I think the essentials would be something to carry your tools, and something to carry your phone, wallet and keys. I offer the Jersey Pouch which can be used for either, although it does assume you're wearing a cycling jersey and thus have somewhere to put it. For inner tube, tyre levers, CO2 and multi-tool storage I offer a Road Roll (which is also available in reflective fabric) which will take an inner tube up to around 32mm, and the Tool Roll which will take almost any size of road/gravel inner tube. Both also have room for a typical size multi-tool. I offer the Road Roll and Jersey Pouch as a combo which saves you a bit of money.
15-50km (10-30 miles)
For a ride of this length many will be happy with just the essentials. However if the weather isn't looking great, you might want to carry a jacket, or a pair of gloves. At this point your cycling jersey will start to overflow - again assuming you're wearing one - and if you're not, where the heck does everything go?
My most popular bag is the Bar Bag. This will quite comfortably take a small rain jacket - or arm warmers - and a pair of gloves, as well as some snacks, and you should also be able to squeeze your mobile phone, wallet, and keys inside, it's also available with a Road Roll as a combo. However if it's not quite big enough then the Roll Top Bar Bag is slightly bigger when rolled down, and the expandable size allows for additional items, be that a bigger jacket, or items for you and maybe your riding partner.
Personally speaking I always have a Top Tube Bag on my bike which I use for my phone, wallet and keys, and maybe a chocolate bar here and there, but it is smaller than either of my handlebar mounted options, so wont take a jacket or gloves. I like to keep mine on my shopping/post office/casual bike so there's always room for my essentials when I'm not wearing cycling gear.
50-100km (30-60 miles)
This is where something like a Stem Bag can come in handy. It's shaped to take a water bottle, but grabbing a pair can let you have one for water, and the other for snacks or a general "dump pouch". I often use a stem bag for my phone if I'm riding somewhere particularly picturesque since it allows for super quick access - no fussing with zips - the only downside being that if the heavens open, your gear will probably get wet!
I also find a Frame Bag useful for these kind of distances. When you're spending a few hours on the bike you never know what you might need, and I always keep a jacket, gloves, and a bit more of a comprehensive set of tools in my frame bag if I'm going out on a longer ride. However, with a combination of something like a Bar Bag and a Stem Bag, you'll probably have adequate storage.
100-160km (60-100 miles)
These kind of distances are often milestone rides for people. Your first "metric century" (100km) or century (100 miles) is a big achievement, and proper planning can help make it significantly easier. Personally speaking I'd probably put a Frame Bag on to ensure I'm prepared in case the weather turns, and a Bar Bag for my snacks and phone.
If multi-day rides are your thing then you're probably going to be carrying "casual" clothes for when you're off the bike. At this point I would recommend something like a Saddle Bag or Bar Roll (or both depending on the length of the ride). These are designed to carry bigger loads, such as camping equipment, sleeping bags, warm jackets, etc.
Ultimately, it's personal choice. There are people who are content riding 100+ mile days with nothing but a very basic set of tools and their phone, and people who are riding 30 miles with their bike loaded to the hilt. I would rather be over-prepared than under, which is why all three of my bikes always have a frame bag fitted so I can throw some additional clothing in there as I always struggle with being too cold.
If you're preparing for a ride and have read this far and still have no idea what you might need, then please get in touch and I'll do my best to help out.