What to pack to go trail running

When you're first starting out with trail running it can be confusing to know what you should carry with you. And I will start answering this question by saying, it depends. That may add even more confusion, but exactly what you carry depends on many factors such as where you're going, what the weather will be like, how long you plan to be out for etc. What you carry will change from run to run, so for instance, you would take a lot less on a low level 10 km loop compared to a run in the hills. What is listed below will give you a good idea of where to start, but I certainly don't carry all this on every trail run I do.

For context, this is what I carried for a run over a couple of mountains in the Scottish Highlands in November.

Pack
Lets start with what everything is going in, because it's a pretty important piece of kit. The size of pack or running vest you choose will depend on what you're doing and how much kit you need to take, and over time you'll probably end up with two or three packs of different sizes. You want your pack to be just big enough to fit in everything that you need. If it's too big the contents will bounce around as you run and get very annoying. Fortunately running vests are made out of stretchy materials so you can squeeze a surprising amount of kit into a small bag. They also have a variety of pockets and pouches where you can store items that you want quick access too. There's a lot of choice out there so go and try some on, check that you can reach all the pockets without dislocating your shoulders and if possible, fill it with stuff and check that it doesn't bounce around on your body when you run.
(To give you an idea of size, the pack in the photo is 12 litres and everything there went in with no issues.)

Clothing
This is another one that really does depend on where you're going, how long you're planning to be out, how fast you're planning to run and of course, what the weather forecast says. You should always have at least one warm layer so you have something to put on if you have to stop. The speed at which you cool down when you stop running is quite alarming so it is essential to have a warm layer to stick on, even if you're just stopping for five minutes to sort out a blister. It's worth adding that the warm layers I carry are almost always synthetic. Synthetic insulation isn't as warm as down for the same weight and it doesn't pack down as small, but if it gets wet it maintains it's insulating properties. Down, on the other hand, clumps together, loses it's loft and therefore also loses it's insulating properties. For a damp climate like we have in the UK, synthetic insulation is a clear winner. If there is any chance of rain then definitely pack your waterproofs, and if you're heading over any hills or mountains then gloves and a hat or buff are a must.

Navigation
Again, it depends on where you're going and how well you know the trails, but if you're heading into the hills then it's definitely worth having a map and compass. For the size and weight of them is there any reason not to stick them in your bag? As a back up you might also have a hand held GPS or mapping on your watch or phone. These are definitely back ups though and shouldn't be your only means of navigation. Batteries run out of juice unexpectedly, especially in the winter when it's cold, so you need to have something that doesn't rely on battery power.

Poles
Some people love them, some people hate them, so they are a very personal choice. However, over long distances or on steep or uneven terrain poles can be very beneficial. They put some of the work into your arms back and shoulders so your leg muscles can keep going for longer, and they can really help with balance, particularly on more technical sections or steep downhills. Your pack should have a way of attaching your poles so you can easily stash them away when you get to an easier section where you don't need them.

Emergency Kit
A little emergency kit bag is absolutely essential, and this should include a first aid kit, some kind of shelter and perhaps a head torch if you're going out for a longer run. In the winter your head torch may be an essential piece of kit given how short the days can be but you might also want to have one in your pack in the summer. Your first aid kit should contain things to fix common running injuries, which is mainly blisters. Zinc oxide tape, climbing tape and K tape all work really well, along with a tiny knife to cut the tape. I also carry a few small plasters and a small ambulance dressing for any trips that result in bigger wounds. For a shelter you can get a simple foil bivvy bag - basically a tinfoil sleeping bag. It doesn't sound like much and the the aim is to not use it, but if everything goes a bit pear shaped then you have something to get you out the wind and rain until help arrives. A mobile phone also forms a part of your emergency kit, but most people don't go anywhere without one so it's a bit of a given.
There is one other emergency item that I carry, but it won't be essential for most trail runners, and that is a PLB (personal locator beacon). I only carry this because a lot of my running is solo and/or in places with no or poor phone signal. A PLB allows me to contact the emergency services via the satellites rather than the phone network. If you plan to head out into more remote areas then it might be worth investing in a PLB.

Hydration
This could be a bladder, soft flasks or just a little soft cup so you can easily grab water from streams. This one depends on how long you're going out for, the temperature and whether or not the water in any streams you pass is drinkable - if you're not sure then err on the side of caution and just carry enough water! What you take to drink is also worth considering. For shorter runs in cooler temperatures water will be just fine. However, in hot weather or if you're out for a long run then you need to replace salts lost through sweat so you might want to add an electrolyte mix.

Nutrition
I won't add any more here because sports nutrition is a huge topic and also incredibly personal. What works for one person won't work for the next. If your run is longer than 60-90 minutes then you should take some food. Exactly what you take is up to you, but I am a big advocate for taking real food. By that I mean a jam sandwich rather than a sports gel, but you see, it's already turned into a personal preference. In general you want carbohydrates that are easy to eat and easy to digest, so go and try some foods out on your next run and see what works for you.