22 Essential Things to Carry for a Hiking or Trekking Trip

Following are the essential things which I feel are you should pack/ check/ buy/ rent before going for trekking:

In summer if you are in Delhi, Bangalore, Chennai it's going to be 40 degrees, but in the mountains, it's going to be cold so if its summer you will need to take three warm clothes at least. So first thing, take a -
1. Woollen sweater - it needs to be light, and compact don't take something heavy, and it needs to serve the purpose of keeping you warm.
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2. Fleece jacket - Its very light and I don't think it weighs more than 50-60 grams. Its is inexpensive and easily available.
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3. Padded jacket - When you are climbing at high altitude, as you go higher it is going to get colder so in the night the temperature could drop around even below 0 degrees. So take a warm padded jacket which will keep you warm and insulated. (Don't miss out on it because it is beneficial if you climb higher).
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Now, these were the things you will be taking apart from your T-shirts.
4. T-shirts - Take three T-shirts at least, collared ones preferably because it will cover your neck as well, which will reduce the chances of sunburn.
I want to point out that this is for summer trek and if you are going during October-November, add one more layer to this. And during December you should be carrying a total of five layers, that's mandatory, don't forget it.
5. Trekking Pants - Take two pants at least. Trekking Pants are not the same as Track pants. These are cotton pants which are light and compact, and even if it gets wet, it will dry up quickly. And if you are going on with track pants, make sure to get the thin ones.
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6. Thermals - at night you can wear them, but in summers it is not necessary, but in winters you should take the thermals which will keep you warm and cosy at night. Don't wear them while trekking because you will need it fresh at night, don't let it get sweaty and keep it for the night.
7. Undergarments - Take three pairs at least

Accessories:

8. Sun cap - In the mountains, it is going to be very sunny. You can wear a hat/cap with flaps that will protect your neck and back of your ears.
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9. Balaclava or Woollen cap - At night you will need something to keep your head warm, remember most of the heat escapes through your head. So you need to cover your head and ears.
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10. Neck warmer - This you'll be using most in the evening or when it's windy. Buy a neck warmer which uses fleece.
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11. Sunglasses - Don't forget these because you will be trekking in snow whether its summer or winter and you will need to protect your eyes. Snow blindness is very common if you don't take sunglasses.
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12. Gloves and Socks - to keep your hands and feet warm you need gloves, don't take woollen gloves because you will be handling snow and they will get wet will be of no use. Take synthetic ones which are water-resistant. For your feet you need a good pair of socks, I use sports socks always because they are of cotton and are more comfortable than any other fabric and for the night keep woollen socks to keep you warm.
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13. Trekking Shoes - Make sure your shoe has a good grip. It should be soft and flexible. And if it comes with ankle support and water-resistant, it will do more good to you.
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14. Poncho - It is like a raincoat, but it covers you from your head to toe, including your backpack. Carry a poncho as you don't know when it's going to rain or snow in the mountains.
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15. Headlamp or torch - Headlamp is more convenient than the flashlight and readily available at the sports stores.
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16. Trekking pole - On a high altitude trek, you will be climbing up and down, and it's going to be quite tiring. And according to some research, the use of a trekking pole effectively saves up to 40% of your energy.
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17. Medical kit - Carry essential medicines, bandages and ointment
18. Toilet kit - It consists of Toothbrush, toothpaste, moisturizer(petroleum jelly is preferable) and facewash, hand sanitizer, toilet paper and chapsticks because your lips will chap a lot at high altitude. Sunscreen is one crucial thing and make it above SPF 40, and use it every 3 hours during a trek
19. Water Bottles - Take two bottles and avoid mineral water bottles which you can discard your trek. Do not leave anything that you take behind and be a responsible trekker.
20. Extra slippers - It can be floaters or flip-flops, whatever works for you. You will need them at the campsite because its challenging to get in or out of your shoes
21. Spare plastic bags - You will need it to keep used undergarments, socks etc
22. Day Pack - A small backpack that will be used to keep water bottles and basic requirements like medical kit etc.
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Now I know most of the people will say that some things are missing from the list, but they are left on purpose because during the trek you are the one who is going to carry your backpack, so try to take essential things only. And believe me, treks are no joke and if you are a beginner don't start your adventure with high altitude treks. Go for the shorter and low altitude ones like - Kheerganga, Triund, Kareri lake, Prashar lake etc.

See also:

5 Best Rucksacks in India - Reviews & Buying Guide
Top 3 Best 40 Liter Rucksacks/Bags in India
Top 5 Best Trekking Shoes in India - Buying Guide
24 Important Tips for Beginners While Trekking and Hiking
Health Benefits of Hiking and Trekking
Acute Mountain Sickness, HAPE & HACE: Causes, Symptoms, Treatment, Prevention