I've flown 48 hours on long-haul flights in the past year and always brought one crucial item onboard. It saved me over $100.

From New Zealand to Japan, I've traveled all over the world for my job as a travel reporter. And to get to many of those places, I've taken a long-haul flight.

On those flights, I've been both spoiled in business class and squished in economy. 

But regardless of the cabin class, I've made plenty of packing mistakes.

Last summer, when I traveled to Auckland, New Zealand, I spent 13 hours on a plane regretting the snacks I packed and my fast-food lunch choice. 

But the one mistake I never make is forgetting to pack a reusable water bottle.

On my most recent flight from Denver, Colorado, to Tokyo, I decided that a water bottle is the most helpful item I can have with me on any flight.

Flying — no matter the distance — dehydrates you.

Bob Bacheler, managing director and in-flight nurse at Flying Angels, told CNN that the airplane's dry cabin is to blame. Since cabins often have low humidity levels, they cause passengers to lose 8 ounces of water an hour, which mainly happens through breathing. 

With that in mind, the Aerospace Medical Association states that travelers should drink at least 8 ounces of water for every hour they're in the air. 

On my most recent long-haul flight, a flight attendant and beverage cart stopped at my seat four times during the 12 hours. But the four 8-ounce cups of water they served to other passengers don't collectively come close to the 96 ounces recommended by The Aerospace Medical Association for my flight.

That's where my 32-ounce reusable water bottle comes in. I can board a plane with it full thanks to airport water fountains, and flight attendants have always been happy to refill my water bottle instead of handing me a plastic cup. This means I can easily drink the recommended 96 ounces of water during the flight — as long as I remember to constantly keep sipping.

And once I disembark, I've found that my body thanks me. Staying hydrated has been proven to help with energy levels, which is exactly what I need when it comes to fighting jet lag. Both the Mayo Clinic and Harvard Medical School state that staying hydrated is one way to prevent jet lag.

So the easiest way for me to stay hydrated is by bringing a water bottle that holds plenty of water. 

As I sat at my gate in the Narita International Airport in Japan, I watched a steady stream of travelers stop at a nearby vending machine.

I noticed that most were grabbing a bottle of water for the long flight ahead of them.

The sound of change dropping into the machine and water bottles tumbling down were reminders of the money I was saving.

US News and Travel reports that the average water bottle at an airport costs between $2.50 to $5.

Since I've spent 48 hours on flights in the past year for work, based on the Aerospace Medical Association recommendations, that means I should have had at least 384 ounces of water.

And since most regular bottles of water are 16 ounces, that comes out to be 24 bottles of water that I would have purchased in airports to stay fully hydrated.

If I bought all those water bottles and paid $5 at the high end for each, I would have spent $120.

When I travel, I try to find ways I can be more responsible. And in the past, I've spoken to experts about how I can make trips like my one to Japan more eco-conscious.

Those experts told me it often involves taking a direct flight, booking tours with locals, and using public transportation.

But they also said there are small things I can do to lessen my carbon footprint when I travel. One way is to avoid single-use plastic.

According to a report published by the United Nations University Institute for Water, Environment and Health, the world generated 600 billion plastic bottles and containers in 2021, which is about 25 million tons of waste. Most of this isn't recycled, the same report states. 

So by squeezing a reusable water bottle into my bag instead of purchasing those 24 aforementioned bottles of water, I'm preventing more waste from ending up in a landfill. 

Every time I squeeze my airplane pillow into my personal item brought onboard a flight, I feel like I'm wasting precious packing space.

A travel pillow will only be helpful for the flights, which is really a small sliver of my entire trip. And since I like to pack light, I struggle to justify packing one.

The opposite is true for my reusable water bottle. The single item is one of the few things I will use and carry with me every day of any trip. 

For me, staying hydrated while traveling is just as important as staying hydrated on a plane. I want to feel energized and refreshed for each day of my trip, and water helps accomplish that.

So no matter where the plane is taking me, I make sure to add a reusable water bottle to my packing list.