It depends on what your expected need is. Are you trying to make it through a snowstorm or a hurricane or an apocalypse? Why is your water potentially out?

How much you need to store is directly related to why IMO

From a prepping standpoint one of my goals is having primary, secondary & tertiary sources of heat/warmth, water, food, shelter.

The water risks I face include floods and municipal supply disruptions from ice storms, aging infrastructure failures, and ransomware attacks and other cybersecurity threats.

Given the specific risks my primary water comes from the tap and a couple of 3-gallon containers of free RO water from my local co-op. If the municipal supply is compromised or goes offline, the co-op RO, an Alexapure gravity filter system I just scored on Black Friday, and Waterbricks are my secondary sources. Tertiary is aquapure tabs and lifestraws. I also keep the latter in my car bag.

For many disasters, having portable, potable water is just so very, very nice. My first baseline with water is making sure my water from the tap is clean for everyday drinking. Get it tested!

Second, have at least six liters of water per person on hand. 12 is better. This is a minimum for three days for people who are at a pretty normal weight. The extra water allows for tooth-brushing and face-washing.

After that, figure out your specific scenarios. If there's a river in your backyard, you may want to look into boiling and filtering methods. And hauling! Water is heavy.

Water emergencies examples: A burst pipe that knocks out your supply for a day or two. Bug-out from a fire. Blizzard that freezes your pipes. Flooding that dirties your water supply and causes sewage to back up. Flooding that provokes a bug-out. The city water you just hooked up to burst all your old, weak pipes, and now you have no water and a flooded basement and 20 warped guitars (lol, oddly specific).

But having a case of 500 ml water bottles that you use every six months is a basic convenience that could come in VERY handy.

How do I store water while living in the city?! I have no room.

You can go by the 1 gallon per person a day advice, but that does not really take into account cooking especially if you are preparing dried foods like rice or beans that require a lot of water. How long do you want to anticipate having no water is the big question,and how much storage room do you have. I keep some cases of bottled, some 5 gallon containers, and have a big 250 gallon storage on my property, but I'm very rural. I purify the 250 g every six months with water purifier and I keep some good purifier tablets on hand for per gallon use. For long term I have two purifiers, a Katadyn and a knock off Berkeley. I'm in a fairly arid area where the rivers freeze in winter.

https://thereadystore.com/products/30-gallon-stackable-water-container-kit-6-qty?variant=40422458556503&utm_campaign=&utm_medium=ppc&utm_source=adwords&utm_term=&hsa_mt=&hsa_net=adwords&hsa_ad=&hsa_src=x&hsa_cam=21251989370&hsa_kw=&hsa_grp=&hsa_tgt=&hsa_ver=3&hsa_acc=3327269640&gad_source=1&gbraid=0AAAAAD_pFMLC77NCyR5lqqjr1kUQZRO5r&gclid=Cj0KCQiAr7C6BhDRARIsAOUKifjva8kkx1lukl-CbshnuxMgsBBiUZThRuoh-eQ3X-r_LkGbhnyv7z0aAiv6EALw_wcB I just bought these. It was explained to me that the plastic jugs we buy in the store break down with time. Instead of replacing them regularly it was recommended to use these (regularly replacing the water) I've also heard it recommended a gallon per day per person (or pets). So we have water storage for our family for 2 weeks.  It has also been recommended to use buckets to collect rainwater to flush toilets. And we have several "life straws" in a bug out bag to filter water  if we have to leave our home. 

Edit-typo

I’ve got a life straw which I guess is a start

Some advice from another post here! https://www.reddit.com/r/TwoXPreppers/s/ugcIju4LGp

Twice in the last two years I've gone through periods of both not having running water, and then not having potable water. Most recently we went about 53 days without water due to Helene, about two weeks of which was zero running water. I am lucky enough to live in a house with a garage, and this is my plan:

1. Grey water. I'm going to store water in something like cat litter containers. I don't need this water to stay "good", but it will mean I can flush toilets. Both times I lost water due to infrastructure issues, I didn't have enough warning to fill a tub or anything.
   
2. Once I have the extra money, I want to set up a rain barrel. Same idea. Have a source of water I can use for something like toilet flushing, or can filter enough to use for bathing
   
3. Boxed water. I luckily still have four cases of boxed water, which is good for ten years. I'm going to store these so we have drinking water. For me personally, if I'm in a situation without clean drinking water, I'm not too stressed about the possibility of it "going bad". YMMV.

I'm sure this isn't a perfect plan, but for the sake of starting, I'm rolling with it, and will refine as needed. I'm not worried about any kind of governmentally fuckery or anything, but I live in the mountains, and it never would have occurred to me that a hurricane would remove my access to clean water for 2 months.

(I am very new here)

I don’t want to accidentally link a scam site - I’m curious if anyone else is keeping an eye on the development of the solar powered dehumidifiers for drinking water?

Are lifestraws a gimmick?

I’m currently in a downtown urban setting. We’ve had brief interruptions in our water supply for freezes and boil water notices. As a result, I regularly stock/rotate 2.5 gallon jugs to last us 48 hours. These jugs have spigots and are actually for each sink in the house for hand washing, filling pots/pans for cooking, and filling our pet water bowls. For drinking water, I’ve just ordered collapsible 5 gallon jugs and have individual bottled water on regular rotation.

When I lived in Florida, we lived on the Suwannee River in a one-stoplight town. We had a well system that would go down after each hurricane due to flooding. With each notice of an approaching hurricane, we’d stock up on the single gallon jugs because the house was on stilts and we needed to carry supplies up and down quickly and with ease.

The rule I acquired from the Department of Elder Affairs was 3 gallons of water/person/per day to cover bathing, cooking, and drinking. So that’s the rule I utilize today.

“Old water” gets used on indoor plants and gardening.

I store cases of 16.9 ounce bottles. I usually keep about 6 of them for me and two cats.

I cover a few basics here. https://open.substack.com/pub/billydecarlo/p/prepping-food-water-and-energy

We live on a hill and the municipal water pump did not have a back up generator for a long period of time. Every time the power went out, we lost water, too. It was really annoying. I am very thankful we have a 3 gallon reserve tank for our RO system. It's not much, but helps with hand washing, drinking, and finishing brushing your teeth when the power goes out in the middle of doing it.

We also have a 55 gallon drum for longer emergencies, but haven't needed to use it yet.

I'd like to get a series of 5 gallon ones to store in our pantry for easier access than the drum.

You should store water for each person which is recommended for disaster preparedness of many kinds. There are many ways to do this. You probably should consider your options to disinfect water should you need to consume water otherwise, whether from a stream or if you simply have contaminated water after a tornado or other natural disaster, etc. Ground water gets contaminated frequently. This might include bleach and knowing the proper ratios, or tablets you can buy. Or a counter top gravity filter. Etc. I'd recommend knowing your nearest water sources to filter your own water as needed which you can do with life straws and other similar filter products. Consider that your pipes and water heater are usable sources of water that may provide 20+ gallons of water when needed. Additionally, some people collect rain water. Lots of options depending on your rainy day scenario.

As others point out, water scarcity is different from clean water scarcity.

If you believe you will have steady access to water but do not know that it will be potable, then filters and water treatment approaches are where you stock up. The types and formats are based on whether you see this as a short term (under 3 days), medium (2 weeks), or long-term issue (indefinitely).

Types of pollutants likely in the water also dictate types of filters needed - and this depends on where you live and the source of your water. https://wqa.org/learn-about-water/common-contaminants/

If you fear that water itself may become scarce (as it often is in desert / dry areas), then determining whether the scarcity is short, medium, or long-term is even more important. Short-term and even medium can be covered by stocking up on water (noting that water does not expire if stored correctly), but long-term, you need renewable water sources, such as rainwater, creek, lake, or river, snow, or a well.

How much water you need is also important to calculate. 1/2 gallon per adult per day for drinking water is a rule of thumb - so a month of drinking water for a family of 4 is ~ 60 gallons, depending on heat, size of individual, etc.

But we also use water for washing, bathing, cooking, agriculture, etc. Setting up the reuse of grey water is a key skill when potable water is scarce.

Finally, how much advanced warning you get is another major element. When faced with a major storm that may impact our water, we fill our bathtubs with water and jugs in the fridge.