Random  Access Memory, usually shortened to “RAM” or simply “memory,” is one of  the most important parts of any computer. But how much do you need?  Current new PCs and similar devices range from around the two gigabyte  mark to 16GB or more.
How  much memory you really require will depend on two factors — how much  you want to do, and how much you’re willing to spend. This article will  focus on computers running a desktop operating systems like Windows, Mac  OS X, Linux, or Chrome OS.
An introduction to RAM
Memory capacity is often confused with the long-term storage offered by a solid state or mechanical hard drive.  Sometimes even manufacturers or retailers will mix up these terms. A  desk is a useful analogy to consider the difference between memory and  storage. Think of RAM as the top of the desk. The bigger it is, the more  papers you can spread out and read at once. Hard drives are more like  the drawers underneath the desk, capable of storing papers you’re not  using.
The  more RAM your system has, the more programs it can handle  simultaneously. RAM isn’t the only determining factor, and you can  technically open dozens of programs at once even with a very small  amount of RAM, but doing so will slow your system down. Think of the  desk again. If you have far too many papers on it, it becomes cluttered,  and your work will slow as you try to find whatever paper you need at a  particular moment. You’ll be forced to frequently dig into the drawers  to store what won’t fit on top of the desk and retrieve papers you need.
A computer with more RAM might feel like  it’s performing faster, especially when you use many programs at once,  but more memory doesn’t actually increase its processing speed. Only a  faster CPU can do that. More RAM won’t increase the amount of files or  programs your computer can hold, either. That’s what the desk drawers —  the hard drive or solid state drive – are for.
Standard RAM shouldn’t be confused with video memory, a statistic associated with computer video cards.  High-end 3D games rely on video RAM, usually expressed as “GDDR3” or  similar, whereas standard memory will simply be referred to as memory,  RAM or DDR3/4.
RAM-heavy applications
The biggest RAM-hogs on most home computers are the operating system itself and the web browser.  There’s not much you can do to make Windows or OS X use less memory, but  more RAM in your computer means that you can have more browser tabs open in Chrome, Firefox, Internet Explorer,  et cetera. In addition, some websites will use more RAM than others. A  simple text news story is relatively light on memory, while something  like Gmail or Netflix uses quite a lot.

(Bill Roberson/Digital Trends)
Other  programs tend to use more RAM as they increase in complexity. A chat  program or a game like Minesweeper will use almost no RAM, while a  gigantic Excel spreadsheet or a huge Photoshop project might use more  than a gigabyte all by itself. Modern 3D games also use quite a lot of  RAM — often three or four gigabytes, and some use a bit more than that.
That  means your need for RAM is entirely dependent on the programs you use.  Let’s say you have two Windows computers, one with two gigabytes of RAM,  and one with sixteen, that are otherwise identical. If you have no  programs open they’ll operate similarily. Load a big Excel spreadsheet,  though, and only the latter will still seem responsive.
Choosing RAM for PC tablets
Most  Windows tablets will come with somewhere between two and four gigabytes  of RAM. This is enough for basic tasks only. A few browser tabs, some  email, and one video at a time is fine, but heavy multitasking will  quickly become frustrating on most tablets. Because basic PC tablets are  intended for these specific light-duty tasks, they’re usually fine  (though not exactly quick) with two to four gigabytes.
A  few more expensive Windows tablets (usually ones that use Core i3  processors or better) are more similar to laptops without the keyboard.  In these machines, try to get as much RAM as your budget will allow. For  example, Microsoft’s Surface Pro series comes with up to 8GB. If you intend to use your tablet for  occasional work or intense web browsing sessions, it might be worth the  upgrade.
Choosing RAM for laptops
New  laptops start at two gigabytes of RAM (especially for budget machines  and Chromebooks) and go up to eight gigabytes, with some very expensive  models offering 12 or 16GB. Systems towards the bottom of this scale are  suited only to general web browsing, email, video, and perhaps some  casual gaming.
Four  gigabytes is about as low as you want to go in Windows or OS X. This is  an ideal combination of capability and price for most people. If you  intend to run dozens of browser tabs at once, or you plan to do more  intensive tasks like high-resolution photo editing, consider a bump up  to eight gigs.
An  upgrade from 4GB to 8GB usually costs about $100 from the manufacturer,  but can be more expensive if it’s paired with a faster processor or  bigger storage drive. 12GB or more is ideal for powerful laptops often  referred to as “desktop replacements,” and is usually only available in  the most expensive models.
Choosing RAM for desktops
Small  desktop PCs and inexpensive all-in-one models actually have more in  common with laptops than typical “tower” PCs, and might come with as  little as two gigabytes of memory. Again, four gigabytes is about as low  as you want to go for a traditional computer.
RAM  for desktops is less expensive than RAM for tablets or laptops, so it’s  often easier to find computers with more memory at lower prices. Eight  gigabytes is a comfortable middle ground for desktops. If you’re  planning on using it as your primary work machine, especially for  intense programs or dozens of browser tabs, Office documents, and  similar uses, consider upgrading to 12 or 16GB.

For  the tip top of the desktop world, the sky’s the limit. The most  expensive desktops can handle huge amounts of RAM, up to 64GB or more.  Most users won’t be able to effectively use more than 16GB even if they  try, but high-end programs like 3D modeling or 4K video editing may  benefit from these pricey upgrades.
Conclusion
In  summary, two gigabytes should be considered the bare minimum, and is  only suited for entry-level specialty tablets and laptops, like  Chromebooks. Four gigabytes is the comfortable minimum for a Windows or  OS X machine, and eight gigabytes provides room to grow. Anything beyond  that is only of interest to enthusiasts and power users.
HTHC Notes:  I disagree with some of the points here. To run Windows 8.1 or Mac OS Yosemite, less than 8GB's of memory ensures a completely useless system. When pricing a computer, remember that upgrading a 4GB system (2x 2GB ram sticks) means discarding this memory and replacing it with 8GB's (2x 4GB's ram sticks). Price the memory online making sure the memory you are looking at will work in your system. If it is an Apple computer, ensure that the memory is Apple compatible. 
Tablets with less than 16GB's of storage will become full before you know it. Look for tablets that allow you to add more storage by inserting a micro SDD. Cheaper tablets and iPads do not have this feature. Most Android tablets have this ability.
I have the same advice for Smartphones. The entry level Smartphone with just 8GB's will fill rapidly. Smartphones with the ability to add a micro SDD are preferred. iPhones do not have this feature. Samsung Galaxy S5's do. The recently released Samsung Galaxy S does not.
Are advice on what type of computer, Smartphone or tablet to buy is free. 
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