Why doesn't my Windows® PC recognize the whole 4GB of memory I installed?

Not only is there a maximum amount of memory that your system's motherboard can accept, there is also a maximum amount of memory that your operating system (OS) can accept. For instance, when you install 4GB of memory in a 32-bit Microsoft® Windows® OS, your system may report only 3GB or 3.5GB of available memory. If this occurs, don't worry. The memory you purchased and installed is fine. The problem revolves around how much memory the OS can address.

While the Windows OS allows for a maximum 4GB of installed physical memory, this does not equate to 4GB of availablememory. The reason? A portion of your system's memory (regardless of how much you have installed) is reserved to run devices, such as any graphics cards, PCI cards, integrated network connections, etc., meaning that certain amounts of installed memory may be unavailable for use as available memory.

Upon startup, your system calculates the amount of memory needed to run devices. If you've maxed out the amount of physically installed memory in your system, the amount of memory necessary to run devices will be deducted from your system's available memory, meaning that you won't be able to use all of your physical memory. However, if you haven't maxed out your physical memory, all of it will be available for use.

The maximum amount of memory that an OS will recognize varies by type and version of the operating system. In terms of differentiating between OS types, 32-bit operating systems are typically the lower-end consumer versions, while 64-bit operating systems are designed for high-use consumers and business users. Here's how to determine whether a computer is running a 32-bit version or 64-bit version of the Windows operating system.

Physical memory limits for recent Windows versions are as follows.

Windows 8

Windows 8 (32-bit)

Windows 8 (64-bit)

Standard: 4GB

Pro: 4GB

Enterprise: 4GB

Standard: 128GB

Pro: 512GB

Enterprise: 512GB

Windows Server

Windows Server 2012 (64-bit)

Windows Server 2008 (32-bit)

Storage Server Workgroup: 32GB

Foundation: 32GB

Essentials: 64GB

Standard: 4TB

Storage Server Standard: 4TB

Datacenter: 4TB

Hyper-V: 4TB





Standard: 4 GB

Web Server: 4 GB

Enterprise: 64 GB

Datacenter: 64 GB


Windows Server 2008 (64-bit)

Standard: 32 GB

Web Server: 32 GB

Small Business Server: 32GB

Enterprise: 1 TB

Datacenter: 1 TB

Note to Windows Server Users: Certain 32-bit Microsoft server operating systems can support over 4GB of memory via Physical Address Extension (PAE). Please refer to a Microsoft knowledge base article located here for more information.


Windows 7

Windows 7 (32-bit)

Windows 7 (64-bit)

Starter: 2GB

Home Basic: 4GB

Home Premium: 4GB

Professional: 4GB

Ultimate: 4GB

Enterprise: 4GB

Home Basic: 8GB

Home Premium: 16GB

Professional: 192GB

Ultimate: 192GB

Enterprise: 192GB

Windows Vista

Windows Vista (32-bit)

Windows Vista (64-bit)

Starter: 1 GB

Home Basic: 4 GB

Home Premium: 4 GB

Business: 4 GB

Enterprise: 4 GB

Ultimate: 4 GB

Home Basic: 8 GB

Home Premium: 16 GB

Business: 128 GB

Enterprise: 128 GB

Ultimate: 128 GB

Note to Windows Vista users: Microsoft addressed the installed memory limitation problem in Service Pack 1. If you have a system board that can handle more than 4GB of memory and a processor capable of handling x64 instructions and memory remapping, Vista SP1 can help. Because of Windows and the driver stacks, Windows loaded them into 'high' memory locations to avoid potential driver compatibility issues. (Meanwhile, the 32-bit versions of Windows Vista limit the total available memory to 3.12 GB.) VistaSP1 has other features to enhance your computing, so we recommend you add it, if you haven't done so already.


Windows XP

Windows XP (32-bit)

Windows XP (64-bit)

Starter Edition: 512 MB

Home: 4 GB

Professional: 4 GB

Home: 128GB

Professional: 128GB

Note to Windows XP Users: You can cosmetically correct the installed memory limitation problem by editing the Physical Address Extension settings.


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