Please understand what I am asking here. I'm not asking how much RAM can theoretically be apportioned to a single VM in Hyper-V, that's 1TB. What I'm looking for is a figure or formula showing how much a given host with xGB of RAM can apportion to a single Virtual Machine via Hyper-V, since no hypervisor is able to give 100% of itself to any guest.

Are the requirements fixed – ie, does Hyper-V always require xMB and from there anything goes? Or is it always a percentage of the Host's RAM? How does video memory factor in? I don't mind if the results are a bit give-and-take – it'd be ridiculous to expect exact figures down to the megabyte – but any ballpark indication would be immensely useful.

I'm not concerned with multiple VMs at this time. Let's work from the assumption that this is the only device being virtualised.

  • 1
    On my 16GB RAM machine if I go to create a new VM then it says I can allocate up to 251,658,240MB which I suspect is the virtual memory limit for my edition of Windows and more than my actual RAM. There is no real limit except usability. You don't want to allocate more memory than you can spare while leaving your host usable. If your host needs a minimum of 4GB to even function (Windows 10 does) and you need some space for the Hyper-V program and it's virtual components, then I would have a good rule of thumb that you should never leave your host with less than 8GB to itself.
    – Mokubai
    Jul 23 at 11:10
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    Windows can work in 4GB of RAM, but is somewhat less performant and relies heavily on the page file. If you only have 8GB of RAM then giving half to your VM might be acceptable to you. You can give the VM far more than that, but then everything will grind to a halt as your system tries to use the host page file for everything. What you can do and what you should do are two entirely different things and the latter depends entirely on your system and your requirements.
    – Mokubai
    Jul 23 at 11:18

The information is to be found in the article How Much Memory Does My Hyper-V Host Require?

The amount that Hyper-V reserves for the host, called host reserve, is calculated in the following table:

enter image description here

An example taken from Microsoft:

  • You have a host with 16 GB RAM
  • The Mangement OS uses 2 GB
  • The host reserve is up to 2.5 GB
  • That leaves you with 11.5 GB RAM for VMs.
  • Thanks harrymc. In my testing this table isn't close to correct, but it's Microsoft's official statistics, so that's good enough for me to direct people to if they get back strange results.
    – seagull
    Jul 23 at 13:44
  • Hyper-V algorithms change with time, so better use this table just as approximation.
    – harrymc
    Jul 23 at 13:47
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    @seagull that page is for Windows Server 2012 which will be a "streamlined" version of Windows 8. It will have significantly reduced memory footprint to a modern Windows 10 or Server 2019 based machine. Memory requirements have gone up significantly in almost 10 years.
    – Mokubai
    Jul 23 at 15:31
  • Thanks for sharing this. Nice to have that at hand even as an approximation.
    – Strepsils
    Jul 30 at 12:39

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