DPI display settings

The DPI setting of a display monitor is a measure of the number of dots that are visible in a single inch (Dots-per-Inch or Pixels-per-inch).

DPI is not the same as resolution. (Resolution is the number of horizontal and vertical pixels. For example, 1024 x 768 pixels.) An image with 100 x 100 pixel resolution would be 1 x 1 inch when printed at 100 DPI.

When a display DPI is chosen in Microsoft Windows, you are telling the system how the attached monitor will display images, Windows will then rescale to take this into account.

For example changing from 96 DPI to 120 DPI, Windows will assume that you now have a display with closely packed (dense) pixels that make everything 20% smaller, so to compensate will scale everything to be 25% larger.

The changes made by adjusting DPI are more subtle than changing screen resolution, but setting DPI correctly can be important for preventing eye strain when using high pixel density displays.

The ideal* DPI for a display can be calculated using pythagoras as follows:

Given:
W = Width in Pixels
H = Height in Pixels
S = Diagonal Size in Inches (display size)

Then the DPI = √(W2 + H2) / S
examples
An iPad with a 9.7" screen √(10242 + 7682) / 9.7= 132 DPI
An IBM Thinkpad with a 15" screen √(19202 + 12002) / 15.4= 147 DPI

* In practice you may select DPI based on individuals eyesight/preference.
It is not advisable to set a DPI lower than 96 as the majority of Windows fonts are not hinted to display correctly at very small sizes.

Windows XP and Vista DPI settings:

96 to 120 DPI
Control Panel | Display | Settings | Advanced | General

Windows 7/2008 DPI settings:

96 to 500 DPI
Control Panel | Display | Set Custom Text Size (DPI)
or
Control Panel | Personalisation | Adjust Font Size (DPI)

For custom sizes, tick: “Use Windows XP style DPI Scaling” un-ticking this option will turn on full scaling of all non-DPI aware applications, which may cause fonts to become blurry.

Citrix and Terminal servers

Setting the DPI for individual users connected to a terminal server (or Citrix) can only be done in the registry, if the DPI is changed in the Control Panel as above, then the change will apply to all users.

Registry settings for Per-User DPI setting (Windows 2003/2008):

Small 96 DPI
Medium 120 DPI
Large 144 DPI
Default 96 DPI (includes AppliedDPI value)

“There are three classes of people: those who see. Those who see when they are shown. Those who do not see” ~ Leonardo da Vinci

Related

Q2726399 - Cannot change the DPI setting through an RDP session.
REGEDIT - Import or export registry settings.
DPI ruler
DPI Calculator & list of common display sizes.
Detect your monitor DPI


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