CHANGE DPI IN UBUNTU

There is no option in Ubuntu with Unity to change the default DPI which is

considered to be 96 DPI (run in a terminalxrdb -query). But there are two

parameters which control the user interface DPI (this affects font rendering too)

and font rendering only.

The parameter that controls user interface and fonts DPI is scaling-factor and the

font rendering parameter is text-scaling factor. These parameters can be modified

by the Displays application, by GNOME Tweak Tool and by Unity Tweak Tool,

but none of these allows you to set a value with 3-4 decimals.

What's the scaling factor's value

The scaling factor is 1 for a 96 DPI. My display has a DPI of 103, so the scaling

factor is 103 / 96 = 1.072917. GNOME Tweak Tool rounded this value to 1.10 so

it's useless.

If you don't know what your display DPI is, simply measure its width in inches and

divide your horizontal resolution in pixels by the measured value. Another way of

getting the width and height of your display is by running xrandr and looking at the

connected monitor. Example output: DVI-I-2 connected primary 1920x1080+0+0

(normal left inverted right x axis y axis) 477mm x 268mm.

Dconf Editor allows you to set this value with 16 decimals. It can be installed by

running sudo apt-get install dconf-editor in a terminal. These parameters are found

in org.gnome.desktop.interface.

I didn't change the interface DPI (scaling-factor) as my DPI is not so high to make

the interface look too small. I only changed fonts DPI (text-scaling-factor) as I like

for the user interface the font Ubuntu size 8 which looked too small.

What are the effects

 If you open a LibreOffice Writer document at 100% size and overlay a sheet of

paper on the screen it should be exactly the same size.

 A font with the size equal to screen DPI will have a height of 1 inch on the

display.

 Usually (depends on program - e.g. qpdfview follows DPI, Evince doesn't)

documents displayed at 100% are the same size as if they were on paper.

Source: http://onetransistor.blogspot.in/2014/09/change-dpi-in-ubuntu.html