http://necrotic.deadbeast.net/xsf/XFree86/trunk/debian/local/FAQ.xhtml#settingdpi

On my AmiloA Laptop's LCD Screen : 15 inches 1024 * 768 = 786432 pixels ;( d=1280 / 15 = 85 dpi )

$ xdpyinfo
# dimensions:    1024x768 pixels (306x229 millimeters)
# resolution:    75x75 dots per inch

dimensions:    1024x768 pixels (271x203 millimeters)
resolution:    96x96 dots per inch

# file://etc/X11/XF86Config#Monitor # set correct dpi
DisplaySize 304 228 # d = 15 inches = 380 mm  # w = 0.8 * d # h = d * (3/5)

# DisplaySize 288 216 # d^2 = w^2 + ( (3/4)*w ) ^2 )

• (260×195 millimeters) resolution: 100×100 dots per inch
• (271×203 millimeters) resolution: 96×96 dots per inch

• http://www.mozilla.org/unix/dpi.html