Pixels, PPI, DPI, and LPI
for Scanning, Printing,
and Web Publishing
A pixel on your computer monitor is
a point of light
What is the maximum resolution of
Probably 1024 X 768
How many pixels per inch is this?
~ 91 ppi
What is standard for computer
A pixel in an image file is a piece of data.
It could be represented in 1 bit, 1 byte, or
How many colors can you represent if a
• 21 = 2
• 28 = 256
• 224 = ~16 million
Resolution of your computer screen
is measured in pixels per inch (ppi).
Resolution of an image file is
measured in pixels per inch.
What’s the relationship?
There’s a one-to-one relationship between pixels in the
image file and pixels displayed on the computer screen.
However, this does NOT mean that the picture, when it
is printed out, will be the same size as the picture
displayed on the computer screen.
Resolution of a scanner is measured in
dots per inch (dpi). This is essentially the
same as pixels per inch (ppi) in the sense
that it is pieces of information per inch.
If you scan at a resolution of 72 dpi (or
ppi) you’re getting 72 pixels of
information about the picture in each
direction for each inch of the picture.
(5184 pixels per inch2)
The resolution of a printer is measured in
dots per inch (dpi).
Each dot is a spot of color output by the
The dot is the smallest dot of color the
Your printer is probably 300 dpi, 600 dpi,
or 1200 dpi. Note that this is really dots
per inch in both the x and y directions.
The dot of ink sprayed out by an inkjet
printer is wet. One dot blends into
In general, when you’re scanning a
picture to be printed out on an inkjet
printer, a resolution of no more than 1/3
the resolution of the printer is sufficient.
Thus, for a 600 dpi inkjet printer, 200 dpi
resolution should be enough.
Laser Printers, Halftone
Dots, and LPI
A dot is the smallest printable unit
for a laser printer.
A line is created from a group of
dots. The line is a series of half-
tone dots. (p. 59, Avoiding the Scanning Blues, p. 87, The Non-
Designer’s Scan and Print Book.)
Thus, laser printers are
characterized by the lpi at which
they print as well as the dpi.
Each line printed out
by a laser printer is a
series of half-tone
dots. For a grayscale
picture, a half-tone
dot looks more or less
gray depending on
how many primitive
dots it is composed of.
From p. 59, Avoiding the Scanning
Blues by Taz Tally, Prentice-Hall
Laser vs. Inkjet Printers
Laser printers use AM screening
(amplitude modulation). That is, the size
of the halftone dot changes to change the
Inkjet printers use FM screening
(frequency modulation). That is, all dots
are the same size, but the number of dots
in the picture changes.
(p. 44, Avoiding the Scanning Blues)
Common DPI and LPI for
300 dpi 60 lpi (5 pixels2 per
600 dpi 85 lpi (~7 pixels2
per halftone dot)
1200 dpi 110 lpi (~11 pixels2
per halftone dot)
Resolution When Printing
From a Laser Printer
Experience shows that a good rule of
thumb is to scan at a resolution that
is 1.5 to 2 times the lpi of the laser
printer you’ll be printing on.
e.g., for a 600 dpi laser printer, scan
your picture at about 130-170 ppi