I'm seeing it over and over again, people just don't know the difference between certain technical units.
So here's a short explanation of this stuff.
Bits and Bytes
1 bit = a binary digit (just a zero or a one)
1 byte = how many bits it takes to form a character (depends on encoding but in today's use it's mainly 8 bits)
1kb = 1 kilobit = 1'000 bits (yes, not 1'024!)
1kB = 1 kilobyte = 1'000 bytes = 8'000 bits.
So if e.g. your ISP is promising 50Mbit/s this means:
50Mbit/s (megabits per second) ≠ 50MB/s (megabytes per second).
50Mbit/s = (50/8)MB/s = 6.25MB/s
Kilo and Kibi
1 kilo = 1'000 (like in kilogram, kilometer,...) [normal SI-unit]
1 kibi = 2^10 = 1'024. [IEC-unit]
Going further we have
1 mega = 1 million vs. 1 mebi = 2^20
1 giga = 1 billion vs. 1 gibi = 2^30
1 tera = 1 trillion vs. 1 tebi = 2^40
Even though byte is not one of the SI-units you should strictly use 1'000 for kilo.
And just because everybody’s doing it wrong it doesn’t mean you have to do so too :)
DPI and PPI
DPI [dots per (square-)inch] is mainly used in print, because (inkjet) printers spray dots in irregular distances onto the paper and DPI means how many dots the printer could spray in one square-inch.
PPI [pixels per (square-)inch] is used everywhere you have a screen, because on a screen the pixels do have a fixed and regular position and distance.
If you use DPI with screens you would have to count all of the subpixels too.
On a normal RGB-style screen this would mean that DPI = 3 * PPI.
written by: Takashi Yoshi