aka Système Internationale is a decimal system of measures used nearly everywhere but in the USA. Even in the USA, it is used in medicine, scientific and engineering work, and for international trade.
You might wonder why the USA alone sticks with the old complicated Imperial system of measure. The Christian right lobby in the USA hates metric because it was invented by atheists, and the worst kind, French during the French revolution.
It is based on standard prefixes. e.g gram (the mass of about a sugar cube of water), kilogram (a 100 grams), milligram (one thousandth of a gram) or metre (a little longer than a yardstick), kilometre (1000 metres, about 5/8 mile), millimetre (one thousandth of a metre). I use both forms of measure on this website, since there are so many American visitors.
Because RAM (Random Access Memory) necessarily comes in modules sizes of even powers of two, the prefix scheme was modified for measuring bytes by even powers of two. Unfortunately, vendors use the binary scheme for measuring RAM capacity but the decimal scheme for measuring hard disk capacity. I suspect they do this primarily to inflate the apparent disk capacity by 7%, and because their competitors also do this, rather than as protest against the heretic binary RAM byte measuring prefix convention.
Metric Equivalents  

Unit  Metric  Imperial 
milligram  1 mg  0.02 gr 
gram  1 gram  15.43 gr 
kilogram  1 kg  2.20 lbs 
millimetre  1 mm  0.04 in 
centimetre  1 cm  0.39 in 
metre  1 metre  3.28 ft 
kilometre  1 km  0.62 mile 
millilitre  1 ml  0.20 US teaspoon 
litre  1 litre  1.06 US quarts 
metre/sec  1 metre/sec  2.24 mph 
You can improve your metric weight intuition by asking the post office to weigh a few objects for you.
SI prefixes  



prefix  American English  Standard Decimal Multiplier 
Power of 10  Byte Measure Multiplier 
Byte Measure Power of 2 

exa  quintillion (British trillion) 
1,000,000,000,000,000,000  10^{18}  1,152,921,504,606,846,976  2^{60} 
peta  quadrillion  1,000,000,000,000,000  10^{15}  1,125,899,906,842,624  2^{50} 
tera  trillion (British billion) 
1,000,000,000,000  10^{12}  1,099,511,627,776  2^{40} 
giga  billion  1,000,000,000  10^{9}  1,073,741,824  2^{30} 
mega  million  1,000,000  10^{6}  1,048,576  2^{20} 
kilo  thousand  1000  10^{3}  1024  2^{10} 
hecto  hundred  100  10^{2}  
deca  ten  10  10^{1}  
deci  tenth  1/10  10^{1}  
centi  hundredth  1/100  10^{2}  
milli  thousandth  1/1000  10^{3}  
micro  millionth  1/1,000,000  10^{6}  
nano  billionth  1/1,000,000,000  10^{9}  
pico  trillionth (British billionth) 
1/1,000,000,000,000  10^{12}  
femto  quadrillionth  1/1,000,000,000,000,000  10^{15}  
atto  quintillionth (British trillionth) 
1/1,000,000,000,000,000,000  10^{18} 
There are a series of Greek, Latin, Danish and Spanish prefixes used for specifying large numbers. The prefixes have different meaning when talking about RAM than when talking about disk space.
However, Windows measures hard disk capacity using the binary RAM definition.
For measuring RAM, a gigabyte is a little over a billion bytes, or more precisely, 1024 megabytes = 2^{30} bytes = 1024x1024x1024 bytes = 1,073,741,824 bytes. A byte is approximately one character of information.
A megabyte on disk is precisely a million, namely 10^{9} =1000x1000x1000 = 1,000,000 bytes. For RAM it is a little over a million bytes, namely 2^{20} = 1024 × 1024 = 1,048,576 bytes.
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