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I have 4 x 2gb (or gib?) memory sticks. I bought them in pairs, so they might be different.

$ free -m
Mem: 7918

$ free -g
Mem: 7

I was expecting 8192mb and 8gb.

Is it due to the difference between gb and gib? But I still don't get it, can anyone explain this?

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GB (gigabytes) and GiB (gibibytes) are very similar but GiB are slightly larger than GB. It's gigabits that is extremely different, and is usually written as "Gb" or "gb" (emphasis on the lowercase b; the case of the G varies). free definitely does not answer in gigabits, or it'd answer 64 gigabits (that's how many gigabits there are in 8 gigabytes). – allquixotic Oct 20 '12 at 0:31
up vote 2 down vote accepted

The -g switch displays the amount of memory in gigabytes (GiB), but rounded down. I notice you are displaying the totals, so unlike most of the other output of the free command, these are the sum of RAM that is (a) in use; (b) holding page cache; or (c) not in use at all.

The "totals" you are seeing are the amount of usable memory on the sticks. Memory is advertised at even powers of two, but almost never has every last byte as advertised.

Also, system memory that is reserved for hardware devices, such as an integrated graphics chip with a unified memory architecture, might be subtracted from this total.

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