# HDD's - the Advertized size vs the Actual size.

1. ## HDD's - the Advertized size vs the Actual size.

If you wonder why your 500 Gbyte disk only reports 465 Gbyte in your system, read on.
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A drive that is advertised as a 500 Gbyte actually contains 500.000.000.000 bytes
The manufacturers just divides it three times by 1000 to show the value in Gbytes, but that's not accurate.
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1 Gbyte = 1024 Mbyte,
1 Mbyte = 1024 Kbyte
1 Kbyte = 1024 byte
To calculate the actual size we need to divide it three times by 1024.
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500.000.000.000 byte / 1024 = 488.281.250 Kbyte

. . 488.281.250 Kbyte / 1024 = . . . 476.837 Mbyte

. . . . .476.837 Mbyte / 1024 = . . . . . . 465 Gbyte
The actual size of the disk is not 500 Gb, but 465 Gb
With a 1TB disk it's the same thing, because 1 Tbyte = 1024 Gbyte
A 1Tb disk is actually only 1000.000.000.000 byte, which equals 0,9 Tb (931 Gb)
You actually get 69 Gb less than you thought you paid for.
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An alternative method to calculate the real size is to use ratios: thanks to Airbot
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For Gbyte the ratio is 0.9313226

500 Gbyte multiplied by 0.9313226 = 465.6613 Gbyte

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For Tbyte the ratio is 0.9094947

2 Tbyte multiplied by 0.9094947 = 1.8189894 Tbyte or 1,819 Gbyte

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2. Excellent explanation for non informed people.
+reps
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3. mmmm, from looking at this, you get 1extra gig if you buy a TB disk vs. two 500's ?
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4. myzr7 said:
mmmm, from looking at this, you get 1extra gig if you buy a TB disk vs. two 500's ?
Sorry myzr7, actually not.

The real right size of 465 gb = 456,60
If I applied strict rounding, it would be 466 gb
But to not make it to confusing I rounded them the way Windows shows them.

Greetz
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5. Darn just lost that gig. thanks for clearing that up. As said before very good info.
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6. In case anyone is interested in why this difference exists, here's a bit of background.
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As used in science and measurement and as defined by the international standards organization ISO, the prefix "giga" means 10^9.

Computers have address registers that are binary objects, and thus can address quantities as powers of 2. So, in the early days, when "large" numbers like 1024 came into common use, the prefix "kilo" was used because 1024 is "close enough" to 1000 (the "real" kilo) to "borrow" the prefix for talking about RAM.

There were objections even then to that use of the "kilo" prefix but the usage continued.

Time goes on and capacities increase. Now we talk about Kilobytes, Megabytes, Gigabytes, Terabytes, each time multiplying the previous number by 1024. Do that a few times and the difference between the decimal meaning of the prefix and the commonly used computer meaning of the prefix drifts further and further apart until you get a "gigabyte" that is 1,073,741,824 bytes.

But the non-computing world still uses these prefixes in their historical, decimal sense of 10^3, 10^6, 10^8, 10^12 etc. And the computing and non-computing worlds intersect.

One of those intersections is in description of disk capacity. Hard drive, CD, and DVD capacity are given using the decimal meaning of giga- etc. prefixes. Interestingly, flash cards, which are a kind of RAM, have capacities stated with decimal prefixes like hard drives.

Data transmission rates (such as ethernet speeds) and volumes are also stated using decimal meanings of prefixes.

It was proposed that the prefixes for computer/binary-derived counts be changed, e.g., using "gebi-" instead of "giga-". I've not seen that proposal in any use.

Guy
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7. I've always liked the reviews at NewEgg where the person says, "This drive is awesome, but it only formatted to xxx GB's". As if some are bigger and some are smaller.
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9. Nice job squonksc. I thought this looked familiar.

There's another way to calculate the advertised to actual though with the ratios you might want to add to your above post.

Just multiply the advertised size with the correct ratio.

(kilobyte Example - 500.000.000.000 byte multiplied by 0.9765625 = 488.28125
488.281.250 kbyte multiplied by 0.9765625 = 476.837158203125
476.837 Mbyte multiplied by 0.9765625 = 465.6611328125)

Kilobyte - 0.9765625

Megabyte - 0.9536743

Gigabyte - 0.9313226

Terabyte - 0.9094947

Petabyte - 0.8881784

So, let's say I have an advertised HDD size of 500GB, and want to find the actual size that you'll see..

500GB multiplied by 0.9313226 = 465.6613
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10. Thanks Airbot,
I am doing some shopping for a external drive and your numbers will help me with my cost analysis.
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