SD media card 64MB or really 59MB?

  astral traveller 09:10 11 May 03

I've just bought an SD media card for use with my camera. It is supposedly 64MB. When I checked in 'properties' it showed as 63,500,112 bytes or 60.5MB. I had already taken a couple of pictures so decided to format the card. After formatting it now shows as 62,554,624 bytes or 59.6MB. It clearly says 64MB on the card and the packaging. Does anyone know of a reasonable explanation for this or have I been short changed?

  jazzypop 10:51 11 May 03

I will venture a guess that two things have happened.

1. The 64MB refers to the unformatted capacity of the card. Formatting always carries an overhead (a loss of storage capacity).

2. The card manufacturer is using the value of 1000 bytes for a kilobyte (KB), and 1000 kilobytes for a megabyte (MB).

If you take 64 x 1000 x 1000, you end up with 64,000,000 bytes (manufacturer states as 64MB).

However, the operating system (Windows) uses the values of 1024 bytes = 1 KB, and 1024 KB = 1MB.

So, 64,000,000 divided by 1024 = 62,500 (KB)

62,500 / 1024 = 61.035 (MB)

Therefore, Windows always would have reported your '64MB' card as having a capacity of 61MB, before losing capacity to the formatting overhead.

Exactly the same thing happens with hard disks.

  astral traveller 14:20 11 May 03

Sounds like a good explanation to me. Thanks

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