How many GB is 581.724 MB

Simple answer: 0.581724 GB, since 1 GB = 103 MB, or 1 MB = 10-3 GB, so 581.724 MB gets its decimal point shifted three places left to be quoted in gigabytes.

The problem is whether you're talking about true gigabytes (109 bytes, which it appears you are) or "Gibibytes" 1073741824bytes.

See the "Gibibyte" entry in Wikipedia for a more complete explanation.