Mac and Windows report space differently by using different units of measure.
On Mac, space is calculated using decimal/Base 10, where 1 GB=1,000,000,000B (so 480,000,000,000/1,000,000,000=480GB).
In Windows, space is calculated using binary/Base 2, where 1GB=1,073,741,824B (so 480,000,000,000/1,073,741,824=447GB).
An useful analogy would be considering distance in miles vs. kilometers; the distance traveled is the same, but the number used to represent that distance is different.
Base 2 vs Base 10
Three historical systems are commonly used, and sometimes confused, for measuring sizes and performance in computers. The SI Decimal system uses prefixes like 'kilo' for 1,000, or 'mega' for 1,000,000 this is a base-10 system that is most commonly used in metric measurements (kilometer, milimeter) etc.
The International Electrotechnical Commission (IEC) uses base-2 math with similar prefixes 'kibi' for kilobinary (2^10), 'mebi' for megabinary (2^20), gibi for gigabinary (2^30), and tebi for terabinary (2^40).
JEDEC units use the same Base 2 as IEC but uses the same symbols as SI, excepting the k (kilo) is replaced by a K (kilo).
These units of measurement are often mixed by computer operating system and hardware manufacturers, in some cases these will use the same symbol, for example "KB" may be either Kilobyte or Kibibyte depending on the context.
| SI Unit | SI Value | IEC Unit | IEC Value | JEDEC Unit | JEDEC Value |
|---|---|---|---|---|---|
| kilo (k) | 1000 (103) | kibi (ki) | 1024 (210) | kilo (K) | 1024 (210) |
| mega (M) | 1,000,000 (106) | mebi (Mi) | 1,048,576 (220) | mega (M) | 1,048,576 (220) |
| giga (G) | 1,000,000,000 (109) | gibi (Gi) | 1,073,741,824 (230) | giga (G) | 1,073,741,824 (230) |
| tera (T) | 1,000,000,000,000 (1012) | tebi (Ti) | 1,099,511,627,776 (240) | tera (T) | 1,099,511,627,776 (240) |
This wasn't a huge issues for early computers where storage and RAM were both measured in bytes and kibibytes, but as capacities have gotten larger the difference between base 2 and base 10 math has created some noticeable differences in representing the same physical value.
If you have any questions please feel free to contact us at support@plugable.com and we'll be happy to help!
