Convert a data size between decimal and binary units — the reason a "1 TB" disk shows 931 GB — and work out how long it takes to move at the speed of your connection.
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There are two systems for counting bytes, and they use nearly the same names. In the decimal (SI) system a kilobyte is 1,000 bytes, a megabyte is a million and a gigabyte a billion. In the binary system each step is 1,024: a kibibyte (KiB) is 1,024 bytes, a mebibyte (MiB) 1,048,576 and a gibibyte (GiB) 1,073,741,824.
The gap grows with each prefix — 2.4% at kilo, 7.4% at giga, 10% at tera. Disk makers, network speeds and cloud bills use decimal units. Memory, and many operating systems when they report disk space, use binary ones while still writing "GB". So the 1 TB drive you bought holds 1,000,000,000,000 bytes, which Windows displays as 931 "GB". Nothing is missing; the units differ.
Powers of 1000. Used for disk and SSD capacity, network bandwidth, file sizes on macOS and most cloud pricing.
Powers of 1024. Used for RAM, Kubernetes memory limits (Mi, Gi), Linux tools and Windows Explorer.
Network speed is in bits per second (Mbps); file size is in bytes. Eight bits make a byte, so 100 Mbps moves at most 12.5 MB each second.
Mb is megabits, MB megabytes. An ISP's "500 Mb" and a download dialog's "62 MB/s" are the same speed.
Transfer time is size in bits divided by speed in bits per second, but the rated speed is a ceiling. Protocol overhead, encryption, other traffic on the link, disk speed at either end and distance all reduce it — which is what the "usable share" setting is for. For a backup or a migration, measure a small test transfer first and plan with the measured rate. And for very large data sets, compare the result with shipping disks: a few hundred terabytes over a 100 Mbps link takes months.
1,000 in decimal units (1 GB = 1,000 MB). In binary units, 1 GiB = 1,024 MiB. Operating systems often show binary values with decimal labels, which is the source of the confusion.
The drive holds 1,000,000,000,000 bytes, as labelled. The operating system divides by 1,024 three times and gets 931 — that is 931 GiB, displayed as "GB".
About 22 hours at the full rated speed, and closer to 25 hours on a real link with overhead. At 1 Gbps it takes a little over 2 hours.
Mbps is megabits per second and MB/s is megabytes per second. Divide Mbps by 8 to get MB/s: a 100 Mbps connection downloads at up to 12.5 MB/s.
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