Mbps vs MB/s — why the numbers differ

Bits and bytes

The lowercase b in Mbps means bits. The uppercase B in MB/s means bytes. One byte contains eight bits, so a rate of 100 Mbps corresponds to 12.5 MB/s before overhead. The two values express the same theoretical rate, not a loss of seven eighths of the connection.

Decimal and binary units

Mbps means one million bits per second, and Gbps means one billion. MB and GB use decimal multipliers. MiB and GiB use binary multipliers: one MiB is 1,048,576 bytes. A file-size display that uses binary units can therefore produce a different number from a decimal display of the same file.

The NIST guide to binary prefixes documents the distinction. Our speed unit converter keeps MB/s and MiB/s separate.

Estimate a file transfer

Divide the file’s size in bits by the connection’s rate in bits per second. A decimal 1 GB file at 100 Mbps takes 80 seconds in an ideal calculation. Use the transfer calculator to avoid mixing units.

Why a real download can take longer

The application shares available capacity with protocol traffic and other activity. The source server, Wi-Fi, storage and service limits can reduce the observed file-transfer rate. Compare units first, then investigate a persistent gap under controlled conditions.