Two industries, two units
Networking evolved speaking bits (serial transmission, one bit at a time); storage evolved speaking bytes (eight bits, one character). Neither is wrong; together they produce the internet's most successful confusion — a "300 Mbps" plan that moves 37.5 MB/s, described accurately by both parties.
The one-line conversion
Divide megabits by 8 for megabytes. That is the entire conversion, plus a small correction: networking "M" is decimal (1,000,000) while file "M" is binary (1,048,576), so the honest factor is 8.19 — close enough that 8 is the working number and the calculator applies the precise one.
Where the confusion costs
Download estimates (8× wrong when naive), upload expectations (asymmetric plans confuse it further), and hardware shopping ("1 Gbps router" vs actual 110 MB/s NAS throughput). The defense is one habit: convert to MB/s before any comparison, and let the calculators here keep the units honest.