Bandwidth Calculator
Estimate file transfer/download time from file size and connection speed
Frequently Asked Questions
Estimating file transfer or download time from a connection speed involves one detail that trips up almost everyone doing the math by hand: internet speeds are quoted in bits per second (Mbps), while file sizes are quoted in bytes (MB or GB) — and a bit is one-eighth of a byte, so skipping that unit conversion produces an estimate that's off by a factor of eight.
Why Mbps and MB look similar but aren't
Internet service providers advertise connection speed in megabits per second (Mbps) — lowercase "b" for bits. File sizes, meanwhile, are almost universally displayed in megabytes (MB) — uppercase "B" for bytes. Since one byte equals eight bits, a "100 Mbps" connection transfers roughly 12.5 megabytes per second at that connection's theoretical maximum, not 100 megabytes per second — a distinction that's easy to miss given how visually similar "Mbps" and "MB" look, but that changes an estimated download time by a full factor of eight if conflated.
A worked example
A large file download estimated using the connection's Mbps figure directly as if it were MB/s will predict a download roughly eight times faster than what will actually happen — this is exactly the kind of error that makes a "why is my download taking so long" moment feel like a broken connection, when in fact the connection is behaving normally and the estimate itself was wrong.
How this connects to your other technical calculations
Real-world transfer speed is also typically lower than a connection's theoretical maximum, due to network overhead, contention from other devices on the same connection, and server-side limits — this calculator provides a theoretical-maximum estimate, and actual transfer time in practice is often somewhat longer. For converting between other technical units entirely, the Unit Converter handles a broader range of measurement conversions beyond data transfer specifically.
Common mistakes
The single most common mistake is treating a connection's Mbps rating as if it were the file size unit (MB), skipping the divide-by-eight bits-to-bytes conversion entirely — always convert connection speed and file size to the same unit basis (both in bits, or both in bytes) before dividing one by the other to estimate transfer time.