Bandwidth Calculator
Calculate file transfer time given a file size and connection speed. Supports binary (KiB, MiB) and decimal (KB, MB) prefixes.
Assumes ~80% of the theoretical rate due to protocol overhead, handshakes, and contention.
Understanding Bandwidth and File Transfer Time
Network speeds are measured in bits per second, while file sizes are measured in bytes. One byte equals eight bits, so a 100 Mbps (megabit) connection delivers a maximum of 12.5 MBps (megabytes) per second. This bits-versus-bytes distinction is the single most common source of confusion when estimating transfer times, and ISPs advertising in megabits makes the gap feel even wider than the 8:1 ratio suggests.
Real-world throughput is always lower than the advertised link speed. Every TCP segment carries headers for addressing, sequencing, and error checking. TLS encryption adds its own handshake and per-record overhead. Flow control and congestion avoidance algorithms throttle the sender to prevent packet loss. Taken together, these protocol costs typically consume 15-25% of the raw bandwidth, which is why this calculator applies an 80% efficiency factor for its realistic estimate.
File sizes add their own layer of ambiguity. The IEC 80000-13 standard, published in 1998, introduced binary prefixes (KiB, MiB, GiB, TiB) to distinguish powers of 1,024 from the SI decimal prefixes (KB, MB, GB, TB) based on powers of 1,000. A 1 TB drive holds 1,000,000,000,000 bytes by the manufacturer's decimal definition, but an operating system reporting in binary units displays it as roughly 931 GiB. This calculator supports both systems so you can match whichever convention your tools use.
Frequently Asked Questions
What is the difference between Mbps and MBps?
Mbps = megabits per second (network speeds). MBps = megabytes per second (file system speeds). 1 MBps = 8 Mbps. ISPs advertise in Mbps. A 100 Mbps connection transfers data at 12.5 MBps before overhead.
Why is my actual transfer speed slower than advertised?
Protocol overhead (TCP/IP headers, encryption handshakes, acknowledgements), network congestion, server-side throttling, and shared bandwidth. This calculator uses an 80% efficiency factor as a reasonable real-world estimate.
What is the difference between KB and KiB?
KB (kilobyte) = 1,000 bytes (decimal/SI). KiB (kibibyte) = 1,024 bytes (binary/IEC 80000-13). The IEC standard was introduced in 1998 to end the ambiguity. Storage vendors use decimal units; operating systems historically used binary, leading to the "missing space" confusion.
How long does it take to download 1 TB at 100 Mbps?
About 22.2 hours theoretical (1 TB = 8×10¹² bits / 10⁸ bps). Realistically 27-28 hours with overhead, assuming no interruptions. Gigabit connections reduce this to about 2.2 hours.
Does this calculator account for latency?
No. This calculator measures throughput-limited transfer time. Latency (ping) affects how quickly a connection starts and how TCP ramps up, but for large file transfers the total time is dominated by throughput, not latency.