Calculate required or available network bandwidth from data size and time.
Network link speeds are quoted in bits per second (bps), but file sizes are quoted in bytes — and the two are not the same unit. The fundamental conversion is 1 byte = 8 bits. Additionally, there are two size systems: SI (decimal, 1 MB = 1,000,000 B, used by hard drives and ISPs) and IEC (binary, 1 MiB = 1,048,576 B, used by operating systems). This calculator handles both, applies an optional protocol-overhead deduction for TCP/IP headers and retransmissions, then computes the raw transfer time and a theoretical monthly capacity at full utilisation.
Transfer time
Transfer time (s) = File size (bytes) × 8 ÷ (Link speed (bps) × (1 − overhead%))
Monthly throughput at full utilisation
Monthly bytes = Effective bps × 2,592,000 ÷ 8
ISPs advertise speeds in megabits per second (Mbps), not megabytes. Your 100 Mbps connection transfers a maximum of 12.5 MB/s (100 ÷ 8). Beyond that, protocol overhead (TCP headers, retransmissions), server throttling, shared contention and Wi-Fi half-duplex losses all reduce real throughput. The overhead field lets you model a realistic figure.
MB (megabyte) follows the SI decimal standard: 1 MB = 1,000,000 bytes. MiB (mebibyte) follows the IEC binary standard: 1 MiB = 1,048,576 bytes. Hard drives and ISPs use decimal; operating systems typically use binary. A 500 GB hard drive is about 465 GiB as reported by Windows.
Because almost all consumer speed benchmarks (Speedtest, ISP plans) display megabits, but file sizes are in megabytes. Forgetting the factor of 8 is the single most common mistake when estimating download times.
TCP/IP adds header bytes to every packet (typically 40–60 bytes per 1,460-byte payload), and retransmissions add extra cost on lossy links. A 5–10% overhead is realistic for most Internet traffic; 0% gives the theoretical maximum the wire can carry.