A 1 GB file at 100 Mbps takes 80 seconds in theory. Assuming 80% of that rate is available for file data, it takes 100 seconds. The file has not changed: you need to distinguish the units and decide which rate belongs in the formula. This guide covers duration, required bandwidth, and how much data fits a time window.
Mbps and MB/s: bits and bytes
A byte contains 8 bits. 100 Mbps = 100,000,000 bit/s = 12.5 MB/s. Lowercase b means bit, while uppercase B means byte. A rate of 100 MB/s corresponds to 800 Mbps. The calculator also labels megabytes per second as MBps.
MB and MiB: two size scales
| Unit | Bytes |
|---|---|
| 1 MB | 1,000,000 |
| 1 MiB | 1,048,576 |
| 1 GB | 1,000,000,000 |
| 1 GiB | 1,073,741,824 |
NIST distinguishes decimal SI prefixes from binary IEC prefixes. If an application reports an ambiguous “GB”, check the exact byte count in the file information. 1 GiB is about 7.37% larger than 1 GB; at the same rate, it takes proportionally longer to transfer.
Duration and efficiency formula
time [s] = size [B] × 8 / (rate [bit/s] × η), where η = efficiency [%] / 100. Substitute 0.8 for 80%. This model assumes a constant rate and efficiency throughout the transfer.
- Convert the size to bytes using the correct MB, MiB, GB, or GiB scale.
- Multiply by 8 to obtain bits.
- Convert the rate to bits per second and multiply it by efficiency.
- Divide the bit count by useful throughput. Divide seconds by 60 if you need minutes.
80% efficiency is an illustrative assumption, not a universal protocol overhead or a measurement of your connection. If the entered rate is already measured file-data throughput, use 100% to avoid reducing it twice.
Example 1: the same file at different efficiencies
1 GB × 8 = 8,000,000,000 bits. At 100 Mbps and 100%: 8,000,000,000 / 100,000,000 = 80 s. At 80%: 8,000,000,000 / 80,000,000 = 100 s. A 20% reduction in useful throughput increases duration by 25%, rather than 20%.
Example 2: 250 MB or 250 MiB at 20 Mbps
| Size | Duration at 100% | Duration at 80% |
|---|---|---|
| 250 MB | 100 s | 125 s |
| 250 MiB | 104.8576 s | 131.072 s |
The second row uses 250 × 1,048,576 × 8 = 2,097,152,000 bits in the numerator. This explains the duration difference caused solely by the size unit.
Required rate and transferable size
Transferring 1 GB in 60 s at 80% efficiency requires 8,000,000,000 / (60 × 0.8) ≈ 166,666,667 bit/s, or about 166.667 Mbps. In the other direction, 100 Mbps for 180 s at 80% transfers 100,000,000 × 180 × 0.8 / 8 = 1,800,000,000 B, or 1.8 GB.
Use the calculator and assess the result
Open the download time and bandwidth calculator, choose time, rate, or size, then select units and efficiency. For the 250 MiB example, choose MiB, 20 Mbps, and 80% efficiency: the result is 131.072 s. Display rounding may shorten the number but does not change the selected unit scale.
Actual transfers may vary over time with the server, disk, Wi-Fi, shared bandwidth, retransmissions, and connection setup. This calculator does not measure those effects or run a speed test. Compare the estimate with a known file transfer and choose assumptions that match your scenario.