Bandwidth (Download Time) Calculator

How This Calculator Works

Download time is simply file size divided by connection speed — but the two are usually quoted in different, easy-to-confuse units. File sizes are conventionally measured in binary kilobytes/megabytes/gigabytes (1 GB = 1,024 MB, matching how your computer reports file sizes), while internet connection speeds are quoted in decimal megabits/gigabits per second (1 Mbps = 1,000,000 bits per second — the actual industry convention ISPs use). This calculator handles both conventions correctly so you don’t have to convert by hand.

The Formula

$$\text{Transfer Time} = \frac{\vA{\text{File Size (bytes)}}}{\vB{\text{Connection Speed (bytes/second)}}}$$

Since connection speeds are usually quoted in bits per second, not bytes, converting requires dividing by 8 (there are 8 bits in a byte): $\text{Speed (bytes/s)} = \text{Speed (bits/s)} \div 8$.

Worked Example

Downloading a 1 GB file over a 100 Mbps connection:

1. File size in bytes: $1 \text{ GB} = 1{,}073{,}741{,}824 \text{ bytes}$ (binary gigabyte).
2. Speed in bytes per second: $100{,}000{,}000 \text{ bits/s} \div 8 = 12{,}500{,}000 \text{ bytes/s}$ (decimal megabit).
3. Transfer time: $1{,}073{,}741{,}824 \div 12{,}500{,}000 \approx 85.9 \text{ seconds}$ — about 1 minute, 26 seconds.

Real-world transfer speeds are usually somewhat lower than the connection’s rated speed due to overhead, network conditions, and server-side limits, so treat this as a best-case estimate.