How this calculator works
This calculator converts a file size and a connection speed into an estimated download time. The one detail that trips up almost every manual calculation is that internet speeds are advertised in bits per second (Mbps, Gbps) while file sizes are measured in bytes — and there are 8 bits in a byte. Get that division wrong and every estimate is off by a factor of 8.
File sizes here use the decimal convention (1 GB = 1,000,000,000 bytes), the same convention internet service providers use for speed and the one operating systems increasingly use for network transfers. A 'MB/s' speed is already in bytes per second and needs no conversion.
The formula
Time (seconds) = file size in bytes ÷ speed in bytes per secondBits-to-bytes conversion: speed in bytes/sec = speed in Mbps × 1,000,000 ÷ 8100 Mbps and 12.5 MB/s are the same speed. A common mistake is treating '100 Mbps' as '100 MB/s,' which understates the true download time by 8×.
Worked example: a 1 GB file on a 100 Mbps connection
- Convert the speed to bytes per second: 100 Mbps ÷ 8 = 12.5 MB/s = 12,500,000 bytes/sec.
- Convert the file size to bytes: 1 GB = 1,000,000,000 bytes.
- Divide: 1,000,000,000 ÷ 12,500,000 = 80 seconds.
- 80 seconds is 1 minute 20 seconds — shown as 0:01:20.
Frequently asked questions
Why is my download slower than my advertised speed suggests?
Advertised speed is the theoretical maximum of the connection itself. Actual throughput is reduced by protocol overhead, Wi-Fi conditions, the server's own upload speed, and other traffic sharing the line. This calculator's 'realistic estimate' illustrates one commonly cited rule of thumb — about 80% of rated speed — but real results vary by network.
What is the difference between Mbps and MB/s?
Mbps is megabits per second, the unit ISPs advertise. MB/s is megabytes per second, the unit file managers and browsers show progress in. There are 8 bits per byte, so 100 Mbps = 12.5 MB/s.
Why use decimal GB (1,000,000,000 bytes) instead of GiB?
Network speeds and ISP billing use decimal (SI) units throughout. Some operating systems report file and disk sizes in binary GiB (1,073,741,824 bytes) instead, which is a different, larger unit — worth checking if your file manager's number does not match this calculator's input.
Does Wi-Fi affect the estimate?
Yes. Wired Ethernet generally gets closer to the rated speed than Wi-Fi, which loses throughput to signal conditions, distance from the router, and interference. Enter your actual measured speed (from a speed test) for a more accurate result than the plan's advertised number.
How long does a 10 GB file take to download on a 50 Mbps connection?
About 26 minutes 40 seconds. Converting 50 Mbps to bytes per second gives 50 × 1,000,000 ÷ 8 = 6,250,000 bytes/sec, and a 10 GB file is 10,000,000,000 bytes, so 10,000,000,000 ÷ 6,250,000 = 1,600 seconds, which works out to 26 minutes 40 seconds.