Download Time Calculator

About the Download Time Calculator

Find out how long a download, upload or backup will take: enter the file size (bytes, kB, MB, GB, TB or the binary KiB, MiB, GiB, TiB) and the connection speed (kbit/s, Mbit/s, Gbit/s, or MB/s as a download manager shows it). Or choose Speed needed to find the connection speed that moves the file within a time you set.

Internet speeds are counted in bits and file sizes in bytes, and a byte is 8 bits, so 100 Mbit/s moves at most 12.5 MB of data a second. Every packet also carries headers: with TCP/IP over Ethernet and the usual 1500-byte packets, 5.07% of the line carries headers and framing rather than your file (RFC 6349). That is the starting overhead allowance, and you can change it to match what you see. The calculator uses the speed you enter — it does not measure your connection.

Common uses

• Checking whether a game, an update or a film will finish downloading before you need it.
• Planning a cloud backup or a large upload on a slower upload speed.
• Choosing an internet plan: the speed needed to move your files in the time you have.
• Estimating how long copying a disk image over the network will take.

The formula

time = size in bits ÷ (speed × (1 − overhead))

• size in bits = bytes × 8 (1 kB = 1,000 bytes, 1 MB = 1,000,000 bytes, 1 KiB = 1,024 bytes, 1 MiB = 1,048,576 bytes)
• speed in bit/s: 1 kbit/s = 1,000 bit/s, 1 Mbit/s = 1,000,000 bit/s, 1 MB/s = 8,000,000 bit/s
• speed needed = size in bits ÷ time ÷ (1 − overhead)

For example, 4.7 GB is 37,600,000,000 bits. At 100 Mbit/s with 5.07% overhead the file moves at 94.93 Mbit/s, so it takes 37,600 megabits ÷ 94.93 Mbit/s = 396 seconds, or 6 min 36 s.

Bits, bytes, MB and MiB

Internet plans and network equipment count bits per second (Mbit/s, often written Mbps); file sizes are in bytes (MB, GB). Divide Mbit/s by 8 to get MB/s: 100 Mbit/s is 12.5 MB/s at most.

Storage makers use decimal units — 1 GB is 1,000,000,000 bytes — while some software counts in binary units, where 1 GiB is 1,073,741,824 bytes, sometimes still labelled GB. If your file size comes from such a program, pick GiB, MiB or TiB: a 4.7 GB DVD is about 4.38 GiB. The binary prefixes kibi, mebi, gibi and tebi are defined in IEC 80000-13 (NIST: prefixes for binary multiples).

Why a download is slower than the line speed

Each packet carries headers as well as your data. RFC 6349 works this out for Ethernet with the usual 1500-byte packets: every packet takes 1,538 bytes of line time (with the Ethernet header, checksum, gap and preamble) to carry 1,460 bytes of data after the IP and TCP headers. A 100 Mbit/s link therefore moves at most 94.9 Mbit/s of data — 5.07% overhead.

Wi-Fi, VPNs, a busy line, a slow or distant server and other devices sharing the connection take more; allow for them with a higher overhead. Speeds shown in MB/s by a browser or download manager are already the data rate, so enter them with 0% overhead.

Limitations

• The calculator does not measure your connection: the result is only as good as the speed you enter. Measure it with the internet speed test.
• It assumes the speed stays the same for the whole transfer. Real downloads speed up and slow down, and servers often limit the speed of each download.
• Compression, downloads in several parts at once and resumed downloads are not modelled.

Privacy

Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.