Data transfer formula
Transfer time equals the total amount of data divided by the rate at which that data is transferred.
Keep units consistent
The most common mistake is dividing a value measured in bytes by a speed measured in bits per second without converting one of them first.
Example with gigabytes and megabits
A decimal 10 GB transfer contains 80 gigabits of data. At a theoretical 100 Mbps transfer rate, the idealized transfer time is approximately 800 seconds, or about 13 minutes and 20 seconds.
Bandwidth vs throughput
Bandwidth describes potential data-carrying capacity. Throughput describes the rate actually achieved. Real transfer time should therefore be based on effective throughput when that value is available.
What can reduce transfer speed?
- Network congestion
- Protocol overhead
- Wi-Fi interference
- Server or cloud-service limits
- Disk read and write performance
- Encryption or processing overhead
- Shared network usage
Uploads and downloads
Many internet connections have different upload and download speeds. Use the relevant direction's speed when estimating how long a transfer will take.
Large transfer planning
For backups, migrations and large datasets, it is useful to calculate both the theoretical time and a more conservative estimate using a lower expected throughput.
A mathematical transfer estimate assumes a sustained rate. Actual throughput can vary throughout a transfer.