The calculation
A worked example
A 10 GB file at 100 Mbps takes 13 minutes 20 seconds at 100% efficiency. At 85% efficiency, allow about 15 minutes 41 seconds.
Estimate a window, not a promise
Use the upload speed for sending a file and the download speed for receiving it. The largest number on an internet plan is often its download rate; it is not necessarily the rate available for an upload.
Efficiency represents the share of nominal speed available to the transfer before competing traffic. It is a planning assumption, not a measurement. If you already measured sustained application throughput, use that rate with 100% efficiency to avoid discounting it twice. Reserved traffic is then subtracted in Mbps.
What can stretch the estimate?
A remote service may rate-limit a transfer. Wi-Fi interference, a busy storage device, many small files, restarts and connection interruptions add time. Encryption, scanning and server-side processing are separate stages and may continue after all bytes have arrived. Leave a buffer for time-sensitive deliveries.
Source & further reading
NIST: decimal and binary prefixes
Model reviewed October 25, 2026. Read our methodology and limitations.