What this answers
File sizes are in bytes and internet lines are sold in bits, so the sum is never quite obvious. The calculator takes the size and the speed, takes off the share of the line that never becomes file, and gives the time. Switch what it works out and it gives the speed you would need for a target time, or the size that fits in a given time.
Formula
Time = size × 8 / (speed × (1 − overhead / 100))
Speed = size × 8 / (time × (1 − overhead / 100))
Size = time × speed × (1 − overhead / 100) / 8
- size — in megabytes, which is 10⁶ bytes; time in seconds; speed in Mbit/s
- a byte is 8 bits
Worked examples
1 GB over 100 Mbit/s, no overhead. 1 GB is 8,000 Mbit, and 8,000 / 100 = 80 s, 1 min 20 s.
The same with 10% overhead. 90 Mbit/s arrives: 8,000 / 90 = 88.9 s, 1 min 29 s.
The speed needed. 1 GB in 80 s with no overhead needs 100 Mbit/s; with 20% overhead, 125 Mbit/s.
What this doesn’t cover
- Speed that changes during the download. The calculator uses one number for the whole time.
- The overhead of your own line. 10% is an example; compare with a real download to find yours.
Common questions
Why does 1 GiB take longer than 1 GB?
A gibibyte is 1,024³ bytes, a gigabyte 10⁹. The GiB is 1.0737 times as large, and takes that much
longer. Windows often labels GiB as “GB”.