Convert the file size to bits, divide it by the connection speed in bits per second, then convert the resulting seconds into minutes or hours.
Key takeaways
- File size and connection speed must use compatible bit or byte units.
- A 10 GB file at a sustained 100 Mbps takes an ideal 13 minutes 20 seconds.
- For planning, use a realistic sustained speed rather than the highest advertised rate.
The download-time formula
The core formula is time equals data divided by rate. When a file is measured in bytes and speed is measured in bits per second, multiply the file size by eight before dividing.
- Time (seconds) = file size (bits) ÷ speed (bits per second)
- File size (bits) = file size (bytes) × 8
Worked example: 10 GB at 100 Mbps
Using decimal units, 10 GB is 10,000,000,000 bytes. Multiplying by eight gives 80,000,000,000 bits. Dividing by 100,000,000 bits per second gives 800 seconds.
Eight hundred seconds is 13 minutes 20 seconds. That is the ideal result if the connection holds 100 Mbps for the entire transfer.
| Speed | Ideal time for 10 GB |
|---|---|
| 25 Mbps | 53 min 20 sec |
| 50 Mbps | 26 min 40 sec |
| 100 Mbps | 13 min 20 sec |
| 500 Mbps | 2 min 40 sec |
| 1 Gbps | 1 min 20 sec |
Choose the right speed input
For an internet download, use the sustained speed available to that device. For a cloud upload, use upload speed. For a LAN or storage transfer, use the slowest sustained rate in the path.
A speed test can be useful, but a single peak result is not always representative of a long download. Testing more than once can reveal how stable the connection is.
What makes a download take longer?
The formula gives an ideal baseline. Actual duration grows whenever another part of the path cannot maintain the entered rate.
- Wi-Fi interference or weak signal
- Busy or rate-limited source servers
- Other traffic sharing the connection
- Protocol and encryption overhead
- Slow storage or many small files
- Retries caused by packet loss
Use the numbers
Related calculators
Common questions
Questions, answered.
Should I use Mbps or MB/s?+
Use whichever unit matches your known speed, then make sure the calculator is set to the same unit.
Is file size measured in decimal or binary units?+
NetCalcKit uses decimal MB, GB, and TB on transfer calculators. Some operating systems may display binary units differently.
Can the same formula calculate upload time?+
Yes. Replace download speed with the sustained upload speed.