Data Transfer Time Calculator
How long a download, upload or backup will take at a given speed — and why it is always slower than the eight-times-smaller number you expected.
File sizes are quoted in bytes and connection speeds in bits per second, and there are 8 bits in a byte — so a 100 Mbps line moves at most 12.
How the data transfer time calculator works
File sizes are quoted in bytes and connection speeds in bits per second, and there are 8 bits in a byte — so a 100 Mbps line moves at most 12.5 MB per second. The theoretical time is size ÷ speed; the realistic time adds an efficiency allowance for protocol overhead, congestion and the fact that few transfers ever reach the advertised rate. TCP overhead alone typically costs 3–8%.
Formula: seconds = (size in bytes × 8) ÷ (speed in bits per second)
Worked examples
| Inputs | Realistic transfer time | Note |
|---|---|---|
| 5 GB over 100 Mbps | 8 min 20 s | 400 s at full speed, 500 s realistically |
| 1 GB over 1 Gbps | 8 s | exactly 8 seconds |
| A 2 TB backup over 1 Gbps | 5 h 33 min | about 5½ hours |
FAQFrequently asked questions
Why is my download slower than my plan?
Plans are sold in megabits and files measured in megabytes — divide by eight. A 100 Mbps line delivers at most 12.5 MB/s, and real transfers reach 70–90% of that.
What is a realistic efficiency figure?
About 80% over a good wired connection; lower over Wi-Fi, mobile, or long distances where latency limits throughput.
Why does my 1 TB drive show as 931 GB?
Drives are sold in decimal terabytes (10¹² bytes) but operating systems display binary units (2⁴⁰ bytes). The last row shows both.
Does the file size include overhead?
No — the efficiency percentage covers protocol overhead, retransmissions and congestion together.
How long for a large backup?
Set the size in TB. Beyond a few terabytes, shipping physical disks is often faster than any network.
Where these figures come from
- IEC 80000-13 — Information science and technology (quantities and units) — the decimal (kB, MB) versus binary (KiB, MiB) prefixes used throughout
- RFC 4632 — Classless Inter-domain Routing (CIDR) — the address-plan arithmetic behind the subnet calculator
- RFC 1918 — Address Allocation for Private Internets — the private ranges the subnet calculator recognises
- NIST SP 800-63B — Digital Identity Guidelines, Authentication — length over composition rules; the basis of the password guidance here
- NIST SP 800-57 Part 1 — Recommendation for Key Management — key-strength comparisons used by the key-space calculator
- National Cyber Security Centre — password guidance — UK national guidance on password policy
Last checked: September 2026. Units follow the SI decimal convention (IEC 80000-13 defines the binary alternatives); network and security figures cite the defining standard.