Download and data transfer time calculator
How long to download 100 GB, upload a 4 TB backup or copy a virtual machine to another site? Enter the size and the speed: the calculation counts network overhead, tells GB from GiB, and also gives the bandwidth needed to meet a deadline.
The calculation runs in your browser: nothing you type is sent anywhere.
Download or data transfer time
Result
Transfer time
2 h 21 min
100 GB · 100 Mbps
Effective throughput
94 Mbps
i.e. 11.8 MB per second
Size in bits: 800 Gb. 1 byte = 8 bits: a 100 Mbps link moves at most 12.5 MB per second.
The same data over common links
| Link | Time |
|---|---|
| ADSL (8 Mbps) | 1 d 5 h 33 min |
| VDSL2 (50 Mbps) | 4 h 43 min |
| 100 Mbps | 2 h 21 min |
| 1 Gbps fibre | 14 min |
| 10 Gbps | 1 min 25 s |
With the same 94% efficiency. The ADSL and VDSL2 speeds are examples: yours depends on line length.
Transfer slower than expected?
Latency, TCP window and packet loss often cap a single stream well below the link speed. The TCP throughput calculator puts a number on that ceiling.
Reverse calculation: what bandwidth for what deadline?
Result
Nominal bandwidth needed
246.3 Mbps
of which 231.5 Mbps effective throughput
How the calculation works
Speeds are expressed in bits per second, sizes in bytes. The size is converted to bits, then divided by the throughput actually available for data:
bits to transfer = size (bytes) × 8
effective rate = link speed × efficiency
time = bits to transfer / effective rate
bandwidth needed = bits to transfer / deadline / efficiencyDecimal units (MB, GB, TB) count in powers of 10, like drive makers and carriers. Binary units (MiB, GiB, TiB) count in powers of 2, like most operating systems: 1 GiB = 2³⁰ = 1,073,741,824 bytes. The unit selector says explicitly which one you use, because the gap reaches almost 10% for TB.
Efficiency accounts for what the network adds to the data. On Ethernet with a 1,500-byte MTU, each frame takes 1,538 bytes on the wire: 14 bytes of Ethernet header, 4 of checksum (FCS), 8 of preamble and 12 of inter-frame gap. It only carries 1,448 bytes of data, after 20 bytes of IPv4 header, 20 of TCP and 12 of timestamps option. Hence 1,448 / 1,538 = 94%. Jumbo frames (9,000 MTU) reach 99%. A VPN, Wi-Fi or a shared link bring it down.
The first full backup
An offsite backup only sends what changed each day, but the first copy goes in full. At 94% efficiency, 4 TB take 9 h 27 min over a 1 Gbps upload, and almost 4 days at 100 Mbps. For Nimbus, our backup service, the backup window calculator estimates that first upload and the daily incremental; when the first upload runs past a few days, the initial copy can reach us on a disk instead.
After the calculation
Transfer time is only one step of a restore. The RPO / RTO calculator adds it to detection, response and return to service to put a number on real downtime.
Frequently asked questions
How long does it take to download 100 GB?
About 2 h 21 min on a 100 Mbps link, 14 minutes on 1 Gbps fibre and 1 d 5 h 33 min on 8 Mbps ADSL, with 94% efficiency for network overhead. The calculation: 100 GB × 8 = 800 gigabits, divided by the effective throughput.
What is the difference between a GB and a GiB?
A GB (gigabyte) is 10⁹ bytes, the unit drive makers and network speeds use. A GiB (gibibyte) is 2³⁰ bytes, or 1,073,741,824 bytes: 7.4% more. The gap grows with the unit: a TiB is 1.0995 TB, almost 10% more. Windows shows GiB but labels them "GB", which is why a 1 TB drive seems to hold only 931 GB.
Why does 100 Mbps only give 12.5 MB/s?
Because speeds are counted in bits and sizes in bytes, and one byte is 8 bits. 100 Mbps divided by 8 gives 12.5 MB/s at most. Once Ethernet, IP and TCP headers are taken out, about 11.75 MB/s of actual data remains.
Why is my transfer slower than my plan's speed?
Network headers already take 6% on Ethernet. Then latency, TCP window size and packet loss often cap a single stream well below the link speed, followed by disks, CPU or the remote server. And for an upload, the upload speed is what counts, often far lower than the advertised download speed.
What bandwidth do you need to send 5 TB in 48 hours?
About 246 Mbps nominal, i.e. 231 Mbps of effective throughput at 94% efficiency. A 1 Gbps upload link handles it easily; with 100 Mbps upload, the same data takes almost 5 days.
Contact
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