Data Transfer Time Calculator
How long a file takes to transfer at a given connection speed, in bits or bytes, with a real-world overhead allowance and the speed needed to hit a deadline.
Data Transfer Time Calculator: with the default inputs, transfer time is 14 minutes, 48 seconds.
ISPs quote megabits (Mbps); file managers show megabytes (MB/s). They differ by a factor of 8.
Protocol headers, retransmissions and the fact that a link rarely runs at its rated speed. 5–15% is typical on a healthy connection.
At the effective speed, after overhead.
- Transfer time at the rated speed
- 13 minutes, 20 seconds
- Effective speed (Mbps)
- 90
- Effective speed (MB/s)
- 11.25
- Size in bits
- 80,000,000,000
- Transfer time (hours)
- 0.247
- Speed needed to hit the deadline (Mbps)
- 24.69
- How much fits in that window
- 40.5 GB
Assumptions
- Sustained transfer at the effective speed for the whole duration.
- SI units: 1 GB = 10⁹ bytes, 1 Mbps = 10⁶ bits/s. Binary units (GiB, MiB) are available in the dropdowns.
- The overhead percentage stands in for protocol headers, retransmissions and general inefficiency.
- One large transfer — many small files are much slower and are not modelled.
- Latency, TCP window limits and the far end's capacity are not modelled.
| Connection | Rated (Mbps) | Effective (MB/s) | Transfer time |
|---|---|---|---|
| ADSL / rural | 10 | 1.13 | 2.5 h |
| Basic broadband | 50 | 5.63 | 29.6 min |
| Standard broadband | 100 | 11.25 | 14.8 min |
| Fast broadband | 300 | 33.75 | 4.9 min |
| Gigabit fibre | 1,000 | 112.5 | 1.5 min |
| 10 Gigabit LAN | 10,000 | 1,125 | 8.9 s |
All rows include the same overhead allowance. Real transfers of many small files run far below these figures — per-file overhead dominates once the average file drops under a megabyte or so.
How this is worked out
The formula
Time (s) = file size in bits ÷ effective speed in bits/s file size in bits = bytes × 8 effective speed = rated speed × (1 − overhead) Speed needed for a deadline = bits ÷ seconds available ÷ (1 − overhead) 1 byte = 8 bits; 1 GB = 10⁹ bytes; 1 Mbps = 10⁶ bits/s
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- File or dataset size
- A number.in MB, GB, TB, MiB, GiB, TiB · 0 or more · defaults to 10
- Connection speed
- A number.0 or more · defaults to 100
- Speed is in
- ISPs quote megabits (Mbps); file managers show megabytes (MB/s). They differ by a factor of 8.Kbps (kilobits/s) · Mbps (megabits/s) · Gbps (gigabits/s) · KB/s (kilobytes/s) · MB/s (megabytes/s) · GB/s (gigabytes/s)
- Real-world overhead
- Protocol headers, retransmissions and the fact that a link rarely runs at its rated speed. 5–15% is typical on a healthy connection.a percentage · from 0 to 95 · defaults to 10
- Time you have(under More options)
- For the 'speed you would need' output — a backup window, for instance.in s, min, h, day · 0 or more · defaults to 1
What you get back
- Transfer timemain answer
- At the effective speed, after overhead.
- Transfer time at the rated speed
- Effective speed (Mbps)
- Effective speed (MB/s)
- Size in bits
- Transfer time (hours)
- Speed needed to hit the deadline (Mbps)
- How much fits in that window
What this assumes
- Sustained transfer at the effective speed for the whole duration.
- SI units: 1 GB = 10⁹ bytes, 1 Mbps = 10⁶ bits/s. Binary units (GiB, MiB) are available in the dropdowns.
- The overhead percentage stands in for protocol headers, retransmissions and general inefficiency.
- One large transfer — many small files are much slower and are not modelled.
- Latency, TCP window limits and the far end's capacity are not modelled.
About this calculator
Transfer time is file size divided by speed. The only trick is that the two are almost never quoted in the same unit: file sizes are in bytes, connection speeds are in bits, and there are eight bits in a byte. A 10 GB file over a 100 Mbps line is 80,000,000,000 bits ÷ 100,000,000 bits per second = 800 seconds — 13 minutes 20 seconds in theory, closer to 15 in practice.
Why the real number is always worse
The overhead field defaults to 10% because a link never delivers its rated speed:
- Protocol overhead. TCP/IP headers cost about 3–5% of raw throughput on Ethernet; add TLS, and add more again for a VPN's encapsulation.
- TCP behaviour. Slow start means a transfer takes time to reach full speed — irrelevant for a 10 GB file, decisive for a thousand 10 KB ones. Packet loss on a long path can cut throughput dramatically, because the bandwidth-delay product limits a single stream to window size ÷ round-trip time regardless of how much capacity exists.
- The other end. Your 1 Gbps link does not help if the server, the disk, or an intermediate hop is slower. Throughput is set by the narrowest point in the path.
- Many small files. Per-file overhead — open, stat, close, plus a round trip each — dominates. Copying 100,000 small files can run at a tenth of the speed of one large one. Archive first, then transfer.
GB or GiB, Mbps or MBps
This calculator uses SI throughout: 1 GB = 10⁹ bytes, 1 Mbps = 10⁶ bits per second. That matches how drives, ISPs and network equipment are specified. Windows reports file sizes in GiB (2³⁰ bytes) while labelling them "GB", so a file this calls 10 GB shows as 9.31 GB there — a 7.4% difference. Both GiB and MiB are in the unit dropdowns if you want to work in binary units.
For speeds, watch the case: Mbps is megabits and MB/s is megabytes, and they differ by a factor of eight. That is the entire explanation for "my gigabit connection only downloads at 118 MB/s" — 1000 ÷ 8 = 125, minus overhead, so 118 is exactly right.
Planning a backup or migration window
Open More options and enter the time you have; the calculator gives the sustained speed you would need. Two things worth remembering when the answer is uncomfortable:
- Sustained is not peak. A window that needs 90% of your rated speed for eight hours will not survive contention with daytime traffic.
- Sneakernet still wins at scale. A hundred terabytes over a gigabit link is nine days of continuous transfer. A courier with a box of disks is faster, which is why the big cloud providers all sell physical data-shipping appliances.
Delta transfers and compression are also nearly always the better lever than raw speed: sending only the changed blocks turns a nine-day job into an overnight one.
Frequently asked questions
▸How long does it take to download a 10 GB file on 100 Mbps?
About 13 minutes 20 seconds at the theoretical rate, and roughly 15 minutes once you allow 10% for protocol overhead. 10 GB is 80 gigabits, divided by 100 megabits per second.
▸Why is my download slower than my connection speed?
Protocol overhead, TCP slow start, the speed of the server at the other end, Wi-Fi contention, and any slower hop in between. Throughput is set by the narrowest point in the path, not by your ISP's number.
▸How do I convert Mbps to MB/s?
Divide by 8. 100 Mbps is 12.5 MB/s at best; 1 Gbps is 125 MB/s. Bits are the network unit, bytes are the file unit.
▸How long does it take to upload 1 TB?
At 25 Mbps upload, about 4 days of continuous transfer. At gigabit, roughly 2.5 hours. Upload is usually far slower than download on consumer connections, so check the right direction.
▸Why does copying many small files take so much longer?
Each file costs a round trip or several — open, transfer, close — and that overhead swamps the actual data below about a megabyte per file. Put them in an archive first and the transfer runs at link speed.
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