Data Transfer Calculator
Enter your file size and connection speed to find out exactly how long a transfer will take. Switch modes to reverse-solve for required speed or maximum file size. Adjust for real-world protocol overhead, and see the step-by-step math. All major size units (bits, bytes, KB, MB, GB, TB) and speed units (bps, Kbps, Mbps, Gbps) are supported.
Formula
Worked example
A 1 GB file over a 100 Mbps connection: 1 GB = 8,000,000,000 bits; 100 Mbps = 100,000,000 bps. With 10% overhead the effective speed is 90,000,000 bps. Transfer time = 8,000,000,000 / 90,000,000 = 88.9 seconds, roughly 1 minute 29 seconds.
How data transfer time is calculated
The core formula is straightforward: transfer time = file size (in bits) / effective speed (in bits per second). Converting everything to the same unit (bits and bits per second) is the most common source of confusion because file sizes are typically advertised in bytes while connection speeds are quoted in bits.
One byte equals eight bits, so a 1 GB file is 8,000,000,000 bits. A 100 Mbps connection delivers 100,000,000 bits per second. Dividing gives 80 seconds for a pure theoretical transfer with zero overhead.
In practice, every protocol adds headers, checksums, and acknowledgement packets. TCP (used by most internet transfers) typically consumes 5-15% of raw bandwidth just on overhead. Wi-Fi adds more due to collision avoidance. The overhead option in this calculator lets you model that realistically.
Bits vs. bytes: why the units matter
Internet speeds are almost always quoted in bits per second (bps, Kbps, Mbps, Gbps), while storage sizes are quoted in bytes (B, KB, MB, GB, TB). The difference is a factor of 8. This causes widespread confusion: a "10 MB/s" download manager reading and a "100 Mbps" connection rating describe the same speed, because 100 Mbps / 8 = 12.5 MB/s.
- Capital B = bytes (storage, files)
- Lowercase b = bits (network speeds)
- 1 byte = 8 bits
When in doubt, convert everything to bits before calculating. This calculator handles the conversion automatically: select the correct unit for each field and the math is done for you.
Reverse-solve modes: required speed and maximum file size
This calculator has two additional modes beyond the standard "how long will this take" question.
Required speed mode answers: "I need to transfer X GB in Y seconds - how fast a connection do I need?" This is useful when planning infrastructure, comparing ISP plans before a data migration, or checking whether a backup window fits. The result is the minimum rated speed; add 20% headroom for real-world conditions.
Maximum file size mode answers: "Given my connection and a fixed time window (say, an overnight backup), how much data can I move?" This helps with backup scheduling, streaming quality selection, and data-centre replication planning.
Protocol overhead and real-world throughput
Rated connection speeds are theoretical maximums measured under ideal, single-stream laboratory conditions. Real transfers are slower for several reasons:
- Protocol headers: TCP/IP, HTTP, and TLS headers add 20-60 bytes per packet on top of your data.
- Acknowledgements: TCP requires the receiver to send back ACK packets, which consume upstream bandwidth.
- Retransmissions: any packet lost to congestion or errors must be re-sent.
- Wi-Fi contention: 802.11 uses CSMA/CA (listen before transmit), which wastes time waiting for the channel to clear.
- ISP throttling: some plans throttle sustained transfers or deprioritise traffic after a daily cap is reached.
As a rule of thumb: assume 90% efficiency on a wired LAN, 80% on a typical home broadband connection, and 60-70% on Wi-Fi or mobile. The 10%, 20%, and 30% overhead presets in this calculator match those scenarios.
Common connection and interface speeds
| Connection type | Rated speed | Approx. effective speed | 1 GB transfer time |
|---|---|---|---|
| Dial-up modem | 56 Kbps | 50 Kbps | ~41 hours |
| DSL (basic) | 8 Mbps | 6 Mbps | ~22 min |
| ADSL2+ | 24 Mbps | 18 Mbps | ~7 min |
| Cable / VDSL2 | 100 Mbps | 80 Mbps | ~1 min 40 s |
| USB 2.0 | 480 Mbps | 300 Mbps | ~27 s |
| Gigabit Ethernet / Fibre | 1 Gbps | 800 Mbps | ~10 s |
| Wi-Fi 5 (802.11ac) | 867 Mbps | 400 Mbps | ~20 s |
| Wi-Fi 6 (802.11ax) | 2.4 Gbps | 1.2 Gbps | ~7 s |
| USB 3.0 / USB 3.1 Gen 1 | 5 Gbps | 3.5 Gbps | ~2.3 s |
| USB 3.2 Gen 2 | 10 Gbps | 7 Gbps | ~1.1 s |
| Thunderbolt 3/4 | 40 Gbps | 35 Gbps | ~0.23 s |
| 10 Gigabit Ethernet | 10 Gbps | 8.5 Gbps | ~0.94 s |
| 5G (mmWave, peak) | 20 Gbps | 4 Gbps | ~2 s |
| NVMe SSD (PCIe 4.0) | 7 GB/s | 6 GB/s | ~0.17 s |
Theoretical maximum speeds. Real-world throughput is typically 60-90% of these figures.
Frequently asked questions
Why is my actual download speed slower than what my ISP advertises?
ISPs advertise peak theoretical speeds measured under ideal conditions, usually in bits per second. Real-world speeds are reduced by protocol overhead (typically 5-15%), network congestion, the distance to the server, your router and cabling quality, and whether other devices on your network are also downloading. A 100 Mbps plan commonly delivers 70-90 Mbps in practice. Use the overhead control in this calculator to model that gap.
What is the difference between Mbps and MB/s?
Mbps is megabits per second (used for network speeds); MB/s is megabytes per second (used for file transfer rates in Windows, macOS, and download managers). Because 1 byte = 8 bits, you divide Mbps by 8 to get MB/s. A 100 Mbps connection translates to roughly 12.5 MB/s of file data.
How long does it take to transfer 1 TB of data?
At 100 Mbps (a common home broadband speed), 1 TB takes approximately 22 hours. At 1 Gbps (gigabit fibre), roughly 2.2 hours. At 10 Gbps (enterprise fibre or a local NVMe drive over Thunderbolt), around 13 minutes. Enter your exact speed and overhead into this calculator for a precise answer.
What is protocol overhead and should I always include it?
Protocol overhead is the fraction of your connection bandwidth consumed by control data (headers, acknowledgements, checksums) rather than your actual file. For quick estimates, you can ignore it (0% overhead setting). For planning backups, migrations, or checking whether a transfer fits a window, use 10% for broadband or 20% for Wi-Fi to get a more accurate figure.
Is it faster to mail a hard drive than to transfer data over the internet?
For very large datasets, physical media is often faster and cheaper - a concept sometimes called "sneakernet". A single 20 TB hard drive shipped overnight arrives in roughly 24 hours, equivalent to a sustained 1.85 Gbps transfer. If you only have a 100 Mbps connection, that same 20 TB would take about 18 days over the wire. Cloud providers such as AWS Snowball and Azure Data Box exist precisely for this reason.
How do I calculate the bandwidth needed to stream a video?
Streaming is a continuous transfer, so the required bandwidth equals the video bitrate. A 4K HDR stream typically requires 25-40 Mbps; 1080p HD requires 5-15 Mbps; 720p requires 3-8 Mbps. Use the "required speed" mode and set the file size to the data consumed per second (e.g. 5 MB/s for a 40 Mbps stream) and the transfer time to 1 second.
How much faster is USB 3.0 than USB 2.0?
USB 3.0 (SuperSpeed) has a rated speed of 5 Gbps vs. 480 Mbps for USB 2.0, making it roughly 10 times faster in theory. Real-world throughput for USB 3.0 is typically 350-400 MB/s (2.8-3.2 Gbps effective) vs. 30-40 MB/s for USB 2.0. A 50 GB file that takes about 20 minutes on USB 2.0 would transfer in around 2 minutes on USB 3.0.