Bandwidth & Data Transfer Calculator
Calculate how long a file will take to download, find the speed you need to hit a transfer deadline, or estimate total data usage over any duration. Supports all units from bps to Gbps and bytes to terabytes.
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What Is Bandwidth?
Bandwidth is the maximum data rate a network connection can carry, measured in bits per second (bps). The critical detail most people miss: ISPs advertise speeds in megabits per second (Mbps), while file sizes are shown in megabytes (MB). Since 1 byte = 8 bits, a 100 Mbps connection transfers a maximum of 12.5 MB every second — not 100 MB/s.
The core formula
Transfer time (s) = (File size in bytes × 8) / (Speed in bps × (1 − overhead))Why Your Download Is Slower Than Advertised
Even if your ISP delivers the full advertised speed, real-world transfer rates are always lower. Several factors eat into your theoretical maximum:
TCP/IP Protocol Overhead
Every packet carries headers — IP (20 bytes), TCP (20 bytes), Ethernet (26 bytes). On small packets this overhead can exceed 50% of the data being sent.
Wi-Fi Collision Avoidance
Wi-Fi uses CSMA/CA — devices wait their turn before transmitting. Under load, real Wi-Fi throughput is typically 40–70% of the rated speed.
Network Congestion
Contention with other users, shared infrastructure, or server-side rate limiting reduces effective throughput, especially during peak hours.
Storage Write Speed
If your SSD or hard drive cannot keep up with the incoming data rate, downloads throttle to match the write speed — often the real bottleneck on fast connections.
Use the overhead slider (default 5%) to account for protocol waste. Increase it to 20–30% when estimating Wi-Fi transfers.
Unit Reference
Speed (bits per second)
| Unit | Equals |
|---|---|
| 1 Kbps | 1,000 bps |
| 1 Mbps | 1,000,000 bps |
| 1 Gbps | 1,000,000,000 bps |
| 100 Mbps | 12.5 MB/s |
| 1 Gbps | 125 MB/s |
Storage (bytes, decimal SI)
| Unit | Equals |
|---|---|
| 1 KB | 1,000 bytes |
| 1 MB | 1,000,000 bytes |
| 1 GB | 1,000,000,000 bytes |
| 1 TB | 1,000,000,000,000 bytes |
| 1 GiB | 1,073,741,824 bytes (binary) |
This calculator uses decimal SI units (1 KB = 1,000 B), matching how ISPs, network devices, and most storage vendors label their products. Operating systems may use binary units (KiB, MiB, GiB) where 1 GiB = 1,024 MiB — so a file your OS shows as 4.38 GiB is 4.70 GB in decimal.
Common Transfer Speeds
| Connection | Typical Speed | Max MB/s |
|---|---|---|
| Dial-up | 56 Kbps | 0.007 MB/s |
| ADSL2+ | 24 Mbps | 3 MB/s |
| Cable 100 Mbps | 100 Mbps | 12.5 MB/s |
| Cable 500 Mbps | 500 Mbps | 62.5 MB/s |
| Fiber 1 Gbps | 1 Gbps | 125 MB/s |
| USB 3.0 | 5 Gbps | 625 MB/s |
| 10GbE LAN | 10 Gbps | 1,250 MB/s |
| Thunderbolt 4 | 40 Gbps | 5,000 MB/s |
Common File Sizes
| File Type | Typical Size |
|---|---|
| MP3 song (3 min, 320 kbps) | ~7 MB |
| 1080p movie (2 h, H.264) | 4–8 GB |
| 4K HDR movie (2 h, H.265) | 20–50 GB |
| AAA PC game | 50–150 GB |
| Linux / Windows ISO | 1–6 GB |
| Virtual machine image | 10–50 GB |
| 4K Blu-ray rip (uncompressed) | 50–100 GB |
Connection Speed Scale
Logarithmic scale — each major step is roughly 10× faster than the previous one.
Download Time Reference Chart
Estimated times with 5% protocol overhead. Use the calculator above for precise results with your exact speed.
| File Size | 25 Mbps | 100 Mbps | 500 Mbps | 1 Gbps |
|---|---|---|---|---|
| 4 GB | 22m 27s | 5m 37s | 1m 7s | 34s |
| 25 GB | 2h 20m | 35m 5s | 7m 1s | 3m 31s |
| 60 GB | 5h 37m | 1h 24m | 16m 51s | 8m 25s |
| 100 GB | 9h 21m | 2h 20m | 28m 4s | 14m 2s |
Frequently Asked Questions
Why does 100 Mbps not equal 100 MB/s?
Because capital M in Mbps means megabits, and capital M in MB/s means megabytes. There are 8 bits in a byte. A 100 Mbps connection transfers 100,000,000 bits per second — divide by 8 to get 12,500,000 bytes per second, or 12.5 MB/s. This naming convention is intentional by ISPs since bit numbers look larger and more impressive.
What is protocol overhead and what percentage should I use?
Every packet you send includes header bytes that carry routing and error-checking information but carry no actual file data. On a wired TCP/IP connection this overhead is roughly 3–7%. Add Wi-Fi and it climbs to 15–30% because of CSMA/CA collision avoidance, re-transmissions, and beacon frames.
Rule of thumb: use 5% for wired gigabit, 10% for wired 100 Mbps, 20–25% for Wi-Fi 5/6, and up to 30% for congested Wi-Fi 4 networks.
How do I calculate download time manually?
Three steps:
- Convert file size to bits:
bits = GB × 1,000,000,000 × 8 - Apply overhead:
effective_bps = Mbps × 1,000,000 × (1 − overhead) - Divide:
seconds = bits / effective_bps
Example: 4.7 GB on 25 Mbps with 5% overhead → 4.7 × 8,000,000,000 / (25,000,000 × 0.95) = 1,583 seconds ≈ 26 minutes.
What internet speed do I need to download 100 GB in 1 hour?
Using the Required Speed mode with 100 GB, 1 hour, and 5% overhead: you need approximately 234 Mbps raw connection speed. At 10% overhead (typical Wi-Fi) it rises to 247 Mbps. Most home fiber plans at 250 Mbps or above will comfortably handle this over wired ethernet.
What is the difference between MB and MiB?
MB (megabyte) is the decimal unit: 1 MB = 1,000,000 bytes. MiB (mebibyte) is the binary unit: 1 MiB = 1,048,576 bytes (1,024²). Hard drive manufacturers, ISPs, and network hardware use decimal MB/GB. Windows and some older tools display file sizes in binary MiB/GiB but label them as MB/GB — this causes the well-known discrepancy where a "500 GB" drive appears as ~465 GB in Windows. This calculator uses decimal SI throughout.
How do I find my actual internet speed?
Run a speed test from a wired connection to get the most accurate reading — results over Wi-Fi reflect your Wi-Fi link speed, not your ISP line. Most ISPs provide their own speed test servers to minimise routing hops. The download result from a speed test (in Mbps) is what you should enter as your connection speed. For planning large transfers, run the test during the time of day you intend to transfer data — evening peak hours frequently show 20–50% lower speeds than off-peak.
Speed Test Tools & Technical References
Measure your real-world connection speed or explore the technical standards behind how bandwidth works.
Speed Test Tools
Speedtest by Ookla
The most widely used speed test. Measures download, upload, and ping against nearby servers.
Fast.com (Netflix)
Netflix's minimalist speed test. Useful for checking the speed your ISP delivers to major streaming CDNs.
Cloudflare Speed Test
Detailed results including latency, jitter, and download/upload breakdowns across Cloudflare's global network.
iperf3
Open-source tool for LAN and server-to-server throughput testing. The gold standard used by network engineers.
Technical References
RFC 793 — TCP Specification
The original IETF specification defining the Transmission Control Protocol and the packet headers behind protocol overhead.
Wireshark
The world's most-used network protocol analyser. Capture and inspect real packet overhead on your own connections.
FCC Broadband Speed Guide
Official FCC guidance on broadband speed requirements for common household activities such as streaming and video calls.