Bandwidth & Transfer Time Calculator

Frequently Asked Questions

Why does the OS show MB/s but my ISP advertises Mbps?

ISPs advertise in megabits per second because the numbers look larger. Operating systems report in megabytes per second because files are measured in bytes. Divide the Mbps figure by 8 to compare: 100 Mbps = 12.5 MB/s.

What causes the gap between theoretical and realistic speeds?

TCP/IP headers add 3-5% overhead, acknowledgment round-trips limit throughput on long-distance links, packet loss triggers retransmissions, Wi-Fi adds its own overhead, and servers often throttle per-connection speeds.

When would I use Speed Mode instead of Time Mode?

Use Speed Mode when you have a fixed deadline - for example, a nightly backup window - and need to know the minimum link speed required to transfer a given amount of data within that window.

Does this calculator account for compression?

No. It calculates raw transfer time. If your transfer tool compresses data, use the compressed size as your input for a more accurate estimate.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How It Works

The fundamental formula for transfer time is: Time (seconds) = File size (bits) / Connection speed (bps). The critical point is unit alignment: file sizes are usually stated in bytes (KB, MB, GB), while network speeds are stated in bits per second (Mbps, Gbps). Since 1 byte = 8 bits, a 100 MB file is 800 megabits. Dividing by 100 Mbps gives 8 seconds of theoretical transfer time.

The realistic estimate applies an 80% efficiency factor, accounting for protocol overhead, TCP retransmissions, and network congestion. Real-world efficiency ranges from 50% on congested Wi-Fi to 95% on direct fiber with minimal load. Speed mode inverts the formula to find the link rate needed to hit a time target.

Use Cases

Network engineers and system administrators use this calculator to:

Estimate how long a large database backup or VM image will take to transfer to offsite storage.

Determine the minimum WAN link speed required to meet a replication window for a DR site.

Explain to end users why a "100 Mbps" connection downloads files at 12.5 MB/s, not 100 MB/s.

Size internet uplinks before deploying a file-sharing or video-streaming service.

Validate that a migration plan (cloud to cloud, on-prem to cloud) will complete within an agreed maintenance window.

Tips

Divide Mbps by 8 to get MB/s - this single conversion prevents most transfer-time estimation errors.

Use the 80% realistic figure for planning - theoretical speeds are never achieved in production; budget the lower number to avoid missed windows.

Measure actual throughput with iperf3 or a file transfer benchmark rather than relying on ISP-advertised speeds before committing to SLAs.

Consider parallel streams - tools like rclone or AWS DataSync use multiple simultaneous connections to multiply effective throughput on high-latency links.

Account for distance - cross-continental or satellite transfers add latency that caps TCP throughput well below the raw bandwidth; see the Bandwidth-Delay Product calculator.

FAQ