How to use the Bandwidth Calculator
- Enter the file or page size and its unit.
- Enter the connection speed in Mbps or Gbps.
- Read the transfer time, allowing for the overhead figure shown.
- For a site, multiply average page weight by monthly page views to size your egress.
How the calculation works
Link speeds are quoted in bits per second while file sizes are in bytes, so a divide by eight is the first step: a 100 Mbps connection moves at most 12.5 MB per second. Protocol overhead from TCP, TLS and HTTP then removes a further slice, so sustained real throughput typically lands at 85 to 90% of the nominal rate — and lower on high-latency links, where the round-trip delay limits the window before bandwidth does.
Storage and transfer units carry a second ambiguity. Decimal units define a gigabyte as 1,000,000,000 bytes while binary units define a gibibyte as 1,073,741,824, a 7.4% difference that grows at each scale step. Cloud providers bill egress in decimal units, so a monthly estimate should use those rather than the binary figures an operating system reports.
Seconds = size in bits / speed in bits per second ; MB/s = Mbps / 8 ; effective ≈ nominal × 0.85–0.90Source: SI and IEC 80000-13 unit definitions for decimal and binary multiples; TCP/IP overhead per RFC 793 and RFC 8446 framing.
Worked example
Downloading a 4.7 GB image over a 240 Mbps connection, and sizing egress for 180,000 monthly page views at 2.3 MB per page.
- 4.7 GB = 4,700 MB = 37,600 megabits.
- 37,600 / 240 = 157 seconds at the nominal rate; at 88% effective, about 178 seconds.
- Site egress = 180,000 × 2.3 MB = 414,000 MB = 414 GB per month.
About 3 minutes for the download, and roughly 414 GB of monthly egress for the site — enough to matter on a metered plan.
Frequently asked questions
Why is my download slower than my connection speed?+
Speeds are in bits and files in bytes, so divide by eight first. Then subtract protocol overhead, and consider that the server, a shared link or Wi-Fi may be the actual bottleneck.
Why does a 1 TB drive show as 931 GB?+
The manufacturer uses decimal terabytes of 1,000,000,000,000 bytes while the operating system reports binary units. The capacity is unchanged; only the unit definition differs.
How much bandwidth does a website need?+
Multiply average page weight by monthly page views, then add a margin for repeat assets, bots and traffic spikes. A CDN absorbs most of it and changes where the cost lands.
Does latency affect throughput?+
Yes. On high-latency links, TCP's window scaling limits how much data is in flight, so a long-distance connection can fall well short of its nominal rate regardless of available bandwidth.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.