Why is my download slower than my advertised speed?

ISPs advertise speed in megabits per second (Mbps), while file sizes are in megabytes (MB). Divide Mbps by 8 to get MB/s: a 100 Mbps line peaks at 12.5 MB/s. Protocol overhead (TCP/IP headers, retransmits) and Wi-Fi conditions typically cost another 3–10%.

What is protocol overhead?

Every packet carries headers and some bandwidth is lost to acknowledgements and retransmission, so real throughput is below the line rate. The default of 5% is a reasonable estimate for wired connections; try 10–20% for congested Wi-Fi.

Does it use decimal (1000) or binary (1024) units?

Decimal: 1 GB = 1,000 MB and 1 Gbps = 1,000 Mbps, which is how network speeds and most storage vendors count. Operating systems that report GiB will show slightly larger sizes.

How is monthly data calculated?

Your speed is converted to bytes per second, multiplied by the seconds in 30 days, and scaled by the average utilization percentage. It is useful for estimating bandwidth caps and hosting transfer quotas.

BANDWIDTH CALCULATOR

Estimate transfer time, required speed, or monthly data usage from file size and connection speed. Calculated in your browser.

File size

Connection speed

Protocol overhead (%)

Transfer time

7m 1s

How to calculate bandwidth and transfer time

1
Choose a mode: transfer time, required speed, or monthly data.
2
Enter the file size and connection speed (or the time you have, or your average utilization). Pick the units — Mbps and MB/s are not the same thing.
3
Adjust the protocol overhead if you want a more realistic estimate, then read the result.

Questions

Why is my download slower than my advertised speed?: ISPs advertise speed in megabits per second (Mbps), while file sizes are in megabytes (MB). Divide Mbps by 8 to get MB/s: a 100 Mbps line peaks at 12.5 MB/s. Protocol overhead (TCP/IP headers, retransmits) and Wi-Fi conditions typically cost another 3–10%.
What is protocol overhead?: Every packet carries headers and some bandwidth is lost to acknowledgements and retransmission, so real throughput is below the line rate. The default of 5% is a reasonable estimate for wired connections; try 10–20% for congested Wi-Fi.
Does it use decimal (1000) or binary (1024) units?: Decimal: 1 GB = 1,000 MB and 1 Gbps = 1,000 Mbps, which is how network speeds and most storage vendors count. Operating systems that report GiB will show slightly larger sizes.
How is monthly data calculated?: Your speed is converted to bytes per second, multiplied by the seconds in 30 days, and scaled by the average utilization percentage. It is useful for estimating bandwidth caps and hosting transfer quotas.