About

Marketing departments sell internet speeds in Megabits per second (Mbps), but operating systems report file sizes in Megabytes (MB). This fundamental mismatch leads to the common confusion where a 100 Mbps connection only delivers about 12.5 MB/s in a perfect vacuum. In the real world, network protocols like TCP/IP require headers and acknowledgement packets, consuming bandwidth before payload data even moves.

This tool bridges the gap between advertised bandwidth and effective throughput. It accounts for the 8-bit byte conversion and allows for granular simulation of network overhead. By adjusting for protocol inefficiency and shared network usage, accurate estimates for large file transfers - such as 4K video rendering uploads or game client patches - become possible. This precision is critical for network engineers planning capacity, streamers managing upload buffers, and developers optimizing content delivery networks.

Formulas

The core conversion relies on the definition that 1 Byte consists of 8 bits. To find the theoretical transfer rate in Megabytes per second (MB/s) from a bandwidth in Megabits per second (Mbps):

Bandwidth(Mbps)8 = Rate(MB/s)

However, real-world throughput is affected by protocol overhead (O). For TCP/IP over Ethernet, this is typically between 2.5% and 10%. The calculation for Effective Speed (S_eff) is:

S_eff = S_theory × (1 − O%)

Finally, to calculate the Time to Complete (T) for a specific File Size (F):

T = FS_eff

Reference Data

Standard / Interface	Advertised Bandwidth	Theoretical Speed	Realistic Speed (~10% Overhead)	Time to Download 10GB
Dial-up (56k)	56 Kbps	7 KB/s	6.3 KB/s	~19 Days
DSL / ADSL	10 Mbps	1.25 MB/s	1.12 MB/s	2h 32m
4G LTE (Avg)	35 Mbps	4.37 MB/s	3.93 MB/s	43m 22s
Fast Ethernet (Cat5)	100 Mbps	12.5 MB/s	11.25 MB/s	15m 11s
Wi-Fi 5 (802.11ac)	866 Mbps	108.2 MB/s	97.4 MB/s	1m 45s
Gigabit Ethernet	1000 Mbps	125 MB/s	112.5 MB/s	1m 31s
USB 2.0	480 Mbps	60 MB/s	54 MB/s	3m 09s
USB 3.0 / 3.1 Gen 1	5 Gbps	625 MB/s	562.5 MB/s	18s
SATA III	6 Gbps	750 MB/s	675 MB/s	15s
Thunderbolt 3	40 Gbps	5 GB/s	4.5 GB/s	2s

Frequently Asked Questions

This is a unit difference, not a technical fault. Internet Service Providers (ISPs) measure speed in bits (small "b", e.g., Mbps), while your computer measures file storage in Bytes (capital "B", e.g., MB/s). Since 1 Byte equals 8 bits, you must divide your ISP's advertised number by 8 to see the theoretical maximum download speed.

Overhead refers to the extra data sent along with your file to ensure it arrives correctly. This includes packet headers, routing information, and error-checking checksums (like in TCP/IP). While invisible to the user, this data consumes 2% to 15% of your bandwidth, effectively reducing the "pipe" available for the actual file.

Bandwidth is a finite resource. If you have a 100 Mbps connection and 4 users are simultaneously streaming high-definition video, the router partitions the bandwidth. While rarely split perfectly evenly due to Quality of Service (QoS) rules, a simple approximation is to divide the total bandwidth by the number of active heavy users.

Yes. Cables act as the physical bottleneck. A Cat5e cable supports up to 1 Gbps (125 MB/s), while Cat6 can handle up to 10 Gbps over short distances. If you have a Gigabit fiber plan but use an old Cat5 cable (max 100 Mbps), your physical hardware limits your speed to 12.5 MB/s regardless of your plan.