About

In computer hardware, the "advertised speed" is almost never the speed you see in real-world file transfers. This discrepancy is largely due to encoding overhead. Protocols like PCIe 2.0 or SATA III use 8b/10b encoding, meaning for every 8 bits of data, 10 bits are transmitted. This results in a 20% bandwidth loss immediately. Newer standards like PCIe 4.0 use more efficient 128b/130b encoding, reducing overhead to just 1.5%.

This tool is essential for system builders and IT professionals comparing bottlenecks. It calculates the Effective Bandwidth after removing protocol overhead. Use it to determine if a PCIe 3.0 x16 GPU slot is faster than a PCIe 4.0 x8 slot, or to understand why your 10Gbps USB drive only transfers at 1000 MB/s.

Formulas

To convert raw bitrate (R) to effective throughput (T), we first apply the encoding efficiency ratio, then convert bits to Bytes:

T = R × Data BitsTotal Bits ÷ 8

For example, PCIe 2.0 uses 8b/10b encoding:

T_pci2 = 5 GT/s × 0.8 ÷ 8 = 500 MB/s

Reference Data

Interface	Raw Speed	Encoding	Eff. Throughput
USB 2.0	480 Mbps	8b/10b	53 MB/s
USB 3.0 (3.2 Gen1)	5 Gbps	8b/10b	500 MB/s
USB 3.2 Gen2	10 Gbps	128b/132b	1212 MB/s
SATA III	6 Gbps	8b/10b	600 MB/s
PCIe 3.0 x1	8 GT/s	128b/130b	985 MB/s
PCIe 4.0 x1	16 GT/s	128b/130b	1969 MB/s
Thunderbolt 3/4	40 Gbps	64b/66b	3880 MB/s

Frequently Asked Questions

Encoding schemes like 8b/10b ensure "DC balance" on the wire, preventing a long stream of zeros or ones that could cause the receiver to lose synchronization with the sender's clock. The extra bits facilitate this timing recovery.

They are roughly equivalent. PCIe 4.0 doubles the transfer rate per lane compared to 3.0. So, 8 lanes of 4.0 provide nearly the same bandwidth (approx. 16 GB/s) as 16 lanes of 3.0.

The USB-IF has rebranded USB 3.0 (5Gbps) to USB 3.1 Gen 1, and later to USB 3.2 Gen 1x1. They are physically the same 5Gbps standard, just renamed for marketing consistency.

Gbps (Gigabits per second) is typically used for raw transmission speed. MB/s (Megabytes per second) is used for file transfer speed. There are 8 bits in 1 Byte. So, 1 Gbps = 125 MB/s theoretically.