Turn a line rate into real frame rate, Layer 2 throughput and TCP goodput after preamble, IFG, headers and FCS.
Line rate is consumed by every byte time on the wire, not just the frame, so the 20 bytes of preamble and interframe gap are charged before anything else. Dividing line rate by the on-wire frame length gives the frame rate, and multiplying that by the useful payload gives the throughput a user actually sees. A gigabit link never delivers a gigabit: at 1500-byte packets it tops out near 949 Mbps of TCP goodput, and quoting the wrong figure in a capacity plan or an SLA is a habit that leads to surprise congestion.
Ethernet Throughput
Each frame occupies payload + 14-byte header + 4-byte FCS + 4-byte optional tag on the wire, plus a fixed 20 bytes of 7-byte preamble, 1-byte SFD and 12-byte interframe gap.
Application goodput
goodput = frames/s × (payload − IP/TCP headers) × 8.
Each frame occupies payload + 14-byte header + 4-byte FCS + 4-byte optional tag on the wire, plus a fixed 20 bytes of 7-byte preamble, 1-byte SFD and 12-byte interframe gap. Line rate is consumed by every byte time on the wire, not just the frame, so the 20 bytes of preamble and interframe gap are charged before anything else. Dividing line rate by the on-wire frame length gives the frame rate, and multiplying that by the useful payload gives the throughput a user actually sees.
A gigabit link never delivers a gigabit: at 1500-byte packets it tops out near 949 Mbps of TCP goodput, and quoting the wrong figure in a capacity plan or an SLA is a habit that leads to surprise congestion.
This calculator takes 4 inputs: Line rate, IP packet size, 802.1Q tagged, IP + TCP header inside the payload. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
The 38–42 bytes of per-frame overhead are fixed. At a 64-byte frame you spend 84 byte times to carry 46 bytes of payload, so efficiency falls to about 55 percent — which is why packets-per-second, not bits-per-second, is the limit on firewalls and routers.