Network Goodput Calculator

A link advertised as one gigabit per second carries a gigabit of everything, headers included. The part that carries your files, web pages or video is called goodput. The page calculates it for a link speed, an MTU and a protocol stack, and shows how much of the line is spent on overhead.

Where the overhead goes

For each packet the link carries more than the application data. At the bottom, Ethernet adds a preamble of 8 bytes and requires a gap of 12 bytes between frames, both of which occupy the wire without being part of the frame; the frame itself has a 14-byte header and a 4-byte checksum. Above that, an IPv4 header takes 20 bytes (IPv6: 40) and a TCP header 20 bytes (32 with timestamps; UDP: 8). With a standard MTU of 1500, TCP over IPv4 leaves 1460 bytes for data, but the packet costs 1538 bytes of line time.

What you can set

The inputs are link speed in Mbit/s, MTU, the transport (TCP or UDP), the IP version, whether an 802.1Q VLAN tag adds four bytes, and whether TCP timestamps are on. Presets cover gigabit Ethernet with TCP, 10 gigabit with jumbo frames and UDP over IPv6. The result gives data per packet, bytes on the wire, overhead, efficiency, goodput and the packet rate at line rate. A comparison table varies only the MTU.

Reading the comparison

Efficiency rises with packet size because the overhead per packet is fixed. At an MTU of 576 the efficiency is 87.30 %, at 1280 it is 94.08 %, at 1500 it is 94.93 %, at 4000 it is 98.07 % and at 9000 it is 99.14 %. Very small packets, as in VoIP or gaming, run far below those figures, which is why packet rate matters as much as bit rate, as the Packets per Second Calculator shows. Larger frames also cut the number of packets a CPU must handle for the same data.

A worked example

The first preset, gigabit Ethernet with TCP over IPv4, an MTU of 1500 and no timestamps, gives 1460 bytes of application data per packet in 1538 bytes on the wire, an overhead of 78 bytes per packet, an efficiency of 94.93 % and a goodput of 949 Mbit/s. At line rate that is 81,274 packets per second. For jumbo frames at 9000 the same link would carry 8,960 bytes of data per packet and reach 991 Mbit/s.

Limits

This is the best case for a continuous full-size stream. The figure does not include acknowledgement traffic in the reverse direction, retransmissions after loss, TLS or VPN encapsulation, application protocol framing, or limits from disks, CPUs and windows. For tunnels, subtract the encapsulation overhead from the MTU first with the MTU and MSS Calculator, then enter the reduced value here.

From megabits to megabytes

File managers and download tools usually report megabytes per second, not megabits. Divide the goodput by eight: 949 Mbit/s is about 118.6 MB/s in decimal units, or roughly 113 MiB/s. That is the ceiling for a single TCP stream of full-size frames on a gigabit link, which is why copies on a healthy gigabit network settle at about 110 to 118 MB/s and never reach 125. Anything far below that points to a slower disk, a half-duplex or Wi-Fi hop, or a limit on the window size rather than to the link.

Related tools

• MTU and MSS Calculator adjusts the MTU for tunnels before you calculate.
• Packets per Second Calculator shows the packet rate at line speed for small frames.
• TCP Window Calculator covers the delay-related ceiling.
• Byte Converter converts the result into megabytes per second.