How to Measure True LAN Speed Between Devices Using iperf3

iperf3 is an open-source, client-server network testing tool maintained by ESnet that measures actual throughput between two devices on a local network, something internet speed tests cannot do. Unlike services such as Speedtest, iperf3 isolates a specific link — such as phone to NAS or mesh node to access point — by controlling both endpoints directly. One device runs as a server using 'iperf3 -s', while the client connects and pushes traffic for a set duration, reporting sustained throughput, retransmits, jitter, and packet loss. The tool is available via standard package managers on Linux and macOS, and native apps exist for iPhone, iPad, and Android to allow mobile testing across different rooms or locations. Walking through a building while running tests reveals real-world Wi-Fi performance gaps that signal-strength indicators alone cannot expose.