Iperf software and manuals can be found on its official home page https://iperf.fr

You can find a TCP Throughput calculator here


For a more accurate test, use commands like:

iperf -c speedtest.hostkey.ru Ru(for IPv4)
iperf -c speedtest.hostkey.com NL(for IPv4)
iperf -c speedtest.hostkey.us -p 5001 -P 10 (for IPv4)


Also you can use IPERFv3 ( -R reverse mode (server sends, client receives)

iperf3 -c speedtest.hostkey.ru RU(for IPv4)
iperf3 -c speedtest.hostkey.com NL(for IPv4)
iperf3 -c speedtest.hostkey.us -p 5201 -P 10 -4 (for IPv4)


Speedtest while downloading files from FTP or HTTP
This archive is available over Russia http Netherlands http or USA


For a more accurate test, use the command like:
wget -4 -O /dev/null "https://speedtest.hostkey.ru/ftp/2000mb.bin" RU(for IPv4)
wget -4 -O /dev/null "http://speedtest.hostkey.com/2000mb.bin" NL(for IPv4)
wget -4 -O /dev/null http://speedtest.hostkey.us/files/2000mb.bin (for IPv4)

Russian

10mb.bin 100mb.bin 1000mb.bin 2000mb.bin 5000mb.bin 10000mb.bin

Netherlands

10mb.bin 100mb.bin 1000mb.bin 2000mb.bin 5000mb.bin 10000mb.bin