calculate bandwidth usage

how i can calculate bandwidth usage for pppoe client via firewall

client address 1.1.1.254

i’m using this rule to calculate bandwidth usage

/ip firewall filter
add action=passthrough chain=forward disabled=no src-address=1.1.1.254

but it does not accurately calculate bandwidth usage

because when i’m trying to download 2Mb file the filter rule does’nt add file size to the bytes

i mean that when i download 2MB file size the filter rule bytes be some kilobytes only

i need only away to calculate bandwidth usage for one address using pppoe connection via firewall

i’m not good in english so i hope you understand me

thanks