$ vnstat
Database updated: Sun Jan 18 19:26:57 2015

   eth0 since 01/14/15

          rx:  33.36 GiB      tx:  31.33 GiB      total:  64.68 GiB

   monthly
                     rx      |     tx      |    total    |   avg. rate
     ------------------------+-------------+-------------+---------------
       Jan '15     33.36 GiB |   31.33 GiB |   64.68 GiB |  352.61 kbit/s
     ------------------------+-------------+-------------+---------------
     estimated     58.06 GiB |   54.53 GiB |  112.58 GiB |

   daily
                     rx      |     tx      |    total    |   avg. rate
     ------------------------+-------------+-------------+---------------
     yesterday     11.06 MiB |   20.16 MiB |   31.22 MiB |    2.96 kbit/s
         today      8.12 MiB |    7.05 MiB |   15.17 MiB |    1.77 kbit/s
     ------------------------+-------------+-------------+---------------
     estimated         9 MiB |       8 MiB |      17 MiB |