When calculating bandwidth, one can't assume that every channel is used all the time. Normal conversation includes a lot of silence, which often means no packets are sent at all. So even if one voice call sets up two 64 Kbit RTP streams over UDP over IP over Ethernet (which adds overhead), the full bandwidth is not used at all times.
A codec that sends a 64kb stream results in a much larger IP network stream. The main cause of the extra bandwidth usage is IP and UDP headers. VoIP sends small packets and so, many times, the headers are actually much larger than the data part of the packet.
Table below which shows how the codec's theoretical bandwidth usage expands with UDP/IP headers:
| Codec | BR | NEB |
| G.711 | 64 Kbps | 87.2 Kbps |
| G.729 | 8 Kbps | 31.2 Kbps |
| G.723.1 | 6.4 Kbps | 21.9 Kbps |
| G.723.1 | 5.3 Kbps | 20.8 Kbps |
| G.726 | 32 Kbps | 55.2 Kbps |
| G.726 | 24 Kbps | 47.2 Kbps |
| G.728 | 16 Kbps | 31.5 Kbps |
| iLBC | 15 Kbps | 27.7 Kbps |
BR = Bit rate
NEB = Nominal Ethernet Bandwidth (one direction)
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