Where the overhead comes from
At 20 ms packet time a phone sends 50 packets per second in each direction. Every packet carries 40 bytes of headers (RTP 12 + UDP 8 + IPv4 20) plus 18 bytes of Ethernet framing. For G.729, whose payload is only 20 bytes per packet, the headers are almost three times larger than the voice.
| Codec @ 20 ms | Payload bytes | IP-layer kbps | Ethernet kbps |
|---|---|---|---|
| G.711 / G.722 | 160 | 80.0 | 87.2 |
| G.729 | 20 | 24.0 | 31.2 |
| iLBC 15.2 | 38 | 31.2 | 38.4 |
Tips
- Longer packet times (30 or 40 ms) reduce overhead but add delay and make each lost packet more noticeable.
- SRTP adds a 10-byte authentication tag per packet with the common HMAC-SHA1-80 setting.
- VPN overhead varies by protocol and settings; the calculator's IPsec option is an approximation.
- Fax over VoIP (T.38 or G.711 pass-through) and video calls need their own allowance.
Frequently asked questions
How much bandwidth does a VoIP call use?
About 87 kbps in each direction for a G.711 call on Ethernet with 20 ms packets, or 80 kbps counting only the IP layer. G.729 uses about 31 kbps on Ethernet. Wideband G.722 uses the same bandwidth as G.711.
How do I calculate VoIP bandwidth?
Add the codec payload per packet to the 40 bytes of RTP/UDP/IP headers and 18 bytes of Ethernet framing, multiply by 8 bits and by packets per second (50 at 20 ms). For G.711: (160 + 40 + 18) × 8 × 50 = 87,200 bps per direction. Multiply by simultaneous calls and add headroom.
How much bandwidth do I need for 10 VoIP phones?
Plan for simultaneous calls, not phones. If all 10 phones could be on calls at once with G.711, budget about 0.9 Mbps in each direction, or about 1.1 Mbps with 25% headroom, on top of normal data use.
Does HD voice use more bandwidth?
G.722 HD voice uses the same 64 kbps payload as G.711, so bandwidth is the same — about 87 kbps on Ethernet. Opus wideband can use more or less depending on the bitrate it chooses.
Is G.729 better than G.711?
G.729 uses about a third of the bandwidth but has lower audio quality, adds a little delay, and is less tolerant of packet loss and transcoding. On modern broadband most business systems use G.711 or a wideband codec; G.729 is for constrained links.
Does encryption increase VoIP bandwidth?
Slightly. SRTP adds an authentication tag, typically 10 bytes per packet, about 4 kbps at 50 packets per second. Running calls through a VPN adds more because of tunnel headers.
Sources & references
- IETF RFC 3550 — RTP: A Transport Protocol for Real-Time Applications
- IETF RFC 3711 — SRTP
- IETF RFC 6716 — Opus audio codec
Written by Eric · Reviewed November 2026