Session counting
Confirm whether transfers, conferences, hairpin calls or hosted call flows create more than one billable or licensed SIP session.
Free SIP planning tool
SIP Trunk Capacity Calculator
Estimate simultaneous SIP channels and media bandwidth from busy-hour traffic or measured peak concurrency, then add growth and operating headroom.
Calculate capacityBusy-hour modelUse attempts and average call duration when peak concurrency is unknown.
Observed-peak modelStart from a measured concurrent-call high-water mark when available.
Bandwidth allowanceEstimate media bandwidth in each direction using a codec assumption.
Capacity estimate
Plan from the call data you actually have.
The result is a planning estimate. Confirm it against production CDRs, PBX and SBC session limits, provider policies, failover design and a controlled load test.
Calculation method
What the estimate includes and what it cannot know.
The busy-hour mode converts calls and average duration into offered traffic measured in Erlangs. It applies expected growth, then uses the Erlang B loss model to find the smallest channel count at or below the selected blocking target. Operating headroom is added after that engineered count.
The known-peak mode starts from a measured concurrent-call high-water mark. It is usually the stronger input when the observation period represents normal peaks, campaigns, seasonal demand and both call directions.
Erlang B calculation derived from queue-economics 0.2.3, MIT licensed.
CPS limits
Concurrent capacity does not replace a calls-per-second limit. Short calls or dialer bursts can reach a signaling limit before channels are full.
Failover capacity
Size the surviving path for the traffic it must carry during maintenance, carrier failure, SBC failure or a network event.
Network path
Account for codec, packetization, encryption, VPN overhead, QoS, packet loss, jitter and bandwidth shared with other applications.
Capacity planning questions
SIP trunk sizing FAQ
How many SIP trunk channels do I need?
The useful starting point is your measured peak concurrent calls or a busy-hour workload made from call attempts and average duration. Add expected growth and operating headroom, then confirm the result against PBX, SBC, carrier and network limits.
Is one SIP channel the same as one concurrent call?
For ordinary trunk planning, one active call normally consumes one concurrent SIP session or channel. Some call flows create multiple legs, so confirm how your PBX, SBC and provider count sessions.
Do DIDs determine the number of SIP channels?
No. A DID provides an address for inbound calls, while channels or sessions determine how many calls can be active at once. A business can have many DIDs and a smaller shared pool of concurrent channels.
What is Erlang B used for in SIP trunk sizing?
Erlang B estimates the probability that a call is blocked when a fixed number of channels carries a known busy-hour traffic load. It is a planning model, so production high-water data and failover tests should still be used.
How much bandwidth does a SIP call require?
Bandwidth depends on the codec, packet interval and network overhead. This planner provides common per-call allowances and reports media bandwidth in each direction. Validate the final allowance with the actual codec, VPN, encryption and network design.
Validate the estimate
Bring the workload, platform and failover plan to a SIP review.
Share peak concurrency, busy-hour attempts, CPS, codec, PBX or SBC details, DID requirements and the markets your voice service needs to reach.