Capacity inputs
Size your SIP trunk
Use your measured peak concurrency when available. Otherwise, estimate capacity from busy-hour call traffic.
Estimated requirement
Your SIP capacity plan
Your recommended channel capacity will appear here.
Calculation breakdown
How your estimate was calculated
| Step | Formula or method | Result |
|---|---|---|
| Enter your values and calculate to see the breakdown. | ||
What Is a SIP Trunk Channel?
A SIP channel is the capacity needed for one active call. If 15 external calls are active at the same time, the trunk needs at least 15 available channels. A channel is not the same as a phone number, user, extension, or calls-per-second limit.
SIP Channel Calculation Methods
Method 1: Known peak concurrent calls
This is the preferred method when your PBX, SBC, or provider reports actual simultaneous-call peaks. Use the busiest representative period, not the daily average.
Method 2: Busy-hour traffic with Erlang B
The calculator then finds the smallest whole number of channels whose Erlang B blocking probability is at or below your selected target. This model assumes random arrivals and that a call encountering full capacity is blocked rather than queued.
How to Choose the Inputs
Peak concurrent calls
Use the highest number of simultaneous inbound and outbound PSTN calls observed during a normal but demanding period. Exclude purely internal calls if they do not traverse the SIP trunk.
Expected traffic growth
Estimate how much external call demand may increase during the capacity-planning period. Base it on hiring, seasonality, campaigns, new locations, and historical traffic rather than revenue growth alone.
Safety reserve
A reserve protects the direct-concurrency method from short spikes and estimation error. It is not a substitute for a separate redundant trunk, SBC, carrier route, or internet connection.
Blocking probability
In the Erlang B method, the target is the estimated probability that a call finds every channel occupied. A lower target generally requires additional channels.
Bandwidth Estimate
The bandwidth result multiplies recommended channels by the selected per-call codec estimate. It is shown per direction because a two-way conversation sends and receives media simultaneously. The G.711 and G.729 values use Cisco’s 20 ms Ethernet examples and include typical protocol and Layer 2 overhead.
Checks Before Ordering Channels
- Confirm whether the provider counts inbound and outbound calls in one shared limit.
- Check SBC, PBX, session-license, and carrier concurrency limits.
- Check CPS separately; enough channels do not guarantee that a burst of new calls can start.
- Plan carrier, SBC, network, and power redundancy independently.
- Validate emergency calling, number routing, fax, recording, and contact-center behavior.
- Monitor real peak concurrency after deployment and resize when demand changes.
Important Limitations
This calculator provides a planning estimate, not a carrier quotation or guaranteed grade of service. Erlang B assumes independent random call arrivals, stable average holding time, and blocked calls that do not queue or retry. Predictive dialers, campaigns, emergencies, seasonal spikes, callbacks, queueing, and retry behavior can require a different traffic model or additional capacity.
Frequently Asked Questions
How many calls can one SIP channel handle?
One SIP channel normally supports one active call at a time. The same channel can serve many sequential calls, but not multiple simultaneous calls.
Are SIP channels and SIP trunks the same?
No. A SIP trunk is the connection or service, while channels describe how many simultaneous calls that trunk can carry.
Should inbound and outbound calls be counted together?
Usually yes when they share one concurrency pool. Confirm the billing and capacity rules with your provider because some services apply separate limits.
What is the difference between channels and CPS?
Channels measure active simultaneous calls. Calls per second measures the rate at which new calls can be initiated or processed.
Does an internal extension-to-extension call use a SIP channel?
Usually not when the media and signaling remain inside the PBX environment. It may use a channel when the call is routed through an external service.
Is the bandwidth estimate upload or download?
It is the approximate requirement in each direction at full channel utilization. A two-way call consumes both upload and download capacity.
Methodology Sources
The calculation logic and definitions were checked against Cisco’s Erlang B sizing guidance, Cisco’s voice bandwidth calculations, Telnyx’s channel definition, and Twilio’s concurrency and CPS guidance.