Erlang B calculator for E1/PRI and SIP channels
Work out how many voice channels and E1 or T1 PRI lines you need from your busy-hour call volume.
Calculated in your browser. Nothing is sent to our servers.
Enter the number of calls in your busiest hour, the average call length and the blocking you can accept. The calculator uses the Erlang B formula to find the number of voice channels you need, and how many E1 (30 channels) or T1 (23 channels) PRI lines that means. Use the same channel count for SIP trunk licences.
What is an Erlang?
One Erlang is one line kept busy for an hour. 120 calls of 3 minutes in the busy hour is 360 minutes, or 6 Erlangs. Traffic is random, so you need more channels than Erlangs to keep callers from hearing a busy tone.
Grade of service
The grade of service (GoS) is the share of calls that may be blocked in the busy hour. 1% (P.01) is the usual target for business lines; contact centres often use 0.5% or less. A lower GoS needs more channels.
E1 and T1 PRI
An E1 PRI carries 30 voice channels (30B+D) at 2.048 Mbps and is standard in India and Europe. A T1 PRI carries 23 channels (23B+D) at 1.544 Mbps in North America. When you move to SIP, the same channel count sets the number of concurrent call licences.
Common questions
How do I find busy-hour calls?
Look at your PBX call records or carrier bills for the busiest hour of a typical busy day. If you only know daily calls, the busy hour is often 15–20% of the daily total.
How many channels for 10 Erlangs?
18 channels at 1% blocking. That fits in a single E1.
Can we replace E1 lines with SIP?
Yes. We integrate SIP trunks with your existing PBX, or bridge E1/PRI to SIP with a gateway so you keep your numbers.
