Erlang: Call Center Traffic Unit and Staffing Formulas

An Erlang is the standard unit of telecommunications traffic. One Erlang equals one hour of continuous voice activity on a single line, or 60 minutes of aggregate call time distributed across many lines. Call center planners use Erlang formulas to size trunks, seats, and agent schedules against expected call volume.

The unit is named after Agner Krarup Erlang, the Danish mathematician who published the first traffic engineering paper in 1909. His work still frames how contact centers forecast staffing today, even inside cloud-native platforms.

The word carries a second, unrelated meaning. Erlang is also a functional programming language built at Ericsson in 1986 for telecom switching. This entry covers the traffic-unit sense, which is the one call center operators use every day.

Key takeaways

• One Erlang equals one hour of continuous voice activity on a single circuit or agent.
• Erlang B, Extended Erlang B, and Erlang C are the three formulas that convert forecast traffic into required headcount.
• Erlang C, which assumes callers wait in queue, is the default staffing model for inbound customer service.
• The unit is named after Agner Krarup Erlang, whose 1909 paper still frames the mathematics.
• Modern workforce management platforms from NICE, Genesys, and Verint embed Erlang C directly in their forecasting engines.

How it works

An Erlang measures traffic intensity — the average number of concurrent calls in progress over a defined period. If a queue handles 30 hours of talk time within one hour, that’s 30 Erlangs. Planners feed this number into Erlang B or Erlang C to compute the minimum capacity that meets a service target.

Three formulas dominate the field. Each treats blocked calls differently, which is why the choice matters.

Erlang B assumes blocked callers vanish, so it’s the tighter formula. Telcos and outbound teams use it to size trunks where a busy signal ends the call.

Extended Erlang B layers in a retry rate. It’s more realistic for outbound campaigns where dialers redial and callers try again a minute later.

Erlang C assumes blocked callers wait in queue — it’s the default for inbound call center staffing because most customer-service callers hold on hook rather than abandon immediately.

A working planner runs the numbers roughly like this:

1. Forecast call volume for a 30-minute interval. 300 calls at a six-minute average handle time equals 30 Erlangs.
2. Set a service level target. 80% of calls answered within 20 seconds is the industry norm.
3. Run Erlang C to find the minimum agent count that hits the target.
4. Add shrinkage for breaks, adherence, and training. Most operations allow 25% to 35%.
5. Publish the headcount to the workforce management scheduler by interval.

Examples

Erlang math shapes staffing across every major BPO destination. Forecasting teams in Manila, Bogotá, Kraków, and Cape Town convert half-hour call volumes into Erlangs, then compute the seat count that hits contracted service levels.

Concentrix, a Fremont-headquartered CX firm, reported roughly 440,000 employees in its FY2024 annual filing. Its enterprise contact center contracts run Erlang C forecasting inside a NICE workforce management stack, with staffing recalculated at 15- or 30-minute grain.

Teleperformance, headquartered in Paris and one of the world’s largest customer experience providers, operates voice campaigns across the Philippines, Colombia, India, and Portugal. Erlang-based scheduling is standard practice across its front-office queues — the TP Client Interaction Solutions layer sits on top of Verint or Genesys.

Genesys Cloud CX, one of the largest cloud contact-center platforms, documents Erlang C as the default forecasting method inside its workforce management module. Small operators without a full WFM licence use the same maths in free spreadsheet tools published by erlang.com and other independent calculators.

Related terms

• Workforce management: the wider discipline that folds Erlang forecasting into scheduling, adherence, and shrinkage.
• Average handle time: the seconds-per-call input every Erlang calculation depends on.
• Service level agreement: the answer-within-X-seconds target Erlang C solves for.
• Call center: the operational venue where Erlang math is applied every day.
• Occupancy: the utilization output Erlang C produces alongside required agents.
• Shrinkage: the buffer added on top of Erlang output to cover breaks and off-phone time.

FAQ

What does one Erlang mean in a call center?

One Erlang equals one hour of continuous voice traffic. That can be a single 60-minute call, six 10-minute calls, or any mix of calls whose talk time totals an hour within the measurement window.

What’s the difference between Erlang B and Erlang C?

Erlang B assumes blocked callers hang up and never return. Erlang C assumes blocked callers wait in queue. Inbound customer service uses Erlang C. Trunk sizing and outbound dialer capacity typically use Erlang B.

Is Erlang C still accurate for modern contact centers?

Erlang C delivers reliable staffing estimates for stable inbound voice queues. It tends to overstate required agents when call abandonment is high — many WFM platforms now blend Erlang C with discrete-event simulation to correct the skew.

Who invented the Erlang formula?

Danish mathematician Agner Krarup Erlang published the first traffic-loss formula while working at the Copenhagen Telephone Company in 1909. The International Consultative Committee on Telephony adopted his name as the official unit of traffic in 1946.

Do chat and email channels use Erlang?

Erlang C was built for voice, where an agent handles one call at a time. Chat allows concurrent sessions and email is asynchronous, so most modern WFM tools use modified formulas or simulation for those channels rather than pure Erlang C.

Getting the Erlang math right is table stakes for any voice BPO engagement. Explore verified BPO providers on the Outsource Accelerator directory to find partners with mature workforce management practices.