What is the Erlang C Formula?
The Erlang C formula is a mathematical model used in workforce management to calculate the minimum number of agents required to handle a given volume of calls while maintaining a specific service level.
Formula Components
The Erlang C calculation uses:
- Call Volume (λ): Number of calls per hour
- Average Handle Time (AHT): Average duration of each call in seconds
- Service Level (SL): Target percentage of calls answered within threshold
- Answer Time Threshold: Target answer time in seconds
How It Works
The formula calculates traffic intensity (Erlang) and uses probability theory to determine the minimum agents needed. Higher service level targets require more agents.
Example Calculation
If you have:
- 100 calls per hour
- 5 minutes (300 seconds) average handle time
- 80% service level target
- 20 second answer time threshold
The calculator shows you need approximately 27 agents to meet these targets.
Best Practices
- Use historical data to calculate accurate AHT
- Account for shrinkage (breaks, training, etc.)
- Add buffer agents for unexpected spikes
- Review and update calculations monthly
- Consider seasonal variations
Limitations
The Erlang C formula assumes:
- Calls arrive randomly (Poisson distribution)
- Call durations are exponentially distributed
- All calls wait indefinitely if needed
- No abandoned calls
Conclusion
The Erlang C calculator is an essential tool for contact center workforce planning. By using accurate data and understanding the formula's assumptions, you can optimize staffing levels to balance service quality with operational costs.