Erlang C Staffing Calculator
Estimate call center staffing based on arrival volume, average handle time, service level target, and shrinkage.
What is an Erlang?
An Erlang is a unit of telecom traffic intensity. In practical call center terms, it tells you how much concurrent workload you are trying to handle.
The core formula is:
Offered Load (Erlangs) = (Contacts per Hour × Average Handle Time in seconds) / 3600
If your center gets 120 calls per hour and each call takes 300 seconds on average, then your offered load is:
(120 × 300) / 3600 = 10 Erlangs
That means you need roughly 10 agents busy at once just to keep up with work volume, before considering queueing performance, target service level, or staffing realities like shrinkage.
Why use an Erlangs calculator?
Without traffic modeling, staffing is often based on guesswork. An Erlangs calculator helps you move from “we think this is enough” to “this staffing level should hit our SLA under these assumptions.”
- Improves service level planning: Balance customer wait times and cost.
- Controls burnout risk: Avoid unsustainably high occupancy.
- Supports budget conversations: Turn demand into defensible headcount logic.
- Enables scenario testing: Quickly test volume spikes or AHT improvements.
Erlang B vs Erlang C (and why this tool uses C)
Erlang B
Used when blocked contacts are lost (no queue), such as old-school trunk line planning.
Erlang C
Used when contacts can wait in a queue, which is how most support centers operate. That is why this page uses the Erlang C model.
Important: Erlang C assumes random arrivals, stable average handling behavior, and a first-come-first-served queue. Real operations can differ, so treat outputs as planning estimates, then calibrate with actual data.
How this Erlangs calculator works
1) Compute offered load
We calculate Erlangs from hourly volume and average handle time.
2) Find minimum productive agents
The script iterates agent counts upward until it meets:
- Your service level target (for example, 80% within 20 seconds), and
- Your occupancy cap (for example, 85%).
3) Inflate for shrinkage
If productive requirement is 14 and shrinkage is 30%, scheduled agents needed become:
14 / (1 - 0.30) = 20 (rounded up)
Quick example
Suppose your queue receives 180 contacts/hour, AHT is 240 seconds, target is 80/20, max occupancy 85%, and shrinkage is 32%.
The calculator may show a productive staffing requirement in the high teens, with total scheduled headcount in the mid-20s after shrinkage. That is normal: shrinkage can significantly increase scheduled staffing needs.
Best practices for better results
- Use interval-level forecasts (15 or 30 minutes), not daily averages.
- Use true AHT (talk + hold + wrap), not just talk time.
- Model separate queues separately (sales vs support, voice vs chat).
- Apply realistic shrinkage based on your operation’s historical patterns.
- Re-forecast weekly and compare model output with actual SLA performance.
Common pitfalls
Using annual averages
Seasonality and intraday peaks matter. Averages can hide staffing shortages during critical intervals.
Ignoring occupancy
Chasing SLA with 95%+ occupancy can cause burnout and quality decline.
Understating shrinkage
If you use 15% when your true shrinkage is 30%, your schedule will miss plan almost every day.
Final thoughts
A solid Erlangs calculator turns demand and service goals into actionable staffing targets. Use it for planning, then improve it with your real operational data. The result is better customer experience, healthier teams, and cleaner workforce decisions.