How many agents do you need? Erlang C without the maths degree

Every call center has to answer the same question every day: how many people do we need on the phones? Too few and callers wait and hang up. Too many and you pay people to sit idle. The Erlang C formula is the standard way to find the balance.

The three numbers you need

1. Call volume — how many calls arrive in a period, usually an hour.
2. Average handle time (AHT) — talk time plus hold time plus after-call work, in seconds.
3. Service level target — for example, 80% of calls answered within 20 seconds.

Step 1: work out the traffic

Multiply calls by handle time and divide by the length of the period. That gives the traffic in erlangs — the number of agents that would be busy all the time if calls arrived perfectly evenly.

For 120 calls an hour with a 4-minute (240-second) handle time: 120 × 240 ÷ 3600 = 8 erlangs.

Step 2: add agents until the target is met

Calls don't arrive evenly, so 8 agents isn't enough — queues would build up. Erlang C works out the chance a caller has to wait with a given number of agents, and from that the share answered within your target time. You try 9 agents, then 10, and so on, until the service level is met.

In this example, around 11 agents reach 80% in 20 seconds. You don't need to do the maths by hand: the Erlang C calculator does it instantly.

Step 3: add shrinkage

Those 11 agents must be logged in and taking calls. But people take breaks, attend training and are sometimes off sick. If shrinkage is 30%, you need to schedule 11 ÷ 0.7 ≈ 16 agents for that hour.

Mistakes to avoid

• Using daily averages. Plan interval by interval — call volume at 10 am can be double that at 4 pm.
• Forgetting after-call work. Notes and wrap-up are part of handle time.
• Running agents too hot. Very high occupancy for long stretches leads to burnout and staff turnover, which costs more than an extra agent.
• Ignoring abandoned calls. Erlang C assumes nobody hangs up while waiting, so it slightly overstates the agents you need. That makes it a safe, slightly cautious starting point.

Try your own numbers in the free Erlang C calculator.