Erlang C calculator

Classic staffing calculation. Tells you how many agents are needed to meet a service-level target. Runs entirely in your browser — nothing is sent anywhere.

Inputs

Contacts in interval

Number of calls/contacts arriving during the interval below.

Interval length (minutes)

Average handle time (seconds)

AHT = talk time + after-call work, in seconds.

Target service level (%)

Target answer time (seconds)

e.g. “80% in 20 seconds” → SL 80%, time 20s.

Max occupancy (%) — optional

A cap to keep agent utilisation sustainable. Leave high (e.g. 95) to ignore.

Shrinkage (%) — optional

Used to gross net agents up to scheduled headcount.

Results

Agents required (net)—

Scheduled (after shrinkage)—

Achieved service level—

Average speed of answer—

Occupancy—

Probability of waiting—

Offered load (Erlangs)—

Sensitivity

Agents	SL %	ASA (s)	Occ %

Erlang C assumes Poisson arrivals, exponential handle times, infinite queue and infinite patience, and a single skill. Real contact centres rarely satisfy these assumptions perfectly — treat Erlang C as a planning baseline, not a precise prediction. If abandonment matters, use Erlang A instead.

How it works

The Erlang C formula is the workhorse of contact-centre staffing. Give it your contact volume, average handle time and an interval length and it works out the offered load; tell it a service-level target (the classic is 80% of contacts answered within 20 seconds) and it returns the smallest number of agents that meets it, along with average speed of answer, occupancy and the probability of waiting. It assumes calls arrive at random (a Poisson process), every caller waits as long as it takes, and any agent can take any contact — assumptions that hold well for a single-skill voice queue with at least 15-minute intervals.

Common questions

How many agents do I need for 80% answered in 20 seconds?

Enter your contacts, interval length and AHT, set the target to 80% and the target time to 20 seconds, and the calculator returns the minimum agents. As a rule of thumb the number sits a little above your offered load in Erlangs, with the cushion growing as your service target rises.

Does Erlang C account for abandonment?

No — Erlang C assumes every caller waits forever, so it slightly over-staffs an impatient queue. If abandonment is material, use the Erlang A or Erlang X calculators, which model caller patience.

What occupancy should I expect?

Occupancy falls out of the maths rather than being something you set. Larger queues run more efficiently, so a big operation hitting target may sit near 90% while a small one sits nearer 80%. Sustained occupancy in the low-90s is a warning sign, not a goal.