What is Erlang C? The Formula Behind Call Center Staffing

If you’ve ever wondered how call centers figure out how many agents they need on the phones, there’s a formula behind it. It’s called Erlang C, and it’s been the industry standard for decades.

The problem? Most explanations are filled with math so dense it feels like you need a PhD to understand it. Let’s break it down in plain English, what Erlang C is, why it matters, and why you don’t actually need to crunch the numbers yourself anymore.

What is Erlang C?

Erlang C is a probability formula that calculates the chance a caller will wait for an agent. By combining this with call volume, handle time, and target wait times, it helps leaders determine how many agents are needed to meet service level goals (for example: answering 80% of calls within 30 seconds).

How Erlang C Works (Without the Scary Math)

Erlang C takes a few simple inputs:

• Call volume – how many calls you expect.

• Average handle time (AHT) – how long each call takes.

• Service level goal – like answering 80% of calls in 30 seconds.

• Target wait time – how long you’re willing to let customers wait.

It uses those numbers to calculate the probability that a customer will wait and how many agents you need to hit your goals.

The math itself? Complex. That’s why most managers never run it by hand — they either rely on spreadsheets (clunky.. we will get to that later) or software that does it instantly.

The Limitations of Erlang C

Here’s the part most people don’t tell you: Erlang C isn’t perfect.

• Assumes near 100% occupancy. It calculates as if agents are always busy, which isn’t realistic. In practice, healthy occupancy is usually 80–90%.

• Doesn’t include shrinkage. Breaks, meetings, coaching sessions, call-outs — Erlang C doesn’t know those exist.

• Built for a single channel. Modern contact centers handle chat, email, SMS, and social alongside calls. Have your agents in multiple queues or channels? Yeah that will get messy quick.

That’s why most leaders add a 5–10% buffer on top of what the formula spits out. Which.. takes us straight back to the guessing game of staffing. There are better solutions though.

How Spark Queue Solves These Gaps

This is exactly why we built Spark Queue. We take the raw power of Erlang C but design it for real-world contact centers:

• Occupancy adjusted – Spark Queue lets you see what happens when occupancy isn’t 100%.

• Shrinkage built in – enter your % for breaks, PTO, coaching, etc., and the tool automatically adjusts your staffing.

• Multi-channel support – forecast calls, email, and SMS together in one place.

• No spreadsheets – get your staffing numbers in minutes, without wrestling formulas or risking an Excel error.

• Your real team, built in - You won't look at confusing numbers and see who goes where, with Spark Queue, you will add your real team in and see a live over/under.

In short: Erlang C tells you the “perfect world” number. Spark Queue turns that into the real-world schedule you can actually run.