If you manage a call center, you have probably heard of Erlang C. It is the industry standard formula for calculating how many agents you need to handle a certain volume of calls within a desired service level. Erlang C calculators take the math and help you a bit: enter your call volume, average handle time, and service level goal, and most calculators will tell you how many agents you need.

But while these calculators are useful, they only give you part of the picture.

How Erlang C Works

The Erlang C formula calculates the probability that a call will wait before being answered. When used in a staffing calculator, it runs that calculation repeatedly to find the minimum number of agents needed to meet a specific service level target.

Most free Erlang C calculators ask for:

• Call volume – The number of calls you expect in a given period.

• Average handle time (AHT) – How long an average call takes from start to finish.

• Service level target – For example, answering 80% of calls within 20 seconds.

Once you plug in the numbers, Erlang C gives you the minimum agents needed to meet that service level under ideal conditions.

What Erlang C Doesn’t Do

This is where many contact center leaders get tripped up. Erlang C tells you how many agents you’d need if they were available 100% of the time and working a single channel. But in reality, agents take breaks, attend meetings, handle multiple channels, and work different shifts.

Some key limitations of a standalone Erlang C calculator:

• No shrinkage built in – Breaks, PTO, training, and meetings aren’t factored in.

• No multi-channel view – Calls, chat, email, and SMS are planned separately.

• No scheduling context – You can’t load actual shifts to see where you’re over or under staffed.

• Single time block – Doesn’t break coverage down by interval to show gaps during the day.

• Static results – Change a parameter, and you have to re-run everything manually.

That’s why smaller teams often plug numbers into a free calculator, get a number, and still miss their service level goals. Some "free" calculators out there may include shrinkage/occupancy as inputs and provide you with an updated number - but these tools just aren't sustainable for long term staffing needs.

The Gap Between a Number and a Plan

An Erlang C calculator might tell you “you need 17 agents.”

What it doesn’t tell you is:

• Which shifts they should work.

• Where you’ll be short or overstaffed throughout the day.

• How to balance different channels without missing targets.

Staffing is not a one-time calculation — it’s a moving target that changes with volume, handle time, and availability.

How Spark Queue Goes Beyond the Formula

Spark Queue starts with the Erlang math but layers in the real-world factors calculators miss — turning a number into an actionable staffing plan you can trust. In short, Spark Queue takes staffing to the next level.

Here’s what Spark Queue does differently:

• See all channels in one view – Plan for calls, chat, email, and SMS together in a single screen.

• Plan for reality, not theory – Shrinkage is built in, so staffing accounts for breaks, meetings, and training.

• Spot coverage gaps instantly – View staffing by time of day to see exactly where you’re over or under.

• Adapt on the fly – Change volume or handle time and watch staffing needs update in real time.

• Turn math into a schedule – See coverage, add agents, and know exactly where you stand.

The Bottom Line

An Erlang C calculator is a great starting point, but it’s just that — a start.
For teams that want to consistently meet service levels and manage all their channels in one place, you need more than a formula.

Spark Queue combines Erlang C accuracy with intelligent, adaptive logic that thinks like a workforce manager — instantly adjusting to every variable so you can staff with confidence.