How Many Agents Do I Need? A Quick Guide for Contact Centers

If you manage a contact center, one of your biggest questions is simple:
How many agents do I need to meet my service levels?

The go-to method for answering that is the Erlang C formula — the industry standard for estimating staffing needs in environments where calls are queued and answered in the order they arrive.

But here’s the catch: Erlang C on its own only gives you the base number of agents under perfect conditions. Real-world staffing requires extra steps, and that’s where most teams trip up.

Step 1 – Gather Your Data

Before you can calculate staffing needs, you’ll need a few key metrics:

• Call Volume: The number of calls (or contacts) you expect in a given time period.

• Average Handle Time (AHT): How long each interaction takes, including talk and after-call work.

• Service Level Goal: The percentage of calls you want answered within a set time (e.g., 80% within 20 seconds).

• Shrinkage: The percentage of time agents are unavailable for calls (breaks, meetings, training, PTO, etc.).

• Occupancy: The percentage of time agents spend actively handling interactions versus waiting for the next one. High occupancy can lead to burnout, while low occupancy may indicate overstaffing.

Pro tip: If you have interval data (X amount of calls per hour), use it. Staffing from daily averages can hide peak periods and lead to missed service goals.

Step 2 – Use the Erlang C Formula

Most workforce planners use the Erlang C formula to determine the minimum number of agents needed to meet service levels. There are free calculators online if you're looking to do this yourself and save a bit of time. The formula itself can be heavy to digest..

While the math can get technical, here’s the concept:

• Inputs: Call volume, AHT, service level target

• Output: Required number of staffed agents

Example:

• Volume: 500 calls in an hour

• AHT: 300 seconds (5 minutes)

• Service Level Goal: 80% in 20 seconds

Plugging these into an Erlang C calculator gives you the minimum number of agents needed to meet your service level under perfect availability — meaning no breaks, no meetings, no absences, and no multi-tasking.

And if you just went “yikes,” you’re not wrong.

That’s one of the biggest problems with using a basic calculator — it gives you the ideal scenario, not the real-world one. If you don’t adjust for things like shrinkage and occupancy, you’ll almost certainly be underestimating your staffing needs.

Step 3 – Factor in Shrinkage & Occupancy

Once you have your base staffing from Erlang C, you need to adjust it for reality.

Shrinkage:

If your shrinkage is 30%, you’ll need to increase your base staffing:

Base Staffing × (1 / (1 - Shrinkage %))

Using our example:

• Base staffing from Erlang C: 19 agents

• Shrinkage: 30%

• Adjusted staffing: 19 ÷ (1 - 0.30) = 27 agents

Occupancy:

Occupancy is the percentage of time agents spend actively handling calls versus waiting for the next one. High occupancy (90%+) can cause burnout, while low occupancy can mean overstaffing.

Formula:

Occupancy = Traffic Intensity (in Erlangs - your calculator should output this number) ÷ Number of Agents

If your calculated occupancy is higher than your target (say, 85%), add agents until you reach the desired balance.

The easy way: Instead of running these adjustments manually, Spark Queue calculates staffing that meets your service level and respects your shrinkage and occupancy limits — in one step — across every interval of the day.

Step 4 – Understand the Limits of Free Erlang Calculators

Free Erlang C calculators are great for quick estimates, but they:

• Only give you a single number, not a full-day staffing plan

• Don’t show coverage by interval or highlight peak-hour gaps

• Don't account for multi-channel work with agents handling multiple queues (calls, chat, email, SMS)

• Require you to re-run everything manually for “what if” scenarios

This is why so many teams plug numbers into a calculator, get “the number,” and still miss service levels.

Step 5 – Bridge the Gap Between a Number and a Plan

That’s where Spark Queue comes in.

We use Erlang C for the math — then layer in the real-world intelligent logic that turns a static number into an actionable plan.

With Spark Queue, you can:

• See all channels in one view – Calls, chat, email, and SMS together

• Plan for reality – Shrinkage is built in automatically

• Spot coverage gaps instantly – Interval-by-interval staffing view

• Adapt on the fly – Change volume or handle time and see updates instantly

• Turn math into a schedule – Add your agents in and know exactly where you’re over or understaffed

Final Thoughts

Erlang C is a proven formula for determining base staffing, but it’s only the starting point. If you stop there, you risk missing your service levels — or burning out your agents.

By layering in shrinkage, occupancy, multi-channel planning, and real-time schedule visibility, Spark Queue gives you the accuracy of Erlang C with the flexibility of a workforce manager’s brain — no spreadsheets, no guesswork.