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Use this template to plan capacity, predict staffing needs, and meet SLAs efficiently with clear, defined metrics.

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product OVERVIEW

Platform
Assembled Workforce Management
Assembled Vendor Management
Assembled Assist (AI)
Integrations

solutions

Enterprise WFM
Scheduling at scale
Short-term forecasting
Real-time management
Performance management
Metrics management
Back office management
Quality assurance
AI workflow builder
Agent copilot
Automated case resolution

tour assembled assist

Explore Assembled Assist, an AI-powered issue resolution engine and agent co-pilot, in a self-guided tour.

See Assembled in action

Chat with sales

Features

product OVERVIEW

Platform
Assembled Workforce Management
Assembled Vendor Management
Assembled Assist (AI)
Integrations

solutions

Enterprise WFM
Scheduling at scale
Short-term forecasting
Real-time management
Performance management
Metrics management
Back office management
Quality assurance
AI workflow builder
Agent copilot
Automated case resolution

tour assembled assist

Explore Assembled Assist, an AI-powered issue resolution engine and agent co-pilot, in a self-guided tour.

See Assembled in action

Chat with sales

resources

product help

Support
Integrations
Product updates
API documentation
Status page
Security

Learn

Blog
Case studies
Resource library
ROI Calculator
WFM Glossary

download the capacity planner

Use this template to plan capacity, predict staffing needs, and meet SLAs efficiently with clear, defined metrics.

pricing

Customers

company

meet assembled

About
Why Assembled?
Careers
WFM Changemakers

Join us Nov. 20 as we unpack Intercom Pioneer, discussing the latest CX trends and how AI is transforming support.

See assembled in action

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Erlang calculator to improve call center staffing

Remove the guesswork of staffing and calculate the hours and people needed to hit SLA with Assembled's Erlang C calculator.

Tell us a little bit about you. We’ll do the math.

AHT (Seconds)

SLA

First Response

Volume

Hours of operation (HOOPs) per week

Net productive hours per agent per week

Erlang hours

Hours of work needed to meet SLA target

People needed

Results based on a single week

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Frequently asked questions

What is an Erlang C calculator?

An Erlang C calculator is a math tool typically used in contact centers and customer service industries to help teams optimize staff levels and manage call volumes. It’s named after Danish mathematician A.K. Erlang, who in 1917 was asked to figure out how many switchboard operators were needed to keep up with the growing telephone system.

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Today, it helps teams determine the ideal number of agents needed to handle incoming calls while still hitting service-level targets (without using a clunky Excel spreadsheet).

How does the Erlang calculator work?

The Erlang C calculator takes into account:

Call volume or traffic intensity
Average call handling time (AHT)
The company’s desired service level (e.g. the percentage of calls answered within a certain time)
Number of agents

It takes those inputs and runs them through some complex math formulas to predict things like the chances of a caller waiting in line, how long they’re likely to wait, and how many agents you’ll need to hit your service level goals.

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Here’s a breakdown of what that looks like in action:

It first determines the total workload, a.k.a. the product of call volume and AHT, which is measured in Erlangs (a unit of traffic intensity).
The calculator then uses the Erlang C formula, a complex math equation, to calculate the probability of a caller having to wait.
Based on this probability, it estimates the average wait time for callers and the percentage of calls that will be answered within the target time frame.
The calculator can then determine if the current staffing level is enough to meet the desired service level or suggest how many agents are needed to hit the target.

How do call centers use Erlang calculators?

Erlang calculators are handy tools for call center managers. Trying to manually juggle team workloads and make sure you’re giving customers a great service experience isn’t easy, but an Erlang calculator can remove some of the tedium from those tasks.

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Here are a few common use cases:

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Forecasting. Managers can use an Erlang calculator to accurately forecast staffing needs for different times of the day or days of the week based on historic call patterns.
Real-time adjustments. You can tweak staffing levels in real time to respond to unexpected spikes or lulls in call volume.
Set targets. Use the staffing calculator to set realistic service level targets based on your available resources and previous call patterns.
Justify staffing requests. Erlang C calculators can be a convincing way to show upper management how staff time off heavily impacts customer service metrics. ‍
Optimize scheduling. Make sure you have appropriate coverage during peak call times while avoiding overstaffing during slower periods.

How do you calculate call handling capacity and average handling time using an Erlang calculator?

Here’s how to calculate call handling capacity using an Erlang calculator:

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Input the total number of agents you have available to handle calls.
Enter your average handling time (AHT) for calls.
Specify your desired service level (e.g. “we’d like 80% of calls answered within 20 seconds”).
Input an acceptable wait time for callers.

The important thing here is to be as realistic and specific as possible. The more realistic you are with your numbers, the better the results will be.

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Once you’ve inputted this info, the calculator can then determine the maximum number of calls that can be handled in a given time, the occupancy rate of agents (a.k.a. the percentage of time they spend on calls), and how likely it is that calls will be answered within the specified wait time.

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For example, if you have 10 agents with an AHT of five minutes and want 80% of calls answered within 20 seconds, the calculator might show that your team can handle about 100 calls per hour at 85% occupancy.

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While the Erlang calculator doesn’t directly calculate AHT, it’s a must-have input. To externally calculate your AHT, sum up the total time agents spend on calls (including after-call work) over a specific period and divide this total by the number of calls handled in that period.

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For example, if your team spent 1000 minutes on 200 calls in a day, the formula would look like this:

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AHT = 1000 minutes / 200 calls = 5 minutes per call

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Once you’ve figured out your AHT, you can input it into the Erlang calculator to plan staffing.

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👉 Assembled automatically calculates your AHT based on previous call history.

How do I calculate call center staffing requirements using an Erlang calculator?

Before you dive in, gather all the data you need. This includes:

Expected call volume: the number of calls you anticipate receiving during a specific time period (this is usually per hour or half-hour).
AHT: the average call duration (including after-call work), typically measured in seconds or minutes.
Service level goal: your target for call answering speed, usually written as a percentage of calls answered within a specific time frame (e.g. 80% of calls in 20 seconds).
Maximum acceptable wait time: the longest amount of time you’re willing to have callers wait before speaking to an agent.

Once you’ve got this data, add each figure to the calculator in the corresponding box. The calculator will then share a few pieces of info, including the required number of agents you’ll need to meet your service level goals, the percentage of time agents actively handle calls, and the expected average time callers wait before speaking to an agent.

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When you’ve got the results, you can adjust your inputs to optimize staffing levels. For example, if the required number of agents is higher than you expected, you might extend the acceptable wait time, lower your service level goal, or find ways to reduce AHT.

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Can an Erlang calculator be used for both inbound and outbound call centers?

In short: yes.

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The longer version is that, while it can be used in both inbound and outbound environments, an Erlang calculator’s application and specific calculations can vary between the two.

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Erlang calculators are mainly used in inbound call centers. They help managers figure out how many agents they need to handle call volumes, predict wait times, and meet service-level goals. The Erlang C formula is often used for this and assumes that if all agents are busy, callers will wait in line.

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While less common, Erlang calculators can be valuable for outbound call centers too. They can help estimate the number of agents needed to make a certain number of calls, calculate the probability of reaching a person on a call, and determine how many calls an agent can make in a given time.

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However, the biggest difference here is that outbound call centers use the Erlang B formula, which assumes that if all lines are busy, the call is lost rather than queued.

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How does Assembled’s Erlang calculator help contact centers improve staffing issues?

Assembled’s Erlang calculator was built to help contact centers address staffing challenges and optimize their workforce management. It does this through:

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Accurate staffing predictions. The calculator uses complex formulas to predict staffing needs so call centers can forecast headcount, adjust staffing levels, and avoid overstaffing.
Optimizing service levels. The calculator helps managers decide how many agents they need on hand to meet service-level goals and make informed decisions about trade-offs between service quality and staffing costs.
Real-time adjustments. The calculator lets you recalculate as conditions change. This means managers can respond quickly to unexpected spikes or lulls in call volume, adjust the number of staff on the fly, and make data-driven decisions about when to bring in additional staff members.
Scenario planning. The calculator facilitates “what-if” analysis, so contact centers can model the impact of changes in call volume, handling time, or service level goals, and plan for seasonal fluctuations.
Considering shrinkage. Assembled’s calculator helps managers account for breaks and training, ensure adequate coverage, and improve the overall efficiency of their workforce.
Data-driven decision making. The Erlang calculator spits out clear, quantitative results. This means managers can easily justify staffing decisions to upper management, identify opportunities, and balance customer satisfaction goals with operational cost constraints.