# Erlang B Calculator - Complete Guide

> Work out how many phone lines, SIP trunk channels or IVR ports you need for your busy hour at a chosen blocking target, using the Erlang B model.

Source: https://edesy.in/docs/tools/voice-ai-tools/erlang-b-calculator

The [Erlang B Calculator](https://edesy.in/tools/erlang-b-calculator) tells you how many lines, SIP trunk channels or IVR ports you need so that only a chosen share of busy-hour calls find every line busy.

## How to Use

1. Under "How do you want to enter traffic?", pick **Calls and duration** or **Erlangs**.
2. For calls and duration, enter **Calls in the busy hour** and **Average call duration** in minutes. For erlangs, enter **Busy-hour traffic**.
3. Choose a **Target blocking (grade of service)**: 0.1%, 0.5%, 1%, 2% or 5%.
4. Optionally enter **Lines you have now** to check your current setup. Leave it at 0 to skip.
5. Read the results and the table of lines needed at every blocking target.

## Inputs

- **Calls in the busy hour**: inbound and outbound calls that need a line during your single busiest hour.
- **Average call duration**: minutes per call. Decimals are fine (2.5 is two and a half minutes).
- **Busy-hour traffic (erlangs)**: one erlang is one line busy for the whole hour. Use this if your phone system already reports it.
- **Target blocking**: the share of calls you accept will get a busy signal or fail to connect.
- **Lines you have now**: your current line or channel count. Decimals are rounded down.

## Reading the results

```
Traffic (erlangs) = Calls in the busy hour x Average minutes / 60
Lines needed = smallest line count where Erlang B blocking <= target
Blocked calls per busy hour = Calls x Blocking at that count
```

- **Lines needed**: the smallest count that meets your target.
- **Blocking at that count**: the actual blocking with that many lines, usually a little under the target.
- **Blocked calls per busy hour** (calls mode) or **Traffic lost to blocking** (erlangs mode).
- If you entered current lines, you also see the blocking with those lines, how many lines short or spare you are, and a warning if you are short.

The table lists lines needed and actual blocking at all five targets, so you can see what a stricter or looser target costs in lines.

## Things to know

- Size for the busiest hour, not a daily average.
- Erlang B assumes blocked calls are lost, not queued or retried. Arrivals are random and independent, and a line is either busy or free.
- If callers wait in a queue for an agent instead of hearing a busy tone, that is Erlang C. Size agents with Erlang C first, then use this tool to size the lines they need.

**Related:** [Erlang C Calculator](https://edesy.in/tools/erlang-c-calculator) | [IVR Cost Calculator](https://edesy.in/tools/ivr-cost-calculator) | [Call Center Shrinkage Calculator](https://edesy.in/tools/call-center-shrinkage-calculator)