# What's The Difference Between Arrival Rates and Inter Arrival Times?

We've heard that sometimes users get confused between arrival rates and inter-arrival times.

Well, let's clarify this for you; the inter arrival time is a key option in the Start Point of any simulation, therefore it's really important that users know the difference before they start simulating.

• The arrival rate is the number of arrivals per unit of time.
• The inter arrival time is the time between each arrival into the system and the next.

The arrival rate is calculated from the following equation:

`arrival rate = 1/inter arrival time `

The neat trick with this equation is that if you invert the arrival rate your answer is the inter arrival time which can then be used in your simulation.

`inter arrival time = 1/arrival rate`

### For example

If 12 customers enter a store per hour, the time between each arrival is:

inter arrival time = 1/arrival rate

= 1/12

= 0.083 (hours)

= 0.083 x 60 minutes

= 5 (minutes)

Therefore from the arrival rate of 12 per hour, the time between each arrival is 5 minutes.

Let us know if you found this tip useful and if there are any more examples you'd like to see, please reach out to us at support@simul8.com