There's an app for that. It is called Crowdsize. You can download the iOS app from the App store here.
This is also a very helpful tool if you are trying to find a location for your event. You can scout as many locations as you like to understand how many people the event location can hold.
Once you've downloaded the app and have it open on your iOS device, follow these simple steps. To estimate the size of a crowd based on the density of attendees, simply find the location of your event on a map. You can search by the address or just drag the map and pinch to zoom in.
Once you find the location, you can pinch and swipe to zoom in and out to be more precise. Next, “lock” the area of your event and outline the perimeter.
Drag the slider to populate the crowd. You can choose from Light, Medium and Heavy to describe the crowd density.
Click ‘done’ and the app will estimate the number of people at your event. If you’re trying to find a location for your event, the app will help you determine if a location is big enough.
You can download the Crowdsize iOS app in the App Store.