Get the MKCoordinateRegion that encompasses a set of MKAnnotations

gistfile1.m

// returns a MKCoordinateRegion that encompasses an array of MKAnnotations

- (MKCoordinateRegion)regionForAnnotations:(NSArray *)annotations {
    
    CLLocationDegrees minLat = 90.0;
    CLLocationDegrees maxLat = -90.0;
    CLLocationDegrees minLon = 180.0;
    CLLocationDegrees maxLon = -180.0;
    
    for (id <MKAnnotation> annotation in annotations) {
        if (annotation.coordinate.latitude < minLat) {
            minLat = annotation.coordinate.latitude;
        }		
        if (annotation.coordinate.longitude < minLon) {
            minLon = annotation.coordinate.longitude;
        }		
        if (annotation.coordinate.latitude > maxLat) {
            maxLat = annotation.coordinate.latitude;
        }		
        if (annotation.coordinate.longitude > maxLon) {
            maxLon = annotation.coordinate.longitude;
        }
    }
    
    MKCoordinateSpan span = MKCoordinateSpanMake(maxLat - minLat, maxLon - minLon);
    
    CLLocationCoordinate2D center = CLLocationCoordinate2DMake((maxLat - span.latitudeDelta / 2), maxLon - span.longitudeDelta / 2);
    
    return MKCoordinateRegionMake(center, span);
}

Here is a version that correctly handles coordinates around the 180th meridian:
https://gist.github.com/dionc/46f7e7ee9db7dbd7bddec56bd5418ca6

Swift 4.2 version of falkobuttler's version. Region is bigger so that coordinates are not at the edges:

import Foundation
import MapKit
extension MKCoordinateRegion {
    init(coordinates: [CLLocationCoordinate2D]) {
        var minLat: CLLocationDegrees = 90.0
        var maxLat: CLLocationDegrees = -90.0
        var minLon: CLLocationDegrees = 180.0
        var maxLon: CLLocationDegrees = -180.0
        
        for coordinate in coordinates {
            let lat = Double(coordinate.latitude)
            let long = Double(coordinate.longitude)
            if lat < minLat {
                minLat = lat
            }
            if long < minLon {
                minLon = long
            }
            if lat > maxLat {
                maxLat = lat
            }
            if long > maxLon {
                maxLon = long
            }
        }
        
        let span = MKCoordinateSpan(latitudeDelta: (maxLat - minLat)*2.0, longitudeDelta: (maxLon - minLon)*2.0)
        let center = CLLocationCoordinate2DMake(maxLat - span.latitudeDelta / 4, maxLon - span.longitudeDelta / 4)
        self.init(center: center, span: span)
    }
}

No need of this anymore, From iOS 7 We can directly use this Api to set the region so all the given annotations are visible.

self.mapView.showAnnotations(annotations, animated: true)

No need of this anymore, From iOS 7 We can directly use this Api to set the region so all the given annotations are visible.

self.mapView.showAnnotations(annotations, animated: true)

That fine if you want to zoom everything to fit in the center of the map. But sometimes you want to offset it (like if you have a bottom sheet that covers half the map).

You need to be able to get a region that fits the coordinates to offset it. So this is still valid code.