Swift5 implementation to get radius from MKMapView's current zoomed in region & zoom-pan to any coordinate with animation.

MKMapView+Ext.swift

//  Created by Varun Parakh on 09/04/19 via Daniel.Burke (https://gist.github.com/d2burke/ad29811b07ae31b378ff)
/*
 Have these defined in your global constants.
 let mercatorRadius = 85445659.44705395
 let maxGoogleZoomLevels = 20
 let mercatorOffset = 268435456.0
 */

//
//  MKMapView+Ext.swift
//

import Foundation
import MapKit

extension MKMapView {

    private func longitudeToPixelSpaceX(longitude: Double) -> Double {
        return round(mercatorOffset + mercatorRadius * longitude * Double.pi / 180.0)
    }

    private func latitudeToPixelSpaceY(latitude: Double) -> Double {
        return round(mercatorOffset - mercatorRadius * log((1 + sin(latitude * Double.pi
            / 180.0)) / (1 - sin(latitude * Double.pi / 180.0))) / 2.0)
    }

    private  func pixelSpaceXToLongitude(pixelX: Double) -> Double {
        return ((round(pixelX) - mercatorOffset) / mercatorRadius) * 180.0 / Double.pi
    }

    private func pixelSpaceYToLatitude(pixelY: Double) -> Double {
        return (Double.pi / 2.0 - 2.0 * atan(exp((round(pixelY) - mercatorOffset / mercatorRadius)  ))) * 180.0 / Double.pi
    }

    private func coordinateSpan(withMapView mapView: MKMapView, centerCoordinate: CLLocationCoordinate2D, zoomLevel: Double) -> MKCoordinateSpan {

        let centerPixelX = longitudeToPixelSpaceX(longitude: centerCoordinate.longitude)
        let centerPixelY = latitudeToPixelSpaceY(latitude: centerCoordinate.latitude)

        let zoomExponent = Double(20 - zoomLevel)
        let zoomScale = pow(2.0, zoomExponent)

        let mapSizeInPixels = mapView.bounds.size
        let scaledMapWidth =  Double(mapSizeInPixels.width) * zoomScale
        let scaledMapHeight = Double(mapSizeInPixels.height) * zoomScale

        let topLeftPixelX = centerPixelX - (scaledMapWidth / 2)
        let topLeftPixelY = centerPixelY - (scaledMapHeight / 2)

        //    // find delta between left and right longitudes
        let minLng = pixelSpaceXToLongitude(pixelX: topLeftPixelX)
        let maxLng = pixelSpaceXToLongitude(pixelX: topLeftPixelX + scaledMapWidth)
        let longitudeDelta = maxLng - minLng

        let minLat = pixelSpaceYToLatitude(pixelY: topLeftPixelY)
        let maxLat = pixelSpaceYToLatitude(pixelY: topLeftPixelY + scaledMapHeight)
        let latitudeDelta = -1 * (maxLat - minLat)

        let span = MKCoordinateSpan(latitudeDelta: latitudeDelta, longitudeDelta: longitudeDelta)
        return span

    }

    func zoom(toCenterCoordinate centerCoordinate: CLLocationCoordinate2D, zoomLevel: Double, animated: Bool) {
        let zoomLevel = min(zoomLevel, Double(maxGoogleZoomLevels))
        let span = self.coordinateSpan(withMapView: self, centerCoordinate: centerCoordinate, zoomLevel: zoomLevel)
        let region = MKCoordinateRegion(center: centerCoordinate, span: span)
        self.setRegion(region, animated: animated)
    }

    func zoomLevel() -> Double {

        let longitudeDelta = self.region.span.longitudeDelta
        let mapWidthInPixels = self.bounds.size.width

        let zoomScale = CGFloat(longitudeDelta * mercatorRadius * Double.pi) / (180.0 * mapWidthInPixels)
        var zoomer = CGFloat(maxGoogleZoomLevels) - log2( zoomScale )
        //Zoom can not be less than 0
        zoomer = max(zoomer, 0)
        return  Double(zoomer)
    }
}