Example of how to convert swift String to Double

ViewController.swift

//
//  ViewController.swift
//  Area Calculator
//
//  Created by Ranjith Nagella on 10/7/14.
//  Copyright (c) 2014 Ranjith Nagella. All rights reserved.
//

import UIKit

class ViewController: UIViewController {
    
    // Input
    @IBOutlet weak var widthTextField: UITextField!
    @IBOutlet weak var heightTextField: UITextField!
    
    //Output
    @IBOutlet weak var areaOutputLabel: UILabel!
    @IBOutlet weak var permiterOutputLabel: UILabel!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    
    // Actions

    @IBAction func buttonPressed(sender: AnyObject) {
        println("Button pressed.");
        
        // String to integer conversion
        
        // let width = widthTextField.text.toInt()!,
        // height = heightTextField.text.toInt()!
        
        // String to double conversion
        let width = NSString(string: widthTextField.text).doubleValue
        let  height = NSString(string: heightTextField.text).doubleValue
        
        // area
        let area = width * height
        
        println("Area for W \(width) and H \(height) is \(area)")
        areaOutputLabel.text =  "\(area)"
        
        // perimter
        let perimter = 2 * (height + width)
        
        println("Perimter of W \(width) and H \(height) is \(perimter)")
        permiterOutputLabel.text =  "\(perimter)"
        
        
    }

}

// Int 
/* Integers are number ranges from -Ve to +ve i.e, -2,147,483,648 to 2,147,483,647
    
*/
// let thisYear: Int = 2014
// String 
/* String is a combination of characters 1 or more.
    
*/
// let myNextBirthdayIs: String = "April 17th 1990"

Thats helpfull , thanks.

Thanks for sharing! It's really helpful!