Get the size of the screen, current web page and browser window

This has everything you need to know: Get viewport/window size

but in short:

var win = window,
    doc = document,
    docElem = doc.documentElement,
    body = doc.getElementsByTagName('body')[0],
    x = win.innerWidth || docElem.clientWidth || body.clientWidth,
    y = win.innerHeight|| docElem.clientHeight|| body.clientHeight;
alert(x + ' × ' + y);

Fiddle

Please stop editing this answer. It's been edited 22 times now by different people to match their code format preference. It's also been pointed out that this isn't required if you only want to target modern browsers - if so you only need the following:

const width  = window.innerWidth || document.documentElement.clientWidth || 
document.body.clientWidth;
const height = window.innerHeight|| document.documentElement.clientHeight|| 
document.body.clientHeight;

console.log(width, height);

Here is a cross browser solution with pure JavaScript (Source):

var width = window.innerWidth
|| document.documentElement.clientWidth
|| document.body.clientWidth;

var height = window.innerHeight
|| document.documentElement.clientHeight
|| document.body.clientHeight;

A non-jQuery way to get the available screen dimension. window.screen.width/height has already been put up, but for responsive webdesign and completeness sake I think its worth to mention those attributes:

alert(window.screen.availWidth);
alert(window.screen.availHeight);

http://www.quirksmode.org/dom/w3c_cssom.html#t10 :

availWidth and availHeight - The available width and height on the screen (excluding OS taskbars and such).

But when we talk about responsive screens and if we want to handle it using jQuery for some reason,

window.innerWidth, window.innerHeight

gives the correct measurement. Even it removes the scroll-bar's extra space and we don't need to worry about adjusting that space :)

To check height and width of your current loaded page of any website using "console" or after clicking "Inspect".

step 1: Click the right button of mouse and click on 'Inspect' and then click 'console'

step 2: Make sure that your browser screen should be not in 'maximize' mode. If the browser screen is in 'maximize' mode, you need to first click the maximize button (present either at right or left top corner) and un-maximize it.

step 3: Now, write the following after the greater than sign ('>') i.e.

       > window.innerWidth
            output : your present window width in px (say 749)

       > window.innerHeight
            output : your present window height in px (say 359)