Breakpoint

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The easiest way to find the break points in your responsive web designs.Breakpoint lets you use your keyboard to resize your browser window - one pixel at a time. Therefore it is the perfect companion when you are trying to find the exact pixel where your responsive websites break.

How to use
These are the default settings, you can change them in "Window" > "Extensions", then scroll down to the bottom and to the right you will find "Keyboard Shortcuts".
Please note: Since Chrome version 29 these are not set by default anymore! You now have to set them manually when you install the extension.

Alt + Left Arrow: Decrease the width of the window by 1px
Alt + Right Arrow: Increase the width of the window by 1px
(Add Shift to any of the combinations to change it by 10px)

Breakpoint also has the ability to change the height of the browser window, but you have to manually activate it in "Window" > "Extensions" > "Keyboard Shortcuts".

When you find the break point you are looking for the tiny icon in the toolbar will tell you the exact size of you browser window.