A topic about the online rulers (aka virtual rulers)
Maybe you know what the rulers are, but I'll give you some short definitions:
1. A ruler, sometimes called a rule or line gauge, is a straightedge with equally spaced markings along its length.
https://en.wikipedia.org/wiki/Ruler Text is available under the Creative Commons Attribution-ShareAlike License
2. A straight strip or cylinder of plastic, wood, metal, or other rigid material, typically marked at regular intervals and used to draw straight lines or measure distances.
https://en.oxforddictionaries.com/definition/ruler
Now you probably know what I mean. To be quite sure, I'll show you the pronunciation(-s) of the word 'ruler': [ˈru:lə(r)], [ˈrulɚ] plus a photograph of different kinds of rulers: (https://upload.wikimedia.org/wikipedia/commons/2/2c/Lineale.jpg)By No machine-readable author provided. Peng assumed (based on copyright claims). - No machine-readable source provided. Own work assumed (based on copyright claims)., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=141695 We published recently a photograph of a flexible ruler (http://www.seo-forum-seo-luntan.com/else-topics/math-thread/msg35448/#msg35448) too.
Concrete online rulers (onscreen rulers, virtual rulers)
The good news -- they already exist! There are different online rulers (also knows as onscreen rulers, virtual rulers, rulers online...) and here I'd like to represent some of them:
1. This website measures the size of your screen and creates the image of a ruler that is the actual size: iRuler.net.
2. Ruler with centimeters and inches. Configurable for different screens: freeonlineruler.com.
3. Website shows ruler in real size based on your monitor size: PreciseRuler.com.
If you wanna more, just let us know! 8) :)