This virtual ruler that can be adjusted to true size,
that can actually measure the actual length,
the upper half is metric ruler (millimetre and centimetre), lower half is inches ruler,
before you use this ruler, please set the pixels per inch to your own device.
Adjusting this virtual ruler to actual size
There are some ways to know the pixels per inch(PPI) to your devie.
My laptop has a wide screen (13.6"x7.6"),
and resolution is 1366x768 pixels, 1366 / 13.6 = 100.44 PPI.
Google ,
i am lucky and find my screen is 100 PPI.
Check what paper money in your wallet, and search "the width of paper money" online, then adjust the PPI by it.
The most accurate way,
after i measure the size of virtual ruler by a actual ruler,
i found the markings are not very accurate at 30cm,
so i adjust the default pixels per inch(PPI) to 100.7,
now i get an online actual size ruler.
Dragging ruler adjuster left or right to fit the size of the reference,
remember to save the setting for next time,
after save the setting, refresh your brower to check the result.
On the most popular browsers you can press the F5 key or click on the refresh button.
Online Virtual Rulers
online real size ruler :
It's a good idea to check the products size, before we buying it from internet, try this virtual actural size ruler