Online Real Size Ruler (mm, cm, inch)
http://customprintlanyards.com/online_tools/measurement/online_real_size_scale_ruler.html
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.
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