Cardstick stickers are 3-inches long and can turn an ordinary credit card into a small ruler. They’re waterproof, weatherproof, durable and made from very thin vinyl material so that they don’t affect card readers. Each sticker has both inch and centimeter markings.
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A set of 3 rulers in clear, yellow and white, are available for about $3.50.
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Do you know what would be even easier? Printed or etched measurement cards, which probably already exist in the form of a home improvement center gift card or similar. Still, these stickers attach to any card already in your wallet, avoiding the need to slip an extra card in there.
Harold
Check out this credit-card sized vinyl ruler. It is marketed to crime scene investigators. Imagine that.
http://www.crimesciences.com/StoreBox/rulersandscales/cs-0245.htm