Simple Ruler

A screen ruler you can calibrate with any credit card

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Hi Product Hunt, I'm the maker of Simple Ruler. One day I held a real ruler against the ruler app on my iPhone. At the 10 cm mark, the app was about 6 mm too long. That surprised me. I had trusted that number for years - for screws, cables, the gap behind a shelf. It isn't one bad app. A phone doesn't tell apps how big its screen physically is, so every screen ruler starts with a guess. On older iPhones the guess happens to be right. On an iPhone 15 Pro or 16 Pro it is about 6% too big. On Android it depends on the model. Most ruler apps never ask you to check. Simple Ruler is my attempt at a ruler that asks first. WHAT IT ACTUALLY DOES FOR YOU You're about to order a replacement part and the listing says 32 mm. There's no ruler in the drawer. You open the app, lay the part on the screen, and you have your answer before you would have found the tape measure. A ruler you can check with what's in your wallet. Credit cards, debit cards and most ID cards share the same short side: 53.98 mm. Hold one against the screen, move a slider until the edges line up, and the scale is now set for your exact phone. If you have a real ruler, you can match it directly instead. More than one line. Two cursors to measure the gap between two edges. Width and height at the same time for anything rectangular. A protractor for tilts and angles. Switch between them from the bar at the bottom. Works wherever you are. No internet, no camera, no location access. Your measurements never leave your phone. It opens in inches in the US and in centimeters almost everywhere else, in 13 languages. Free. No subscription. No account. It's supported by ads - a banner in Settings and an occasional full-screen ad when you switch modes, never a banner on top of the scale. WHY I THINK THIS MATTERS There's a quiet assumption that AR measuring made the on-screen ruler obsolete. Point the camera, done. For a room, a sofa or a doorway, I think that's true. For things smaller than your hand - a screw, a ring, a SIM tray, a button you need to replace - you need millimeters. That's where I've found AR gets shaky, and where a ruler you can lay the object on still wins. What I had missed was simpler. I assumed the screen ruler itself was accurate. I never checked. I suspect most people never have. Two questions for you: 1. When you don't have a ruler, what everyday object do you use as a reference? If there's a standard-size one I haven't thought of, I'll add the good ones as calibration options. 2. Has AR measuring really made screen rulers obsolete, or am I overstating how often we need millimeters? And if you have 30 seconds: hold a card against whatever ruler app you use now. I'd love to hear what you find.