Unless you work in construction, there’s a good chance you don’t always carry a tape measure with you. So what do you do when you stumble upon something, and you really need to know exactly how big it is? Will that discarded futon fit in your hatchback? And how big is that 48-inch TV, really? Well, if you’re an iOS user — and assuming you carry your phone around with you– just snap a picture of the object with the VisualRuler app and it will measure it for you.

Of course, it’s not quite that simple. As detailed by Lifehacker, VisualRuler works by comparing the size of an unknown object to the size of something it knows: a credit card. So before you take a photo, you need to place your card somewhere in the frame. The photo isn’t sent anywhere, but if you’d rather not have pictures of your credit cards lying around, you can use a gift card, store membership card, or anything else of the same dimensions. The app will automatically detect the card and place a box around it, then manually put a second box around the object to be measured. Because the app knows how big the credit card is, it can now determine how big the other object is, regardless of how far away the camera was.

It’s far from an automated process, at least at this stage of development, and the app’s accuracy is not perfect. The card must be on the same plane as the object to be measured, and the app can’t account for perspective distortion. Presumably, it would also be difficult to measure very large objects — such as a building — since the credit card would be so relatively small in the frame.

Still, the $3 VisualRuler provides a much better guesstimate than simply eyeballing things – although, in a sense, that’s exactly what it does.

While VisualRuler is not available for Android, a somewhat similar app is. It’s called Viewlers Free Digital Ruler, and it can either use a database of known objects — such as a coin — to determine the size of other objects, or turn your phone into an actual ruler based on the screen’s DPI. It looks a little janky, but that may have had something to do with the poorly lit demonstration video showing it running on an old HTC phone with a cracked screen.