If you don't have a tape measure, buy one. But even if you do have a tape measure, there are things you just can't measure because they're either too large, or downright inaccessible. Instead of hauling out the ladder, let today's iPhone app, Measured, figure out the height and width for you by taking a photo.
After an initial calibration that includes telling Measured what size shoes you wear—it is a measurement app after all—the app is ready to start giving you the height and width of items you photograph. First you take a general photograph of an area. Then you will take a second photograph to determine the size of objects in the photo. Once that is done, you can measure the height, width, and distance of objects in the photo.
Take all the measurements you've taken within the app with you anywhere. If you're shopping for furniture, your photographic measurements of all four walls will be with you while you're at the store. You can add measurements to any photo at any time once it's been saved to your projects folder. The app will even measure distance so you can see how far you spit that piece of gum.
Measured costs £3.99 for the iPhone