vBionic Measure app allows you to take precise measurements of planar objects using your iPhone and iPad with no ruler required.
Place the object you want to measure along with any rectangular, reference shape of the size you know. It can be A3, A4, A5, Letter, Legal, Business card or anything custom.
From app menu "Take photo or choose some" to load it into the app. You can select up to 4 photos and switch between them at any time. Take photos with your camera or use some from your library. Ask others to send you photos to measure remote objects. Remeber to match your reference object you use with your menu selection (i.e. A4 sheet of paper)
Select corners of your reference object. Use multiple combinations of settings that work for you best. You can zoom to fit the screen size, scale it to 50% or be sure that you move the image by 1 pixel selecting "pixel to pixel". Interact with the loaded image moving it with one finger, zooming in and out with two and bringing it back to "fit to screen" possition with double tap. Decide how you want to use the magnifier option - in the corner, next to the cursor or under it. It helps to mark corners precisely.
When done, switch from "Set Corners" to "Measure" mode. Now you can choose any two points on planar image to know the distance between them.
Have a good time measuring!