EasyScaler is a simple but powerful app that allows you to measure from scale drawings using a standard ruler.
It is simple to set up, you just enter the scale, the units of your ruler (e.g. mm) and the units you want the result in.
The app then calculates the result including conversion to your chosen units (e.g. feet). The calculation is done as you type for speed and ease of use, no need to press a button to get your result.
The most powerful feature of the app is the custom scale function which you can use if you have a drawing that is not to scale, for example it has been photocopied or is on screen on your computer. You find something on the drawing that you know the size of, ideally a scale legend or a dimension that is shown, if these are unavailable then something like a standard doorway. You then enter the dimensions in the settings screen and press calculate and you can measure from the rest of the drawing using the calculated scale.