D Free Calculator is a simple calculator with large keys that are arranged in the ten-key adding machine format and spaced accordingly. Instructions for using the keys, with examples where relevant, are included in the app. There is an extra display that acts like the paper tape of a 10-key calculator to show what digits and operations were touched. This is great for making sure that you are doing what you think you are doing. There is also a backspace key (BS) to undo mistakes while entering digits, and a few extra keys for doing some common calculations; such as, percent off, average, and percentage change.