Add to Current Cell Value in Microsoft Excel
I often see someone asking if they can, for instance, type a value into A1 and have it add to the value already in A1.
Yes, it's possible, but not without the help of a macro. However, this is a really bad method of doing any kind of work in Excel. For one thing, it doesn't provide you with any kind of history. What if you type 12 when you meant to type 21? You have no way of knowing what you just typed because the value in the cell is now the new value.
Use a macro if you must, but I encourage you to find another method.