For quick calculations you don't even have to leave vim or pipe out to anything!
<C-r>= in insert mode lets you type mathematical expressions and have the result inserted into your buffer. You need to add a decimal point if you want a floating point result otherwise it truncates to integers.
[num]<C-a> and [num]<C-x> will add or subtract num from the number under the cursor or the next number on the line if there's no number under the cursor. It defaults to 1 if you leave [num] out.
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