Just enter a simple expression in a numeric field starting with the equal sign
=3*5+2
Then enter or change focus to evaluate the expression.
17
It supports javascript syntax, for examples :
=2*Math.PI*5
31.41593
=Math.log(1024)/Math.log(2)
10
=(function(r) { return 2*Math.PI*r; })(5)
31.41593