Round to nearest multiple of a number

Add half of the multiple, then round down.

result = ((number + multiple/2) / multiple) * multiple;

or

result = number + multiple/2;
result -= result % multiple;

This rounds up if the number is exactly in the middle. You might need to tweak the calculation if you want different behaviour in that case. Also, beware overflow if number might be near the top of the type's range.