Problem
This relates to Round to specified int but I’m separating these methods out since they’re technically different.
Is there a method that already performs a custom floor and ceiling to a specified integer?
// If the interval is 5, 9 > 5.
public static int FloorTo(this int value, int interval)
{
var remainder = value % interval;
return remainder > 0 ? value  remainder : value;
}
// If the interval is 5, 6 > 10.
public static int CeilingTo(this int value, int interval)
{
var remainder = value % interval;
return remainder > 0 ? value + (interval  remainder) : value;
}
Solution

Your methods take
int
parameters but it seems like you only have really thought about positive numbers. This yields some inconsistencies. For example:(28).FloorTo(6) == 28
(28).FloorTo(6) == 28
(28).FloorTo(6) == 24
For
CeilingTo
the results are:(28).CeilingTo(6) == 28
(28).CeilingTo(6) == 28
(28).CeilingTo(6) == 18
If you don’t want to think about or deal with negative numbers you should either change the signature to accept only
uint
parameters (which would require the caller to think about what to do in case he’s got anint
) or you check and throw anArgumentOutOfRange
exception 
Just considering positive numbers
FloorTo
could be changed topublic static int FloorTo(this int value, int interval) { var remainder = value % interval; return value  remainder; }
Less logic to read and verify in your head.