def offset_to_next_multiple(num, mult):
    """Calculates the amount needed to round num upward to the next multiple of mult. 
    If num is divisible by mult, then this returns 0, not mult.
    """
    
    if mult == 0:
        return 0
    else:
        offset = mult - (num % mult)
        if offset == mult:
            offset = 0
        return offset