Get Number Of Business Days Between Two Dates

CREATE FUNCTION fnBusinessDays (
	@StartDate DATETIME
	,@EndDate DATETIME
	)
RETURNS INT
AS
BEGIN
	IF (
			@StartDate IS NULL
			OR @EndDate IS NULL
			)
		RETURN (0)

	DECLARE @i INT = 0;

	WHILE (@StartDate <= @EndDate)
	BEGIN
		SET @i = @i + CASE
				WHEN datepart(dw, @StartDate) BETWEEN 2
						AND 6
					THEN 1
				ELSE 0
				END
		SET @StartDate = @StartDate + 1
	END -- while

	RETURN (@i)
END -- function
GO

SELECT dbo.fnBusinessDays('2016-01-01', '2016-12-31')

Source: http://www.sqlusa.com/bestpractices/datetimeconversion/