I found this on another site:

select @weekDaysInPeri od = (
DATEDIFF(DAY, @StartDate, @EndDate) + 1
+ (DATEDIFF(DAY, '17530101', @StartDate) + 1) / 7 * 2
- (DATEDIFF(DAY, '17530101', @EndDate) + 1) / 7 * 2
- CASE DATEDIFF(DAY, '17530101', @StartDate) % 7
WHEN 6 THEN 1
ELSE 0
END
- CASE DATEDIFF(DAY, '17530101', @EndDate) % 7
...