sql server - SQL finding average between dates -

i have user write in 2 dates, say: '2013-7-8','2013-7-15'
stored procedure executes , returns , updated table.
2 columns include:

avg_paid_volume ---- cns_amt

numb_im_looking_for -------------------------10000
numb_im_looking_for ------------------------ 20000
numb_im_looking_for -------------------------30000
... etc

cns_amt volume on specified time period.
i'm looking average paid volume across between specified 2 dates, called @dt_from , @dt_to -> in case 10000/8, 20000/8 , 30000/8

have values inputed table (into 'numb_im_looking_for')

declare @dt_to datetime declare @dt_from datetime set @dt_to = convert(datetime,'2013-7-8') set @dt_from = convert(datetime,'2013-7-15')  update tablename set avg_paid_volume = (cns_amt / datediff(d,@dt_to,@dt_from)) 

datediff key here