HUD provide data for the median income at the 30%, 50% 80% percentile of median income, by FIPS Code for household sizes 1 thru 8.
See http://www.huduser.org/portal/datasets/il/il14/index.html
Some sample data
STATE:ARIZONA ---------------------------I N C O M E L I M I T S---------------------------- PROGRAM 1 PERSON 2 PERSON 3 PERSON 4 PERSON 5 PERSON 6 PERSON 7 PERSON 8 PERSON Flagstaff, AZ MSA FY 2014 MFI: 57900 EXTR LOW INCOME 12500 15730 19790 23850 27910 31970 36030 39300 VERY LOW INCOME 20850 23800 26800 29750 32150 34550 36900 39300 LOW-INCOME 33350 38100 42850 47600 51450 55250 59050 62850
I'm looking for data or an 'approved' method to get the 100% median income for household sizes 1 thru for each FIPS code.