Cockcroft-Gault Method :

GFR for male :

(140 - age) x wt(kg) / [Serum Creatinine * 72]

GFR for female :

GFR(females) = GFR(males) x 0.85  

Technically, use ideal body weight:

Men: 50kg + 2.3kg/inch over 5 ft

Women:45.5kg + 2.3kg/inch over 5 ft

Renal impairment:

Normal : >100 mL/min

Mild : 40-60 mL/min

Moderate : 10-40 mL/min

Severe : < 10 mL/min