Calculating any value for a potential divider

Vin > R1 > Vout > R2 > 0V

Vin = Vout * (R1 + R2) / R2
        
R1 = (Vin * R2 / Vout) – R2
        
R2 = Vout * R1 / (Vin – Vout)
        
Vout = Vin * R2 / (R1 + R2)

Good Calculators

http://www.raltron.com/cust/tools/voltage_divider.asp