One kilogram is equivalent to 2.2046226 pounds.  Therefore, the formula to convert pounds to kilograms would be:  
X pounds / 2.2046226 kilograms = Y kilograms
In your example, the answer is 1.4106 kilograms (3.11 / 2.2046226).  You can also use this link to make the conversion for you:   
http://www.convert-me.com/en/convert/weight