How do you convert lit into kgs?

If by lit you mean litre you can't because litres are a fluid/volume measure and the weight obviously varies depending on the fluid or material involved.

In old money a gallon(8 pints) of water weighs 10lb. 1.8 pints = 1 litre, 1lb=2.2kgm.

Dividing 10lb by 8, a pint of water weighs 1.25 lbs. By multiplying the weight of a lb of water by 1.8 (pints in a litre) you get the weight of a litre of water as 2.25lb if my arithmetic is correct.

This is just heavier than a kgm. ie 2.25 divide by 2.2= 1.02kg is the weight of 1 litre of water.

Litres can measure anything from different fluids, ie milk, oil etc or cereals and nuts.

You used to be able to buy sea food in pints.