20 kilos flour to litres

There can be no conversion.

A kilogram is a measure of mass. A litre is a measure of volume. The two measure different things and, according to basic principles of dimensional analysis, conversion from one to the other is not valid.

The masses of equal volumes of the two substances will clearly be very different. So there is no direct conversion between mass and volume: you need to know the density of the substance to enable you to carry out the conversion. Flours from different grains have different densities.