Thanks for the answers, I see this question in a lot of places and many cannot give an answer at all basically saying 'it depends' but folk want and idea and that is what we get here. Else where it said divide by 2.4, which matches what I see here, however I had some compost here hand and put it into a measuring jug and there was about 0.52L however I compressed it down to 0.42L it weighted
0.238kg so then I divide by 0.42 and it gives a ratio of 0.56 so 1 litre weighs
0.75kg. Using the uncompressed weight I get a ratio on 0.45.
So from that you can basically divide the litres by 2 to get kilograms.
I would add that this was seed compost with is likely to me more dense than general purpose compost. Incidentally the ratio from the previous answers is
0.426 meaning you divide by 2.34.
As a rough guide I would say divide by 2 and add 1/5th or 20%
DIvide by 2 and add a bit is a good rough guide.