Depending on the variable water content of the compost weighing is not accurate but weight the whole bag and divide by 8 to give you 10 litre weight.
Or have you got a 1 Litre flower pot you could fill 10 times

Maybe use a 10 litre container or a 5 litre or a 1 litre jug and dunk 10 times?

Depends how accurate you want to be - its an eighth of the bag, that's all, you could sort of guess it.

Thanks great help