measure the rough .length x widthx breadth of the bags and multiply them together. They have a mass facitily at the filling stage and then the bag settles slightly. There isnt a basic formula as different composts will weigh different things depending on the amount of grit etc is in it.
welcome to GoY.

SBG is right there is no mathematics that can give you a true reading, but you cold go the long way round and use a litre jug and see if you can fill it 50 times,

I have just been and looked at one of our bales of composted bark. It is a 75 kilo bag and when I buy them I reckon that they weigh at least 25kg (half a hundredweight in old money).
Whilst there is no direct comparison between my bark and your compost, Cammacbusby, I would expect 50 litres of compost to weigh something in the region of 20kg.
As well as the composition of the compost the amount of moisture will also affect the weight.

I just have the fealing that you are as near as it gets BA

Thanks for the welcome and answers. I have been busy with kids and ponies for 20yrs so first year back doing veggies now kids have grown and left. The website had an error which they have now fixed. It is a 25 litre bag that arrived that weighs 7.5 kg. I ordered 2 x 50 litre bags to have enough for the 3 grow bags which take 40 litres each. What arrived was one 25 litre bag, clearly not enough, but especially formulated for potatoes.

So I have ordered 2 x 60 litre general veggie compost. I am used to 20kg bags of horse bedding and hay, so knew it was no where near 50 kgs. I have also got seeds, rotovator, greenhouse (plastic) and multipurpose compost 60 litres, so should have enough to get something growing. fingers crossed.

So it looks like you get 1/3 in kilos compared to litres. i.e. 25 lit = 7.5 kg.

I can't argue with that, Cammacbusby. Remember that this conversion applies to this compost only. It won't be accurate for anything else.
In my first post above the electronic fairies seem to have been busy and written a '75 kilo' bag. It should, of course, be 75 litres ;-)

I did not notice that BA , I just took it for granted that you were talking about litres.