There is not one size fits all answer for this, the weight of compost is not set and get vary with different conditions. One of the main factors that will affect the weight of a bag of compost is whether the bag is wet or dry.
This is why you buy compost by volume rather than weight. It is why bags are sold as 25 litres or 50 litres rather than 25kg or 50kg. All the stores would have to do would be water the bags a little and suddenly the price would shoot up, let’s not go giving them any ideas eh?
Let’s just take reasonably dry bags for now as that is what most people will be picking up. As a general guide, a 50-litre bag could be expected to weigh around 10-12kg.
If you have ever been to B&Q and bought the huge 125-litre bags then these should come in at around 25kg. I have recently bought a lot of these for use on my new allotment and from real-world experience, this sounds about right to me.
Working Out The Weight Of Compost
A rough calculation you can do is to multiply the volume of compost by 0.2. So 20 litres becomes 4kg.
This is obviously just a ballpark figure but it is usually there or thereabouts and gives you an easy way to estimate the weight of a bag of compost.
What Affects The Weight Of Compost
There are two major factors that can change the weight of compost, the first being what the compost was made from and the second being the weight.
The Source Material
Peat compost is normally lighter than peat-free compost, this is just because peat is really light whereas peat-free composts usually use bark as a substitute which is heavier.
There are so many different factors here that can affect the weight of compost that it is pointless getting into the nitty gritty, just know that not all compost weighs the same and that what originally went into it can affect the end weight.
This is the major factor in compost weight and it is so large that it makes everything else redundant. The moisture content of your compost will massively affect its weight.
Compost can act like a sponge soaking up water so if the compost has got wet it can easily double or triple in weight.
If you have ever opened a bag of compost and left it in the rain but still fairly full then you will be all too aware of this.
I have done this myself and then gone unsuspectingly to move the bag of compost only to be astounded by how much it now weighs.