There are approximately 32 small bags of ballast in a bulk bag. This assumes that a small bag contains 25kg and a bulk bag includes 800kg, which is the case for most ballast products sold by builder’s merchants and DIY outlets.
It is important to note that although they are often referred to as tonne bags, bulk bags never actually contain a tonne of material. They got this name as this is how much weight they are capable of safely holding.
Calculation: 800kg / 25kg = 32 small bags of ballast
Suppliers do however include a disclaimer that bulk bags can contain plus or minus 5 per cent, as it is difficult to provide an exact amount at this scale. This means bulk bags can include as little as 760kg and as much as 840kg of ballast.
If a bulk bag were to be five per cent under the advertised weight, then there would be 30.4 bags of 25kg ballast.
Conversely, there would be the equivalent of 33.6 bags of ballast if the dumpy bag were to be five per cent heavier and weigh 840kg.
Measurements for 20kg Bags
While most national merchants supply small bags that weigh 25kg, some regional suppliers sell 20kg bags, which would change the answer.
When using small 20kg bags, there are 40 bags in an 800kg bulk bag.
As mentioned previously, bulk bags can vary either way by 40kg. If the bulk bag were to contain 760kg there would be 38 small bags and if it weighed 840kg there would be 42 bags.
Small Bags vs Bulk Bags
If you are thinking about using 25kg bags for an upcoming project, rather than bulk bags, it will be more expensive. Ballast costs £0.17 per kg when buying a 25kg bag and £0.08 per kg when buying an 800kg bulk bag
While it may be cheaper to use bulk bags, there are a few disadvantages.
First of all, using a bulk bag generally involves a lot more manual labour. Having to shovel ballast into a wheelbarrow and push it across a site, which may not have an even surface, takes a lot of work. This is especially difficult on warm days.
You are also likely to have leftover material, which is difficult to dispose of. In the UK, many waste recycling centres will either not accept ballast at all or impose a charge. Even if you could dispose of ballast, dragging a bulk bag across the site and lifting it into a car will be tough. On the other hand, you can easily return any excess ballast in unopened 25kg bags for a full refund.
We recommend using 25kg bags if you are thinking about using up to 800kg of ballast (one bulk bag). The convenience is worth the extra money. However, if you need more than 800kg and the project requires thousands of kilograms of ballast, then it may not be practical to use small bags.