How the volume and weight are worked out
The area is treated as a rectangle: length multiplied by width multiplied by depth, with the depth converted to the same unit as the length first. That gives the volume in cubic metres.
Weight is the volume multiplied by the density, which for hardcore is 1,800 kg per cubic metre. Loose material is less dense than the same material compacted, so this figure matches what a supplier loads rather than what it becomes in place.
The bag or bulk bag count is the weight divided by the pack size and rounded up, because you cannot buy part of a bag.
Worked example
A sub-base 5 metres by 3 metres at 100 mm is 1.5 cubic metres. At 1,800 kg per cubic metre that is 2.70 tonnes, which is 4 bulk bags of 850 kg.
Change any input and all three outputs move together. The count always rounds up, so a job that needs 2.1 bulk bags is quoted as 3.
Layers, compaction and why the tonnage looks high
Hardcore is compacted in layers, usually 100 to 150 mm at a time, with each layer whacked before the next goes down. A single deep layer will not compact properly however much effort goes into the top of it.
The density used here is for loose material as delivered. Once compacted, the same weight occupies less space, which is why the finished sub-base is thinner than the volume you ordered.
MOT Type 1 is a graded mix that binds when compacted. Crushed concrete is a common substitute and performs similarly, but it can contain reinforcement offcuts and plaster, so it is worth checking before it goes under a drive.
Limits of this calculator
It assumes a flat rectangular area at one depth. It does not account for a slope, a curved edge or a trench, all of which need the area measured separately.
It uses a typical loose density of 1,800 kg per cubic metre. Suppliers vary, and damp material weighs more, so treat the weight as an estimate and the volume as the figure to order on.
It does not include a price, delivery, a crane charge or waste from spillage. Add your own percentage if the site is awkward.
For a decorative surface layer rather than a base, use the UK gravel calculator.
Inputs for the hardcore calculator
Length
The long side of the area, in metres.
Width
The short side of the same area, in metres.
Depth
The layer thickness in millimetres, which is the finished depth, not the loose depth.
CALCULATOR QUESTIONS
UK hardcore calculator questions
How much does a cubic metre of hardcore weigh?
About 1,800 kg per cubic metre. The exact figure depends on moisture and grading, which is why suppliers quote a range.
Should I order extra?
Add 5 to 10 per cent for an irregular shape or an uneven base. For a simple rectangle at a constant depth, the calculated figure is enough.
Is the depth the loose or the finished depth?
Enter the finished depth. If the material compacts, the loose layer you lay will be thicker than the number you enter.
Does this work for a a driveway sub-base?
It works for any flat area. For a a driveway sub-base it gives the quantity; it does not specify the material grade, which your supplier should confirm.