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 aggregate is 1,600 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 1,600 kg per cubic metre that is 2.40 tonnes, which is 3 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.
What the density means for a real order
Aggregate is ordered by weight or by volume, and the two only agree if the density is right. A cubic metre of loose ballast is about 1.6 tonnes, but the same material compacted into a sub-base is closer to 2 tonnes, because the voids close up.
This calculator uses the loose figure, which is what a supplier loads. Compaction reduces the volume on site rather than the weight delivered, so order on the loose volume and compact in layers of about 100 mm.
Type 1 or a graded crushed aggregate binds better than a single-size stone, which is why sub-bases use it and decorative gravel does not.
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 1,600 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 aggregate 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 aggregate calculator questions
How much does a cubic metre of aggregate weigh?
About 1,600 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 or patio sub-base?
It works for any flat area. For a a driveway or patio sub-base it gives the quantity; it does not specify the material grade, which your supplier should confirm.