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 gravel 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 drive 5 metres by 3 metres at 50 mm is 0.75 cubic metres. At 1,600 kg per cubic metre that is 1.20 tonnes, which is 2 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.
Choosing a depth and a stone size
A decorative path needs about 50 mm of gravel over a membrane. A driveway that takes vehicles needs 75 to 100 mm, and a loose 100 mm layer will move under a turning wheel whatever the stone.
Stone size follows the use: 10 mm for paths and borders, 20 mm for drives, and anything larger is a drainage or decorative layer rather than a walking surface.
A weed membrane under the gravel slows growth and stops the stone sinking into the sub-base, which is the usual reason a gravel drive needs topping up after a few years.
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 gravel 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 gravel calculator questions
How much does a cubic metre of gravel 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 path or driveway surface?
It works for any flat area. For a a path or driveway surface it gives the quantity; it does not specify the material grade, which your supplier should confirm.