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 tarmac is 2,400 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 2,400 kg per cubic metre that is 1.80 tonnes of asphalt.
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.
Compacted thickness and what tonnage really means
Asphalt is laid loose and rolled, and it compacts by roughly 20 to 25 per cent. The thickness you enter should be the finished, rolled thickness, because that is what the tonnage is calculated from.
A domestic drive is usually 50 mm of asphalt over a compacted sub-base, or 60 to 75 mm where vehicles turn. Below that it cracks at the edges and along any soft spot.
Tonnage is only part of the job. Planers, a roller, a tack coat and the plant's minimum load all affect the price, and a small drive may be charged for a full load whether it needs one or 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 2,400 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 tarmac 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 tarmac calculator questions
How much does a cubic metre of tarmac weigh?
About 2,400 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 path surface?
It works for any flat area. For a a driveway or path surface it gives the quantity; it does not specify the material grade, which your supplier should confirm.