Concrete Kerb Calculator
Concrete for kerbing, edging and mower strips, from a tapered or square cross-section.
Your result
Enter your kerb dimensions above to see how much concrete you need.
Kerb and channel in a street, and the crossover where a driveway meets it, are usually controlled by the local council. They may specify the profile, the strength class, the reinforcement and who is permitted to lay it. Garden edging inside your own boundary is your business; anything on the verge generally is not.
Why a kerb is not just a long rectangle
Most kerb profiles are wider at the base than at the top. The taper gives the kerb stability against sideways load and makes it easier to strip the form. That makes the cross-section a trapezoid rather than a rectangle:
Cross-section = (top width + bottom width) ÷ 2 × height
Volume = cross-section × length
For a 100 mm top, 150 mm bottom and 300 mm height, that is (0.1 + 0.15) ÷ 2 × 0.3 = 0.0375 m² — so 20 metres needs 0.75 m³.
Using the bottom width for the whole section would give 0.9 m³, which is 20% over. Using the top width would give 0.6 m³, which is 17% under and leaves you short mid-pour. The average is the right answer, and it is exact for a straight taper, not an approximation.
If your kerb is square
Enter the same figure for top and bottom. The trapezoid collapses to a rectangle and the calculator handles it without any special case — width × height × length, as you would expect.
Measuring the run
Measure along the centre line of the kerb, not the outside edge. On a straight run it makes no difference. On a curve it makes a considerable one, and following the outside consistently over-states the length.
For a run that turns corners, add the straight sections together and measure the curves along their centre line. The waste allowance absorbs the small error that leaves.
What this does not include
- The bedding underneath. Kerbs usually sit on compacted base. Work that out with the road base calculator, which allows for compaction.
- Haunching. Many kerbs have a concrete haunch behind them for support. That is an extra triangular or rectangular section — work it out separately and add it.
- A channel or gutter. Kerb-and-channel profiles have a dish section that is not a simple trapezoid. Use the profile drawing.
Frequently asked questions
How do I handle a curved garden edge?
Measure along the centre line of the curve — a tape laid in the trench follows it well enough. The cross-section does not change on a curve, only the length, so the arithmetic is otherwise identical.
Does this work for extruded or machine-laid kerbing?
For volume, yes — a profile is a profile. But extruded kerbing uses a low-slump mix placed by machine, and the contractor will work from their own figures. This is more useful for hand-formed work.
What dimensions suit a mower strip?
Mower strips are commonly 100–150 mm wide and 75–100 mm deep, flush with the lawn so a mower wheel can run along them. Deeper than that is usually wasted concrete unless the strip is also retaining something.