How this calculator works
Edging units sit on a bed of concrete and are held by a wedge of concrete (haunching) behind. The units are a division: the length of the edge, with a margin for cuts, divided by the length of one unit, rounded up. The concrete is the cross-section of the bed and the haunch, multiplied by the length. The cross-section is the bed (as wide as the unit plus the haunch, by its depth) plus the haunch (its width by its height).
Worked example: 10 m of edging with 1 m units is 11 units with 5% extra. The cross-section is 300 cm² (an area of 0.03 m²) so the concrete is 0.32 m³. Mixed 1 : 2 : 4 by volume that is 4 bags of cement, 0.22 tonnes of sharp sand and 0.42 tonnes of aggregate. A 30 m run takes 32 units and 0.95 m³, and a 150 mm bed raises the 10 m job to 0.11 m³.
Assumptions and sources
- Assumptions used: 5% waste added; a concrete bed 100 mm deep under the unit and 100 mm wide haunching behind; 1 : 2 : 4 by volume, dry factor 1.54, cement 1440 kg/m³, sharp sand 1.6 and aggregate 1.5 tonnes/m³, 25 kg bags.
- Edging is bedded in concrete on a compacted sub-base and haunched at the back so that it cannot move when the paving or the drive loads it. The bed is as wide as the unit plus the haunching. A drive takes much more load than a path, so use a deeper bed and a wider haunch for it. The sizes are inputs because they depend on the product and the ground: follow the maker's instructions. Ready-mix is the better buy above about 0.5 m³.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
Edging for common jobs
1 m units, 100 mm bed, 100 × 100 mm haunching, 5% extra.
| Edging length | Units | Concrete | Cement bags |
|---|---|---|---|
| 5 m | 6 | 0.16 m³ | 2 |
| 10 m | 11 | 0.32 m³ | 4 |
| 20 m | 21 | 0.63 m³ | 8 |
| 30 m | 32 | 0.95 m³ | 12 |
| 50 m | 53 | 1.58 m³ | 20 |
Tips before you buy
- Set the edging first, then lay the paving or the drive up to it: the edging fixes the finished level, so string a line along the top.
- Let the concrete go off for a day or two before you load the edge or compact against it.
- Mix the concrete fairly dry so that it holds its shape as haunching and does not slump away from the unit.
- Cut units with a disc cutter or a bolster and leave a gap of a few millimetres at each joint for the concrete to key into.
- Lay a first row without mortar to see where the cuts fall, so the last unit is not a sliver.
Tools & materials for this project
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Frequently asked questions
How many edging units do I need?
Divide the length of the edge by the length of one unit and add about 5% for cuts. 10 m of edging with 1 m units takes 11.
How much concrete do I need to bed and haunch edging?
With a 100 mm bed under the unit and a 100 × 100 mm haunch behind it, 10 m of edging takes 0.32 m³, including 5% extra.
What is haunching?
A wedge of concrete along the back of the edging units that stops them from leaning or moving when the paving or the drive is loaded.
How much cement do I need for 10 m of edging?
4 bags of 25 kg in a 1 : 2 : 4 mix, with 0.22 tonnes of sharp sand and 0.42 tonnes of gravel.
Do I need a deeper bed for a driveway?
A drive carries cars, so use a deeper bed and wider haunching than for a path. Check the edging maker's instructions and use the concrete calculator for the size you pick.
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Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.