Curb and gutter is one continuous section repeated down a length, so the whole calculation hinges on getting the cross-section right. Measure the profile once, carefully, and the run length takes care of itself.
How the section is calculated
Volume = [Curb depth × (Curb height + Flag thickness) + Gutter width × Flag thickness] × Length
Worked example. A 150 mm deep curb standing 150 mm above a 100 mm flag, with a 300 mm gutter, over 20 m: [0.15 × 0.25 + 0.3 × 0.1] × 20 = 1.35 m³. Set the gutter width to zero and you get a plain curb or mowing strip.
Typical dimensions
| Element | Typical size |
|---|---|
| Curb height above gutter | 100–150 mm (4–6 in) |
| Curb depth, front to back | 125–200 mm (5–8 in) |
| Gutter width | 300 mm (12 in) US standard |
| Flag / gutter thickness | 100–150 mm (4–6 in) |
Forming curbs by hand
You do not need a slipform machine for a driveway entrance or a garden edge. Timber forms work fine on short runs, provided you do three things: use a stiff mix so it holds its shape when you strike off, cut control joints every 3 m (10 ft), and keep the back of the form braced against the concrete pushing outwards.
Where road salt is used, specify an air-entrained mix. Salt attacks the surface layer of ordinary concrete and a curb takes the worst of it — spray, ploughing and standing meltwater all at once.
Frequently asked questions
How much concrete per foot of curb and gutter?
A standard 6 in curb with a 12 in gutter uses roughly 0.5 to 0.7 ft³ per linear foot, which is about a cubic yard every 40 to 55 feet.
Can I pour curbs without a machine?
Yes. Timber forms, a stiff mix and control joints every 3 m give a perfectly good result on domestic-length runs.
What grade for kerbs?
C25 to C30, air-entrained where de-icing salt is used. Weak surface concrete on a kerb scales off within a couple of winters.
How deep should a curb go?
The flag is usually 100 to 150 mm thick on a compacted sub-base, with the curb standing 100 to 150 mm proud of it. Frost-prone ground needs a deeper sub-base rather than a deeper curb.
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