Example: 30 m of a stated two-rectangle section
For this arithmetic example, rectangle A is 150 × 150 mm and adjacent rectangle B is 450 × 100 mm. Total area = 0.15 × 0.15 + 0.45 × 0.10 = 0.0675 m². Over 30 m, net volume is 2.025 m³. With an illustrative 5% allowance it is 2.12625 m³; a supplier using 0.1 m³ increments would round to 2.2 m³.
Read the detail before entering dimensions
The tool adds curb height × curb width to gutter width × gutter thickness. Sketch which concrete belongs to each rectangle. The two areas must describe the entire selected section without overlap. A label such as curb reveal is not sufficient if a hidden base under the curb is omitted from both rectangles.
If a gutter slab continues underneath the curb, one valid decomposition is the full bottom slab plus only the curb above it. Another uses adjacent non-overlapping vertical rectangles. Use the combination that matches the actual drawing, not whichever labels produce a familiar-looking result.
Measure length consistently
Record straight runs, curved returns and transitions separately. A curved curb has different inner and outer lengths; use the relevant centreline and section convention from the drawing. Do not use the chord across a curve as if it were the full arc length.
For a taper, dropped crossing or changed gutter width, split the run into sections whose areas can be calculated. Where the area changes linearly, a suitable average may work, but document the endpoints. A uniform-section tool cannot infer that a driveway opening changes the detail.
What the two-rectangle model excludes
Battered faces, channels with curved profiles and chamfers are not automatically represented. Calculate the area of those features from the detail if they materially change the volume, or use the approved cross-sectional area supplied by the designer.
Reinforcement, joints, dowels, forms, bedding and drainage connections are separate quantities. A volume result does not determine curb height, accessibility at a crossing or drainage performance. Those requirements define the inputs rather than emerge from them.
Order the complete pour once
Add the net volumes of all sections receiving the same mix during the same delivery. Apply an explicit allowance to that total, then use the supplier's stated purchase increment. Keep that rounding separate from a cutting or placement allowance so the estimate remains explainable.
Ask whether the material quote includes delivery, small-load charges, pump or other placement equipment. Compare quotes for the same total and mix specification. If the contractor quotes by linear metre, retain the section dimensions because a wider profile is not an equivalent metre of work.
Do it in order
- Decompose the approved section into non-overlapping measured areas.
- Multiply each area by its corresponding run length and sum net volume.
- Add a documented allowance and apply the supplier's order increment once.
Planning boundary
The calculator is a geometric two-rectangle model, not a municipal curb detail or drainage specification.
Sources checked
Does curb reveal include the concrete below the gutter?
Not necessarily. Inspect the detail and ensure all concrete appears exactly once in your chosen rectangles.
Can I use one section for a dropped crossing?
Only if it truly has that section. Measure the transition and changed profile separately.