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Arc / ring slabm³ / yd³Free

Curved Slab
Concrete Volume

Concrete volume for a curved slab — an annular ring or an arc segment of one. Enter the inner and outer radii, the included angle, and the slab thickness. Use 360° for a full ring or a smaller angle for a sector (a curved walkway, ramp, or balcony).

Who it's for: Architects, builders, and contractors estimating concrete for a curved walkway, balcony, or ring foundation — you need the inner and outer radii, the arc angle, and the slab thickness before pouring at construction.

Annular sector × thickness
Plan area = (θ/360) × π × (R² − r²). Volume = area × thickness. R = outer radius, r = inner radius, θ = included angle. For a full ring use θ = 360°.
Curved Slab
Volume

How to use this calculator

1
Choose units
Select metres (m³ output) or feet (ft³ and yd³). All radius and thickness inputs must use the same unit.
2
Enter the outer radius R
The horizontal distance from the centre of the arc to the outer edge of the slab in plan.
3
Enter the inner radius r
The distance from the centre to the inner edge. For a curved walkway this is the inner kerb radius; set to 0 for a solid disc sector with no central void.
4
Set the included angle
Use 360° for a full ring. Enter the arc angle in degrees for a curved walkway, balcony, or ramp — commonly 90°–270°.
5
Enter slab thickness and waste %
Use the uniform vertical depth from the project drawings. Set the editable waste allowance from the supplier’s ordering rules and the contractor’s placement plan.
6
Read the total volume
The result shows plan area, net volume, and the waste-adjusted total — the quantity to order from your concrete supplier.

The formula

The plan area of an annular sector scales with the difference in squared radii; multiply by thickness and the selected allowance factor to get the displayed total.

Volume (with allowance) = (θ / 360) × π × (R² − r²) × t × (1 + waste / 100)
R = outer radius
r = inner radius
θ = included angle in degrees (360 for a full ring)
t = uniform slab thickness
waste = user-entered estimating allowance, not a code-prescribed value

Worked example

Example
A semicircular curved walkway has an outer radius of 5 m and inner radius of 4 m, spanning 180° with a 200 mm slab thickness. Plan area = 0.5 × π × (25 − 16) = 14.137 m². Net volume = 14.137 × 0.2 = 2.827 m³. Adding 7% waste gives a total order volume of 3.025 m³.

When this estimate will be off

  • Assumes constant slab thickness — a sloped curved ramp needs a separate calculation per section using the average thickness.
  • Does not account for edge beams, drop panels, or thickened edges within the slab footprint; add those volumes separately.
  • Plan area formula treats the slab as a flat horizontal surface. Inclined slabs such as ramps have a larger inclined area and require a slope factor applied before multiplying by thickness.
  • Does not include reinforcement, formwork, subbase, or any material other than the concrete pour volume.

Frequently asked questions

A common mistake: Assumes constant slab thickness — a sloped curved ramp needs a separate calculation per section using the average thickness.

A full ring uses θ = 360° and computes the annular area between the two radii. A sector is a pie-slice portion — 180° is a half-ring, 90° is a quarter-ring. Curved walkways, balconies, and ramps typically range from 45° to 270°.

The 7% default is an editable estimating assumption, not a code requirement or guaranteed order factor. Set the percentage using the supplier’s minimum-order and return rules plus the contractor’s experience with the actual formwork, placement method, spillage, and overpour risk. The calculator preserves the net volume for comparison.

Not directly. The formula gives the plan (horizontal projection) area, not the inclined surface. For a ramp with slope angle α, multiply the plan area by 1/cos(α) before multiplying by thickness. Use the average thickness measured perpendicular to the slab face.

Setting inner radius r = 0 turns the annular sector into a solid disc sector. The formula reduces to (θ/360) × π × R² × thickness. This gives the correct volume for a circular pad or a wedge-shaped feature with no central void.

Sources

  • Calculation basis: elementary solid geometry — The slab is modeled as an annular-sector plan area multiplied by uniform thickness. No controlling building standard governs this volume arithmetic.
  • Implemented calculator method — The page calculates (angle ÷ 360) × π × (outer radius² − inner radius²) × thickness, then applies the user-entered percentage allowance. It does not size or structurally check the slab.

This free Curved Slab Concrete Volume Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.