Slab Area Concrete Calculator

When the slab footprint isn’t a clean rectangle, the easiest path is to compute the area separately — by hand for simple shapes, or in plan from a CAD drawing — and feed that area plus the slab depth into this calculator. It’s how the iOS app’s Slab Area tool works, and it scales to any irregular shape from a curved patio to an L-shaped extension.

When to use this calculator

Reach for the area-based calculator when the slab is:

Irregular — bay-window steps, shaped patios, organic curves
Composite — an L-shape or T-shape made of two or more rectangles
Already measured — you have the area from plans, CAD, or a quantity surveyor’s take-off
Curved — pours with rounded edges where breaking down rectangles isn’t practical

For clean rectangles, the Rectangular Slab Calculator is faster.

Calculating area for common irregular shapes

L-shape: split into two rectangles and sum them.

T-shape: split into a long rectangle (the bar of the T) and a short rectangle (the stem); sum.

Right triangle: (base × height) ÷ 2.

Half-circle / semicircle: (π × radius²) ÷ 2.

Trapezoid (one pair of parallel sides): ((a + b) ÷ 2) × height, where a and b are the parallel side lengths and height is the perpendicular distance between them.

For a free-form curve, mark stations every 500 mm along the long axis, measure the perpendicular width at each, multiply by the station spacing and sum. Or treat it as a polygon with one vertex per measurement point.

How the calculation works

Volume is area × depth. The calculator converts area to square millimetres and depth to millimetres internally, multiplies for cubic millimetres, then converts to your chosen output unit:

Metric: cubic metres (m³)
Imperial: cubic yards (yd³) and cubic feet (ft³)

A 10% buffer is added per ACI 318. Order to the buffered figure.

Worked example

A 150 m² coffee shop floor pour at 125 mm depth:

150 × 0.125 = 18.75 m³
with 10% buffer = 20.6 m³

That’s a full pump-truck job, not bagged concrete. Plan the access, the pour sequence, and the screed crew before the truck arrives.

On the jobsite

The Concrete Calculator app for iOS has this calculator plus four others (rectangular slabs, circular slabs, circular columns and post holes), saved projects, and full offline support. Get it on the App Store.

Frequently asked questions

When should I use the Slab Area calculator instead of Rectangular?

Use this when the slab footprint isn't a clean rectangle — irregular outdoor patios, L-shaped garage extensions, paths with curves, or any pour where you've already done the area math on paper or in CAD.

How do I measure an irregular area?

Break the shape into simpler pieces — rectangles, right triangles, half-circles — calculate each piece's area, then add them up. For curved edges, treat the curve as straight line segments and accept a small under-count (the 10% buffer covers it).

Why does this calculator use square metres for area?

Because slab pours are big — even a small irregular patio is several square metres. Metric depth is also in metres for the same reason. If you're working in cm, divide by 100 (e.g. 15 cm = 0.15 m).

What if I only know area in square feet but want metric output?

Stay in imperial mode and the result will show metric values alongside (m³). Or convert: 1 ft² = 0.0929 m². 1 m² = 10.764 ft².

How accurate is the result?

As accurate as your area measurement. The calculation itself is exact — area × depth — but irregular areas are usually under-measured because curves are approximated as straight lines. Always order with the 10% buffer.