How much concrete do I need for a slab?

Multiply length × width × thickness in metres. A 4 m × 3 m slab at 100 mm thick is 1.2 m³ of concrete. Add 5–10% overage for uneven subgrade and spillage before ordering.

How much extra concrete should I order?

Order 5–10% more than the net volume. Small pours run closer to 10% because uneven excavation, form deflection and spillage take a bigger relative bite. Running short on pour day costs far more than a small surplus.

How do you calculate concrete for round footings?

Use π × r² × depth. A 400 mm footing that is 800 mm deep needs π × 0.2² × 0.8 ≈ 0.10 m³ per footing. Multiply by the number of footings, then add overage.

How heavy is a cubic metre of concrete?

Normal-weight structural concrete is about 2,400 kg per cubic metre. The calculator estimates total weight from the density you enter, which matters when checking deck and balcony loads.

Does the concrete calculator work in feet and inches?

Yes. Switch the demo to IN to enter dimensions in inches: volumes come back in cubic yards and weight in pounds. One cubic metre equals 1.31 cubic yards.

Concrete Calculator for Slabs, Footings and Columns

How this concrete calculator works

Each pour shape uses its own volume formula, all taken straight from the StructCalc core that powers the Android app:

Slab: length × width × thickness
Round footing: π × r² × depth
Circular slab: π × r² × thickness
Tube column: π × (R² − r²) × depth
Curb & gutter: length × (curb cross-section + gutter cross-section)
Concrete stairs: stepped prism volume for the riser count, plus the platform

The net total multiplies the per-item volume by the quantity. The order volume adds your overage percentage on top — the figure you actually order. Weight comes from the density, which defaults to 2,400 kg/m³ for normal-weight structural concrete.

Common concrete volumes

Reference volumes for typical pours. Net values — add 5–10% overage before ordering.

Pour	Dimensions	Volume
Shed slab	3 × 3 m @ 100 mm	0.90 m³
Patio slab	5 × 4 m @ 100 mm	2.00 m³
Garage slab	6 × 4 m @ 100 mm	2.40 m³
House slab	12 × 8 m @ 150 mm	14.4 m³
Round footing	Ø 400 mm × 800 mm deep	0.10 m³ each
Round footing	Ø 600 mm × 1200 mm deep	0.34 m³ each
Tube column	Ø 400/300 mm × 1200 mm deep	0.066 m³ each

Worked example

A garden shed slab of 4 m × 3 m at 100 mm thick: 4 × 3 × 0.1 = 1.20 m³ net. Ordering with 10% overage gives 1.32 m³, roughly 2,880 kg of concrete. Enter the same numbers in the demo above and the order volume matches — same core, same result.

Concrete calculator FAQ

How much concrete do I need for a slab?: Multiply length × width × thickness in metres. A 4 m × 3 m slab at 100 mm thick is 1.2 m³ of concrete. Add 5–10% overage for uneven subgrade and spillage before ordering.
How much extra concrete should I order?: Order 5–10% more than the net volume. Small pours run closer to 10% because uneven excavation, form deflection and spillage take a bigger relative bite. Running short on pour day costs far more than a small surplus.
How do you calculate concrete for round footings?: Use π × r² × depth. A 400 mm footing that is 800 mm deep needs π × 0.2² × 0.8 ≈ 0.10 m³ per footing. Multiply by the number of footings, then add overage.
How heavy is a cubic metre of concrete?: Normal-weight structural concrete is about 2,400 kg per cubic metre. The calculator estimates total weight from the density you enter, which matters when checking deck and balcony loads.
Does the concrete calculator work in feet and inches?: Yes. Switch the demo to IN to enter dimensions in inches: volumes come back in cubic yards and weight in pounds. One cubic metre equals 1.31 cubic yards.

Need this on site?

StructCalc for Android includes concrete, stair, rafter, baluster, raked wall and material calculators with a faster jobsite workflow. Use the web demo for a quick number, then use the mobile app when you need the full calculator set offline.

Get StructCalc on Google Play