Concrete Calculator

Estimate how much concrete you need for a slab, footing, square column, or round column. Choose m³ or yd³ for the volume result, plus the exact bag count for 40 lb, 60 lb, 80 lb, or 25 kg pre-mix bags — with a built-in waste buffer so you don't run out mid-pour.

Examples

Patio slab — 4 × 3 m × 10 cm

A 4 m × 3 m × 0.10 m patio slab with 10% waste, 80 lb bags.

Shape: Slab
Length: 4
Width: 3
Thickness: 0.1
Depth: 0.3
Height: 2.5
Radius: 0.15
Length Units: metres (m)

Volume Needed (with waste)

1.32 m³

Raw Volume (no waste)

1.2 m³

Bags Needed

78 80 lb bags

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How It Works

Formula

V_{\text{slab}} = L \cdot W \cdot T

V_{\text{footing}} = L \cdot W \cdot D

V_{\text{column}} = W \cdot D \cdot H

V_{\text{round}} = \pi r^2 H

V_{\text{pour}} = V \cdot \left(1 + \tfrac{w}{100}\right)

N = \lceil V_{\text{pour}} / y \rceil

Variables, symbols and units

$L$: Length
$W$: Width (or column side)
$T$: Slab thickness
$D$: Depth (footings) or column side
$H$: Column height
$r$: Round-column radius
$V$: Raw volume(m³)
$w$: Waste percent
$y$: Yield per bag(m³)
$N$: Bags needed (rounded up)

Calculation method explained

Pick the shape and enter your dimensions in your preferred units. The calculator converts everything to metres, applies the standard volume formula for that shape, multiplies by 1 + waste/100 to get the pour quantity, converts to your selected volume unit, and divides by the per-bag yield (rounded up to whole bags).

References and source material

Examples

Patio slab — 4 × 3 m × 10 cmSlab · 4 m → 1.32 m³

A 4 m × 3 m × 0.10 m patio slab with 10% waste, 80 lb bags.

Shape: Slab
Length: 4 m
Width: 3
Thickness: 0.1
Depth: 0.3
Height: 2.5
Radius: 0.15
Volume Units: cubic metres (m³)

Volume Needed (with waste): 1.32 m³

Strip footing — 10 m × 0.4 m × 0.5 mFooting · 10 m → 2.2 m³

A 10 m strip footing 0.4 m wide, 0.5 m deep, 10% waste, 25 kg bags.

Shape: Footing
Length: 10 m
Width: 0.4
Thickness: 0.1
Depth: 0.5
Height: 2.5
Radius: 0.15
Volume Units: cubic metres (m³)

Volume Needed (with waste): 2.2 m³

Round column — r 0.15 m × h 2.5 mRound Column · 4 m → 0.19 m³

A cylindrical column, radius 0.15 m, height 2.5 m, 10% waste, 60 lb bags.

Shape: Round Column
Length: 4 m
Width: 3
Thickness: 0.1
Depth: 0.3
Height: 2.5
Radius: 0.15
Volume Units: cubic metres (m³)

Volume Needed (with waste): 0.19 m³

Frequently Asked Questions

How much waste should I add?

10% is the standard buffer for spillage, over-excavation, and small over-pours. Bump it to 15% for awkward shapes, hand-mixing on a windy site, or first-time DIY pours. Drop to 5% only when you can return unused ready-mix or are mixing bags one at a time.

Which bag size is best?

80 lb (or 25 kg) bags give the best yield-per-bag and the lowest cost per cubic metre, but they are heavy. 60 lb bags are easier to carry and mix in a wheelbarrow. 40 lb bags are for very small patches. Above ~1 m³ (1.3 yd³), bagged concrete starts losing on price and labour to a ready-mix truck.

How do I measure for a slab?

Length × width is the footprint. Thickness is the depth of the pour, not the gravel base. A typical patio is 4 in (10 cm); a driveway 4–6 in; a garage floor 4–6 in over a compacted gravel sub-base. Measure twice — concrete is unforgiving once it sets.

What does 1 yard of concrete weigh?

1 cubic yard of standard concrete weighs roughly 4,050 lb (about 1,840 kg) at a typical 150 lb/ft³ density. 1 cubic metre weighs about 2,400 kg. Plan transport, formwork, and sub-base compaction for that load.

Should I order extra concrete?

Yes. A small buffer helps cover spillage, uneven subgrade, and small form changes on site. Many pours add 5-10% on top of the measured volume, especially for footings, posts, and jobs where the base is not perfectly level.

Concrete Calculator

Patio slab — 4 × 3 m × 10 cm

How It Works

Examples

Frequently Asked Questions

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