Concrete Calculator — Volume, Bags & Mix

Concrete Volume Calculator

How to Calculate Concrete Volume

Calculating concrete volume is the first step in any pour — whether you are ordering ready-mix delivery or buying bags from a hardware store. The formula depends on the shape of your element, but the principle is always the same: multiply the three dimensions to get the volume, add a waste allowance, then convert to the units your supplier uses. The calculator above does all of this instantly, but understanding the maths behind it helps you spot mistakes and order with confidence.

Rectangular Slab or Footing

For a rectangular slab, footing, or wall the formula is Volume = Length × Width × Thickness. All three dimensions must be in the same unit before you multiply. For example, a slab that is 5 metres long, 3 metres wide, and 100 mm (0.1 m) thick has a volume of 5 × 3 × 0.1 = 1.5 m³. In imperial units, a 10 ft × 10 ft patio at 4 inches thick works out to 10 × 10 × (4 ÷ 12) = 33.3 ft³, which is 1.23 cubic yards.

Round Column or Pier

For a circular column, pier, or sonotube the formula is Volume = π × radius² × height. The radius is half the diameter. A 300 mm diameter column that is 600 mm tall has a volume of π × 0.15² × 0.6 = 0.042 m³ per column. For multiple identical columns, multiply by the quantity — or simply enter the quantity in the calculator.

Converting Between Volume Units

If your drawings mix metric and imperial (a common situation in Australia, where both systems are used on site), convert everything to one system first:

• 1 m³ = 35.3147 ft³
• 1 m³ = 1.30795 yd³
• 1 ft³ = 0.0283168 m³
• 1 yd³ = 0.764555 m³
• 1 yd³ = 27 ft³

The calculator displays results in m³, ft³ and yd³ simultaneously so you can order in whichever unit your supplier uses.

Adding a Waste Allowance

Concrete rarely goes exactly where you plan. Spillage, uneven sub-base, over-excavation and formwork gaps all eat into your supply. The industry standard is to add 5–10% extra:

• 5% — for a well-prepared site with accurate formwork and experienced pourers.
• 10% — for DIY projects, uneven ground, or when mixing bagged concrete by hand.

For our 1.5 m³ example with a 5% waste allowance: 1.5 × 1.05 = 1.575 m³. Always round up — running short mid-pour creates a cold joint that permanently weakens the slab.

Bag Count Reference Table

Use this table to estimate how many bags of pre-mixed concrete you need per cubic metre or cubic yard. Yields are typical for standard pre-mixed bagged concrete; always verify against your supplier's stated yield.

Bags per m³ = 1 ÷ yield. Bags per yd³ = bags per m³ × 0.764555. Figures are rounded to the nearest whole bag.

Worked Examples

Example 1 — Shed Slab (Metric)

Example 2 — Patio Slab (Imperial)

Example 3 — Round Deck Footings (Metric)

Example 4 — Strip Footing (Metric)

Example 5 — Driveway Slab (Imperial)

Concrete Density by Type

Density matters for two reasons: it determines the weight of the pour (important for truck load limits and dead-load calculations), and it affects bag yield — denser mixes produce slightly less volume per bag than lighter ones.

Concrete Mix Ratios Explained

A concrete mix ratio expresses the proportion of cement, sand, and aggregate (coarse + fine) by volume. Choosing the right strength grade depends on what the slab or element will carry. The calculator's mix design option computes the exact quantities of cement, sand, aggregate and water for your total volume at each grade.

The calculator applies a dry-material bulking factor of 1.54 — this accounts for the voids between dry ingredients so you order enough sand and aggregate. For a deeper dive, read our complete guide to concrete mix ratios.

Saw Cuts and Control Joints

As concrete cures it shrinks, and shrinkage causes cracking. Control joints (saw cuts) create planes of weakness so the slab cracks neatly along the joint instead of randomly across the surface. Two rules of thumb govern joint layout:

• Spacing: place joints 24–36 times the slab thickness apart. For a 100 mm slab that is 2.4–3.6 m between joints.
• Depth: cut to roughly one quarter of the slab thickness. For a 100 mm slab, cut 25 mm deep.

Timing matters too — cut as soon as the concrete can withstand the saw without raveling the edges, usually within 4–12 hours after finishing in warm weather, or 12–24 hours in cold weather. The calculator's saw-cut option computes both depth and spacing for your slab thickness automatically. Learn more in our saw cuts guide.

Ordering Tips

• Ready-mix minimums: Most plants have a minimum delivery (typically 1–3 m³) and charge short-load fees for smaller orders. A job that calculates as 0.5 m³ may cost as much as a full 1 m³ load.
• Always add waste: Add 5–10% to the theoretical volume. Footings poured against trench walls lose material to formwork gaps and over-excavation. Running short mid-pour creates a cold joint.
• Round up bag counts: Bag yields on the label are slightly optimistic — hand-mixing loses a little volume. Round up and keep a spare bag for patch-up.
• Plan for pump capacity: If pumping, check the weight against pump limits. A 2 m³ pour at 2,400 kg/m³ weighs 4.8 tonnes.
• Order by volume, not bags: For ready-mix, order in m³ or yd³. For bags, order by count with the waste allowance already included.
• Allow for sub-base: Your excavation depth is not the slab thickness; it is slab thickness plus sub-base (typically 50–75 mm of compacted gravel).

Guides & In-Depth Tutorials

Step-by-step guides that take the maths from the calculator and apply it to real projects. Each guide includes worked examples, diagrams, and common mistakes to avoid.

Frequently Asked Questions

How much concrete do I need for a slab?

Multiply length × width × thickness for the volume, then add a 5–10% waste allowance. A 5 m × 3 m slab at 100 mm needs 1.5 m³ before waste, or 1.575 m³ with 5% allowance. Enter your dimensions into the calculator above for an instant figure in m³, ft³ and yd³.

How many bags of concrete are in a cubic metre?

Approximately 100 × 20 kg bags, 83 × 25 kg bags, 67 × 30 kg bags, or 50 × 40 kg bags per cubic metre of normal-weight concrete. See the bag count reference table above for all bag sizes including imperial 60 lb and 80 lb.

How many 80 lb bags of concrete are in a cubic yard?

Roughly 45 × 80 lb bags per cubic yard. Each 80 lb bag yields about 0.60 ft³, and one cubic yard equals 27 ft³ (27 ÷ 0.60 = 45). For 60 lb bags, you need about 60 bags per cubic yard.

What is the density of concrete?

Normal-weight concrete has a density of about 2,400 kg/m³ (150 lb/ft³) per ACI 318. Lightweight concrete is around 1,762 kg/m³ (110 lb/ft³) per ACI 213R. Pre-mixed bagged concrete typically has a density of about 2,130 kg/m³ (133 lb/ft³).

Should I add a waste allowance to my concrete order?

Yes. Add 5% for a well-prepared site with accurate formwork and experienced pourers, and 10% for DIY projects, uneven ground, or hand-mixing bagged concrete. The waste allowance covers spillage, over-excavation, sub-grade irregularity and formwork gaps. The calculator includes a waste-allowance selector (0%, 5% or 10%).

How do I calculate concrete for a round column or sonotube?

Use the formula π × radius² × height. For a 300 mm diameter column that is 600 mm tall: π × 0.15² × 0.6 ≈ 0.042 m³ per column. Multiply by the number of columns, or enter the quantity in the calculator and it handles the total automatically.

Is it cheaper to use bagged concrete or ready-mix delivery?

For pours under about 1 m³ (roughly one cubic yard), bagged concrete is usually cheaper because ready-mix plants charge minimum-delivery and short-load fees. For pours over 1–2 m³, ready-mix is typically cheaper per cubic metre and far less labour-intensive. A useful threshold: at 60+ bags of 20 kg, seriously consider ready-mix.

What concrete mix ratio should I use?

It depends on the application. A common general-purpose mix is 1:2:3 (cement:sand:aggregate) for 25 MPa concrete. For shed slabs use 20–25 MPa, for driveways 32 MPa, and for heavy-duty or exposed slabs 40 MPa. The calculator's mix design option gives exact cement, sand and aggregate quantities for each strength grade. See our mix ratios guide for the full breakdown.

How far apart should saw cuts (control joints) be in a concrete slab?

Control joints should be spaced 24–36 times the slab thickness, and cut to a depth of roughly one quarter of the slab thickness. For a 100 mm (4 inch) slab, space joints 2.4–3.6 m apart and cut 25 mm deep. Timing is also important — cut as soon as the concrete won't ravel at the edges, usually 4–24 hours after finishing. The calculator's saw-cut option computes depth and spacing for you.

How do I calculate concrete on sloping or uneven ground?

Measure the depth at all four corners and take the average. Multiply the plan area (length × width) by the average depth to get the volume. The calculator's graded-land mode does this automatically — enter the four corner depths and it calculates the average and the volume. See our graded land guide for the full method.

How do I convert between cubic metres, cubic feet and cubic yards?

1 m³ = 35.3147 ft³ = 1.30795 yd³. To convert cubic feet to cubic yards, divide by 27. To convert cubic yards to cubic metres, multiply by 0.764555. The calculator displays results in all three units at once so you can order in whichever unit your supplier uses.

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