Concrete Calculator
Work out how much concrete a slab, footing or pad needs — in cubic yards, cubic metres and whole bags.
How to use this calculator
- 1Measure the area. For an irregular space, split it into rectangles, work each out, and add them.
- 2Set the unit you measured in — the calculator converts everything for you.
- 3Check the coverage figure against the bag or tin you are actually buying; it varies between brands.
- 4Leave the waste allowance on. Running short mid-job costs far more than one spare unit.
How the calculation works
Units = ⌈ (Area × Depth × (1 + waste)) / Volume per unit ⌉- Area
- Surface area of the job
- Depth
- How thick the material is laid
- waste
- Allowance for cuts, breakage and spillage
- ⌈ ⌉
- Round up — you cannot buy a fraction of a unit
Every dimension is converted to metres internally, so feet, yards and metres can be mixed with inches or centimetres without error.
Ready-mix is ordered by volume (cubic yards or metres); bagged concrete is bought by count. Both figures are shown because the right one depends on job size.
Above roughly 1 cubic yard, ready-mix is almost always cheaper and far less work than mixing bags by hand.
Worked example
A 20 × 12 ft area
- 1.20 ft × 12 ft = 240 sq ft, which is 22.30 m².
- 2.At the depth set above, that gives the volume shown in the result.
- 3.A 10% waste allowance is added, then the figure is rounded up to whole bags.
Result: See the result panel — it updates live as you change the numbers.
What concrete actually is
Concrete is a composite material, not a single product: it is cement, aggregate (sand and crushed stone or gravel), and water, mixed in careful proportions and left to harden. Cement itself is just the fine grey powder that acts as the binder — a common mix-up is treating "cement" and "concrete" as the same thing, when cement is only one ingredient of several by weight.
Concrete doesn’t harden by drying out the way paint or glue does. It hardens through hydration, a chemical reaction between cement and water that binds the aggregate into a solid mass. That reaction keeps going for weeks after a pour looks and feels solid, which is why freshly poured concrete needs time — and often deliberate moisture — before it is loaded or built on.
Ready-mix versus bagged concrete
Which one makes sense for a job is mostly a question of volume.
- Bagged concrete — pre-blended cement, sand and aggregate sold in 40, 60 or 80 lb bags, mixed on site with water. Convenient for small jobs — post footings, a stepping-stone path, a patch — but slow and physically demanding at any real scale.
- Ready-mix — batched at a plant and delivered by truck, already wet and ready to pour. It costs more to arrange for a small job, usually with a minimum load, but for anything above roughly a cubic yard it’s almost always cheaper per unit and dramatically less labour.
- Fibre-reinforced mixes — have synthetic or steel fibres blended through the concrete to resist cracking, often used for driveways and slabs that will see vehicle loads.
- High-early-strength mixes — use a different cement blend to gain usable strength faster, useful when formwork needs to come off or the area needs traffic sooner than a standard mix allows.
Planning a pour
A concrete pour is unforgiving of skipped steps, because once the truck arrives — or the bags are mixed — there is no pausing to fix the ground underneath.
- 1Excavate and prepare the subgrade — removing soft or organic soil and compacting a stable base, so the slab doesn’t settle unevenly later.
- 2Build formwork — boards staked around the perimeter to hold the wet concrete to the right shape and thickness until it sets.
- 3Add reinforcement if the job calls for it — rebar or wire mesh, positioned before the pour — steel added afterward does nothing structurally.
- 4Pour and screed — fill the form, then drag a straight board across the top to strike off the excess and level the surface.
- 5Finish the surface — float, then trowel or broom, depending on how smooth or slip-resistant the final surface needs to be.
- 6Cure — keep the surface damp or covered for at least the first several days so hydration completes properly instead of drying out too fast, which weakens the finished slab.
Where estimates commonly go wrong
Concrete is one of the more unforgiving materials to underestimate, because a partial truck delivery or a second small batch rarely matches the first in colour or strength.
- Ignoring the waste margin — spillage, an uneven subgrade, and formwork that isn’t perfectly square all eat into the theoretical volume. Trade practice is to add a margin rather than order the bare calculated amount.
- Treating footings as the same depth as the slab — footings and thickened edges are often poured deeper than the field of the slab, and need to be estimated separately.
- Forgetting ready-mix trucks have a minimum order — small jobs that fall well under a full load may still be billed for one, or charged a short-load fee — worth checking with the supplier before assuming the per-yard price applies.
What this assumes, and where it stops
Assumptions
- The area is flat and regular as described. Slopes, steps and obstructions change the quantity.
- Coverage figures are the manufacturer’s stated values, which assume good conditions and an even surface.
- The pour is a uniform thickness over the whole area.
Limitations
- This estimates material only — not labour, tools, delivery, hire or disposal.
- Manufacturer coverage varies between brands and batches. Always check the packaging.
- Does not size reinforcement, calculate rebar, or account for footings thicker than the slab.
- Structural concrete work should be specified by an engineer, not a calculator.
Common questions
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb, or 60 bags of 60 lb. That is why mixing more than about a cubic yard by hand is rarely worth it — a ready-mix delivery is cheaper and takes a fraction of the time.
How thick should a concrete slab be?
A common guide is 4 inches (100 mm) for patios, paths and shed bases, and 5–6 inches (125–150 mm) where vehicles will drive on it. Local building rules and ground conditions override any rule of thumb.
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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