Estimate concrete volume and the materials needed for a slab or similar project.
Explore other useful tools
Rectangular slab volume, materials, and bag counts for patios, shed bases, and driveways.
Cylinder-volume estimator for fence, deck, and mailbox post holes.
Estimate concrete for strip and pad foundation footings.
Convert between cubic metres, litres, cubic feet, gallons, and more.
This calculator estimates the volume of concrete needed and gives a rough breakdown of cement, sand, gravel, and water based on the selected mix ratio.
Structural requirements vary by project, load, ground conditions, reinforcement, and local code rules. For structural work, the correct specification should always come from the relevant design or site requirements.
This concrete calculator is designed to estimate the amount of concrete needed for a project and provide an approximate breakdown of the main ingredients. It is useful for early planning, budgeting, and deciding whether a job is suitable for mixing on site or ordering as ready-mix.
The calculator focuses on a simple slab-style volume estimate, then applies a selected mix ratio to estimate cement, sand, gravel, and water.
For a rectangular slab or section, concrete volume is usually calculated as: length × width × depth.
In metric, entering all dimensions in metres gives a result in cubic metres. In imperial, cubic feet can be converted into cubic metres for the materials estimate.
This is a useful starting point for patios, shed bases, garage floors, simple footings, and other regular shapes.
The calculator above handles the rectangular slab case directly: enter length, width, and thickness in metric or imperial, and it returns the volume plus an estimated cement, sand, gravel, and water breakdown. For a slab-only tool with thickness presets, bag counts at 20/25/40 kg and 60/80 lb sizes, and a waste allowance, try the dedicated Concrete Slab Calculator.
Typical slab thicknesses give a useful sanity check. Footpaths and patios are commonly 75–100 mm thick. Shed and garage bases sit around 100–150 mm. Domestic driveways are usually 150 mm or more, depending on expected loads and ground conditions.
Reinforcement — mesh sheets, rebar, or fibres — is project-specific and is not estimated by this calculator.
When metric units are selected, the volume output is in cubic metres (m³). One cubic metre is 1 m × 1 m × 1 m, or roughly 35.31 cubic feet.
If imperial units are selected, the calculator takes feet for length, width, and depth and converts internally to m³ using the factor 0.0283168, then derives the materials in pounds and gallons.
Footings come in two common shapes. A strip footing is long and narrow — typically 450 mm wide × 200–300 mm deep — and is estimated as length × width × depth. A pad footing is square or rectangular and used for posts and columns; a small pad often measures around 600 × 600 × 300 mm.
Footing depth, width, and reinforcement are normally governed by ground conditions, expected loads, and local building regulations. Use the calculator for a materials estimate, not as a substitute for a designed footing schedule. The dedicated Concrete Footing Calculator has a strip / pad toggle, a multi-pad count, and a footing-specific reference table.
The volume of concrete needed for a post hole is the hole depth multiplied by its cross-section. A round hole uses π × radius² × depth; a rectangular hole uses length × width × depth.
As a rough guide, hole depth is often about ⅓ of the post height and hole diameter about 3× the post width. Total job volume is the volume per hole × the number of holes, minus the volume the post itself occupies if that is significant. For true cylinder math with a round/square toggle, multi-hole totals, and a bag-size selector, use the dedicated Concrete Post Hole Calculator.
The calculator above returns concrete volume and approximate dry-weight totals. Bagged concrete is usually sold by yield per bag instead, so the table below gives a rough bag-count reference for common bag sizes.
| Bag size | Approx. yield | Bags per m³ |
|---|---|---|
| 20 kg bag | ~0.010 m³ | ~100 |
| 25 kg bag | ~0.0125 m³ | ~80 |
| 40 kg bag | ~0.018 m³ | ~55 |
| 60 lb (US, ~27 kg) | ~0.0127 m³ | ~79 |
| 80 lb (US, ~36 kg) | ~0.017 m³ | ~59 |
Bag yields are manufacturer-specific — always check the bag.
Aggregate — the sand and gravel — makes up the bulk of a concrete mix, typically 60–75% of the total volume. The sand and gravel rows in the materials output are derived from the selected mix ratio.
As an example, a standard 1:2:3 mix is 1 part cement, 2 parts sand, and 3 parts gravel by volume. For 1 m³ of mixed concrete that gives roughly ~250 kg cement, ~500 kg sand, and ~840 kg gravel, plus water.
The densities used internally are 1440 kg/m³ for cement, 1600 kg/m³ for sand, and 1680 kg/m³ for gravel.
Real projects usually need slightly more concrete than the pure geometric volume suggests, because of:
How much to add: typically 5–10% for regular slabs and patios, and 10% or more for irregular trenches, uneven ground, or harder-to-place pours. As a quick example: need 0.9 m³? Order ~1.0 m³ to give a ~10% buffer.
Ready-mix is sometimes priced and supplied in fixed minimum loads — see the mixing vs ready-mix section below.
Patio or shed base slab
Fence post hole
Strip footing
Cubic metres to concrete bags
Concrete is made from cement, sand, coarse aggregate, and water. The mix ratio describes the relative parts of the dry ingredients. Changing that ratio affects the expected strength, workability, and suitability for different jobs.
The right mix is only one part of the job. Ground preparation, reinforcement, curing, and thickness are also important.
Water makes concrete workable, but too much can reduce strength. Too little can make placement and finishing difficult. This calculator uses a practical estimate, but site conditions and material moisture can affect what is actually needed.
A common mistake is adding extra water just to make the mix easier to place. That can weaken the final concrete.
Small pours can often be handled with bagged materials or on-site mixing. Larger projects may be easier, faster, and more consistent with ready-mix delivery.
A simple slab estimate is length × width × depth. In metric this gives cubic metres if all dimensions are entered in metres.
Real projects often need a little extra for uneven ground, spillage, over-excavation, and finishing losses. Many people add a small waste allowance.
It depends on the job. Patios, paths, general slabs, and more demanding structural work can use different mix ratios and specifications.
No. It is a materials estimate tool for planning purposes, not a substitute for engineering design or local code requirements.
Small jobs may be mixed on site, while larger pours are often easier and more consistent with ready-mix delivery.
Approximately 80 × 25 kg bags or 100 × 20 kg bags per m³, depending on bag yield. For US-style bags, around 79 × 60 lb bags or 59 × 80 lb bags per m³. Bag yields are manufacturer-specific, so always check the bag.
Volume ≈ π × radius² × depth for a round hole, or length × width × depth for a rectangular hole. Total concrete needed is the volume per hole multiplied by the number of holes.
A common rough guide is hole depth ≈ ⅓ of the post height, with diameter ≈ 3× the post width. Structural requirements, ground conditions, and local building regulations override this rule of thumb.
Typically 5–10% for regular slabs and patios, and 10% or more for irregular trenches, uneven ground, or awkward pours.
Cement is the binder. Concrete is cement combined with sand, gravel, and water. Estimating concrete volume is not the same as estimating cement alone.
The on-page output is in cubic metres. Multiply m³ by 1.308 to convert to cubic yards (or divide by 0.7646).
For connected calculations, see the Volume Converter, Area Converter, Length Converter, and Weight Converter.
This page is designed for estimating materials and comparing options. It is not a structural design tool, and large or load-bearing work should always follow the relevant specification, supplier guidance, or professional advice.