How this concrete volume calculator works
A rectangular slab, pad or footing uses length × width × thickness. A circular column or post hole uses π × radius² × height or depth. The quantity field multiplies that volume for identical sections, such as several post holes or footings. If a plan already gives the total, choose known concrete volume and enter it directly. The calculator then applies the extra allowance selected by the user.
Metric dimensions are converted to cubic metres; imperial dimensions are converted to cubic yards while cubic feet remain visible as a useful secondary result. Bagged-mix quantity divides adjusted volume by the mixed yield per bag and rounds up to a whole bag. Ready-mix quantity rounds adjusted volume up to the purchasing increment entered. These are separate purchase models, not a claim that both prices include the same services.
Measure the formed or excavated space
Use finished internal form dimensions for a slab or formed footing. Measure excavations carefully when concrete will contact the ground: soil can collapse, the bottom may be uneven and an augered hole may not be perfectly cylindrical. Divide an irregular slab into rectangles, calculate each one, and add the volumes. Use known volume for that combined result.
Enter thickness or depth explicitly rather than relying on a generic project label. A calculator cannot select a safe slab, footing or column size. Loads, soil, frost, reinforcement, drainage and local building rules can change the design. Obtain an appropriate plan or professional advice where structural performance matters, then use this tool only to convert those approved dimensions into purchasing quantities.
Why bag yield matters more than bag weight
A 20 kg, 25 kg, 50 lb, 60 lb or 80 lb label describes dry mass, not a universal mixed volume. Yield varies with product formulation and permitted water content. Two bags with the same weight can publish different yields. Copy the mixed yield from the exact package or technical data sheet rather than assuming a conversion from kilograms or pounds.
The metric default of 10 litres is only a convenient editable example. It is not a statement about every 20 kg product. When a specification states cubic metres, multiply by 1,000 to obtain litres. In imperial mode, enter cubic feet per bag. QUIKRETE states that its published bag calculations are approximate and do not include uneven substrate or waste, reinforcing the need to use the actual product information and a project-specific allowance.
Choose an allowance instead of hiding one
The 5% starting value is visible and editable. An allowance can cover uneven ground, dimensional variation, spillage, residue left in equipment and normal handling loss. A carefully formed small slab may need a different margin from deep rough-sided holes or a large pour. Too little material can create a cold joint or delay; too much becomes costly surplus that must be handled responsibly.
Ask the ready-mix supplier or installer what margin fits the pour and how they handle returns or excess. The calculator applies the percentage once, before both bag and ready-mix rounding. It does not add a concealed safety factor and does not describe a selected percentage as guaranteed.
Compare whole bags with a rounded ready-mix order
Bagged mix is shown as a whole-bag count because partial retail bags are generally not a dependable purchasing unit. The ready-mix result rounds to the supplier increment you enter, such as 0.1 cubic metre or 0.25 cubic yard. Suppliers may also enforce a minimum order that is larger than this increment. If so, replace the calculated order with the supplier's minimum when budgeting.
Material-price results are intentionally separate. Bagged cost equals whole bags × price per bag. Ready-mix material cost equals rounded order volume × price per volume. Delivery, distance, short-load charges, waiting time, pumping, washout arrangements, tax and minimum fees can make the final invoices very different. Obtain actual quotes before deciding which option is economical or practical.
Understand the calculation results
| Result | What it means |
|---|---|
| Base concrete volume | The geometric volume of all entered shapes before any extra allowance. |
| Volume including allowance | Base volume increased once by the percentage you selected. |
| Whole bags needed | Adjusted volume divided by the product's entered mixed yield and rounded up. |
| Bagged mix cost | Whole bags multiplied by price per bag, with unentered charges excluded. |
| Rounded ready-mix order | Adjusted volume rounded up to the supplier increment entered. |
| Ready-mix material cost | Rounded volume multiplied by the entered unit rate, not a delivered quote. |
Do not use volume to choose a concrete mix
The required strength, exposure class, aggregate, slump, setting time and reinforcement belong to the project specification. This tool does not select or validate any of them. A bag labelled concrete mix is not interchangeable with mortar, cement alone, non-shrink grout or resurfacer. Match the product to the approved purpose and follow its mixing, placement, curing and temperature instructions.
Do not add extra water merely to make placement easier: follow the product or supplier limits. Water changes performance and yield. For ready mix, communicate access, placement method, pour schedule and site constraints to the supplier. For bagged concrete, confirm that mixing capacity and working time are realistic for the planned volume.
Concrete work and silica safety
Concrete, mortar, sand and stone can contain crystalline silica. OSHA explains that cutting, sawing, grinding, drilling or crushing these materials can create respirable crystalline silica dust associated with serious disease. Follow applicable local requirements, use suitable engineering controls and protective equipment, and read the product safety data sheet. Dry cutting or cleanup should never be treated as harmless because the project is small.
Wet concrete can also cause chemical burns, and heavy bags create handling risk. Protect skin and eyes, use safe lifting methods and keep the work area controlled. Excavations, forms, reinforcement, powered tools and structural pours can require trained professionals. The calculator supplies arithmetic only.
Current calculation and safety references
- QUIKRETE concrete calculator — rounds bag estimates up and warns that yields are approximate and exclude uneven substrate and waste.
- Sakrete concrete calculators — separates bagged-concrete calculations by project and package yield.
- Cement Australia calculators — includes slab, path and post-setting calculations in metric units.
- CEMEX ready-mix calculator — estimates ready-mix volume for slabs, footings and related shapes.
- OSHA crystalline silica overview — describes dust-generating concrete work, health risks and applicable controls.
Frequently asked questions
How many bags make one cubic metre or cubic yard?
There is no single answer without bag yield. Divide 1,000 litres by the metric yield per bag, or 27 cubic feet by the imperial yield per bag, and round up. Verify the exact product data sheet.
Can I calculate several post holes?
Yes. Choose circular column or post hole, enter the hole diameter and depth, then enter the number of identical holes. Measure unusual or bell-shaped holes separately and combine their volumes.
Does the ready-mix price include delivery?
Only if the rate you enter already includes it. The tool multiplies rounded volume by the entered material rate and does not infer delivery, short-load, pumping, waiting-time or tax charges.
Can this calculator tell me how thick a slab should be?
No. Thickness is a design input, not an output. Use an approved specification suited to the load, soil, climate and local rules before estimating volume.
This calculator is independent and currently contains no affiliate links or paid placements. It does not promise an exact quantity, price, structural result or savings. Future commercial relationships will be clearly labeled and will not change the formula or editorial criteria.