How to use it
- Select rectangle or circle and enter the slab dimensions and thickness.
- Add a practical waste allowance for uneven subgrade, formwork and spillage.
- Choose ready-mix or bags and enter the quoted price or the bag’s stated finished yield.
Construction
Concrete Calculator
Calculate concrete for slabs, including waste, bags and project cost.
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Project inputs
Unit system
Calculator guide
Concrete for slabs, pads and circular pours
Calculate ready-mix volume or bagged concrete for rectangular slabs, circular pads, walkways and repeated pours. Results include base volume, waste, cubic yards and estimated cost.
Measure the finished dimensions of the pour. If thickness varies, split the project into sections rather than using a simple average.
Reviewed August 2026 by the DoMyCalc editorial teamConcrete volume formula
Rectangle: length × width × thickness. Circle: π × (diameter ÷ 2)² × thickness. Order volume = volume × (1 + waste %).
Worked example
A 6 m × 4 m slab at 12 cm thick requires 2.88 m³ before waste. With 7% waste, the order volume is 3.082 m³, equivalent to about 4.03 yd³.
Important checks
- The calculator estimates material quantity, not structural strength or reinforcement.
- Bag yield must be taken from the specific product label.
- Ready-mix pricing may include minimum-load, short-load, pump or waiting fees.
How to interpret the estimate
Base volume is the exact geometric quantity; order volume includes your allowance. Bag count is rounded up because partial bags cannot be purchased reliably.
The result estimates quantity only. It does not validate thickness, strength, reinforcement, joints or curing requirements.
Avoid these estimating errors
- Entering slab thickness in the wrong unit
- Using a generic bag yield instead of the product label
- Averaging a highly uneven excavation
Confirm before placing concrete
- Mix specification and required strength
- Supplier order increment and short-load fees
- Access, crew, discharge and curing plan
Frequently asked questions
How much extra concrete should I order?
Common allowances are project-dependent. Use a higher percentage for uneven excavation or complex forms and confirm the final quantity with the supplier.
Can I calculate concrete bags?
Yes. Select bagged mix and enter the finished liters produced by one bag. The calculator rounds up to whole bags.
Does this calculator design the slab?
No. Thickness, reinforcement, joints, subbase and concrete strength should be specified by a qualified professional when structural performance matters.