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Concrete Calculator

Estimate concrete volume, ready-mix quantities or bag counts.

My ProjectNo saved space selectedSave a room once and reuse it here.

Calculator inputs

Concrete project details

Ready to calculate

Project shape

Choose the closest simple shape. If you select a saved project space, PlanCalq can reuse its length and width as the slab footprint. You can still edit them here.

ShapeLength (m)Width (m)Thickness (cm)Project unitsWaste / overage allowance

How are you buying concrete?

For bagged mix, use the yield printed on the bag. For ready-mix, enter the supplier's price per volume.

Purchase methodYield per bag (L)Price per bagCurrency

Planning estimate only. Verify bag yield, subgrade, reinforcement and pour requirements before ordering.

Your estimate

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Ready when you are

Enter the pour dimensions to estimate concrete volume, overage, purchase quantity and optional cost.

Plan with confidence

How to use the Concrete Calculator

Estimate concrete for a rectangular slab, pad or similar pour from length, width and thickness, then convert the volume into ready-mix or bag quantities.

Step by step

What to enter

Enter slab length, width and thickness. Saved spaces can provide length and width.
Choose a waste allowance for spillage, uneven base conditions and measuring variation.
If using bagged concrete, enter the yield printed on one bag.
Optionally enter a unit price to estimate material cost.

Method

How the calculation works

The core calculation is simply length × width × thickness after converting all dimensions into compatible units.

PlanCalq adds the selected waste allowance and can then divide the recommended volume by your bag yield to determine the number of complete bags to purchase.

Calculation summaryLength × width × thickness = concrete volumeConcrete volume × (1 + waste %) = recommended volumeRecommended volume ÷ bag yield = bags required (rounded up)

Worked example

Example: a 5 m × 4 m patio slab

Length: 5 m
Width: 4 m
Thickness: 10 cm
Overage: 5%

Planning resultThe slab volume is 2.00 m³; with 5% overage, the planning quantity becomes 2.10 m³.

Concrete volume is length × width × thickness after all dimensions are converted to compatible units. The extra 0.10 m³ is an allowance for small variations in excavation, subgrade and placement rather than a change to the slab design.

Interpret the output

How to read the result

Use cubic volume for ready-mix ordering and bag yield only when buying bagged concrete.
If the excavation depth varies, use an average thickness cautiously or split the slab into sections with different depths.
The calculator estimates quantity; it does not decide how thick a slab should be.

Planning limits

What this calculator does not cover

Structural thickness, reinforcement, joints, sub-base and concrete strength must be determined from project requirements, local rules and professional advice where appropriate.
Footings, thickened edges, beams and steps should be measured as separate volumes rather than hidden inside a simple slab estimate.
Supplier minimum orders, truck access, pumping and site wastage can affect the practical order quantity.

PlanCalq methodology & safety boundaries →

GuideConcrete quantity basics

Understand how slab dimensions become concrete volume, why thickness matters, how bag yield differs from ready-mix volume and where overage fits.

GuideRenovation contingency allowance

Understand the difference between material waste and renovation contingency, what creates uncertainty, and how to build a realistic allowance into a project budget.

GuideInterpreting calculator estimates

Learn the difference between theoretical quantity, recommended quantity, purchase quantity and estimated cost so you can use calculator results responsibly.

Common questions