Concrete Slab Calculator

How this is calculated

Concrete is sold by volume, so a slab is simply length x width x thickness. Thickness is entered in inches and divided by 12 to bring it into feet before multiplying. A cubic yard is 27 cubic feet by definition, which is the conversion used for the order figure. The waste allowance is a percentage you set. Some overage is normal on any pour: the sub-base is never perfectly level, forms bow slightly under load, and concrete left in the truck or the barrow does not reach the slab. Ten percent is a common starting point on a small flat pour. This assumes a rectangular slab of uniform thickness. Thickened edges, footings and steps are extra volume and are not included.

Common questions

How thick should a slab be?

Four inches suits a patio, a shed base or a footpath. Six inches is the usual step up for a driveway or anything carrying a vehicle. Load, not size, drives the decision, so check local requirements before pouring anything structural.

Why add a waste allowance?

Because the ground is never perfectly flat. A sub-base that dips half an inch across a 10 by 10 slab is about 4.2 extra cubic feet, roughly an eighth of a 4-inch pour, and concrete left in the truck or barrow never reaches the forms.

Can I order a part yard?

Ready-mix suppliers usually sell in quarter-yard steps and often set a minimum load. Ring them with your figure rather than assuming, because the minimum is frequently the deciding cost on a small pour.

Does this include footings or a thickened edge?

No. It is a rectangular slab of even thickness. Footings, thickened edges and steps are separate volumes, so work them out on their own and add them.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Concrete Slab Calculator at https://www.bfcbrilliance.com/tools/concrete-slab-calculator.

What I entered:
- Slab length (ft): ___
- Slab width (ft): ___
- Thickness (in): ___
- Waste allowance (%): ___

What it calculated:
- Cubic yards to order: ___
- Cubic yards (exact): ___
- Cubic feet (exact): ___
- Slab area: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Where the job touches structure, gas, or electrics, tell me to check local building code and whether a permit or a licensed trade is required.

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