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Concrete & Masonry

Concrete Calculator

Work out how many cubic yards of concrete a slab or pad needs, with an allowance for waste.

Calculate your concrete

For one rectangular slab or pad — a patio, walkway, shed base or similar. Length and width in feet, thickness in inches. The estimate updates as you type.

Slab dimensions
ft
ft
in
Waste
%

Covers spillage, over-excavation and an uneven subgrade. 0% is valid if you want the base volume only.

The method

  1. 01

    Convert thickness to feet

    thickness (in) ÷ 12

    Concrete thickness is normally specified in inches; converting to feet lets it multiply directly against the length and width.

  2. 02

    Base volume

    length (ft) × width (ft) × thickness (ft)

    Gives the volume in cubic feet before any waste allowance.

  3. 03

    Add a waste allowance

    base volume × (1 + waste allowance)

    Covers spillage, over-excavation and an uneven subgrade. There's no single correct figure — it depends on the site.

  4. 04

    Convert to cubic yards

    cubic feet ÷ 27

    Ready-mix concrete is commonly ordered by the cubic yard.

Worth knowing

  • This covers one rectangular slab or pad — a patio, walkway section, shed base, driveway section or rectangular footing all use the same math.
  • Ready-mix suppliers often have order minimums and round up to a set increment — check with your supplier before ordering.
  • Buying bagged concrete instead? Bag yield varies by product, so use the cubic-feet yield printed on the bag rather than a universal count.
  • This calculator covers concrete volume only, not excavation or sub-base material.

Concrete calculator example

Length
10 ft
Width
12 ft
Thickness
4 in
Waste allowance
10%
  1. Footprint10 × 12 = 120 sq ft
  2. Thickness in feet4 ÷ 12 = 0.33 ft
  3. Base volume120 × 0.33 = 40 cu ft
  4. After 10% waste40 × 1.10 = 44 cu ft
  5. Convert to cubic yards44 ÷ 27 = 1.63 cu yd

You need 1.63 cubic yards.

Before waste, the slab itself is 1.48 cubic yards — the extra 0.15 covers spillage and an uneven subgrade.

How much extra concrete should you plan for?

A waste allowance covers spillage, over-excavation, form variation, and a subgrade that isn't perfectly flat or level.

SituationPlanning guidance
Well-prepared, level subgradeAround 5–10% is common
Uneven ground or hand-dug forms10% or more is often planned for
Ordering ready-mix for deliveryCheck your supplier's minimum order and increment

Important: these are planning starting points, not guarantees. The calculator above defaults to 10%, but site conditions vary — a rougher excavation or looser forms can use noticeably more.

How thick should a concrete slab be?

There's no single correct thickness for every project — it depends on what the slab needs to carry, the ground beneath it, and any reinforcement or design requirements.

Lighter, non-structural pours like a stepping-stone path or a small shed base are sometimes planned around 3–4 inches as a general guide. Driveways, structural slabs, foundations, and anything load-bearing or covered by local building code should follow your project's own specification or an inspector's requirement rather than a general figure — soil conditions, expected loads, and reinforcement all affect what thickness is actually appropriate. This calculator works out volume for a thickness you provide; it isn't a substitute for structural or code guidance.

Cubic feet vs. cubic yards

Both units describe the same volume — they just group it differently.

There are 27 cubic feet in a cubic yard. Multiplying length and width in feet by a thickness in feet naturally produces cubic feet, which is why the calculator above works out that figure first. Ready-mix concrete is commonly discussed and ordered in cubic yards, so the calculator converts to that as its main result. Bagged concrete is different again — bag yield is product-specific, so don't infer a bag count directly from a weight or volume figure alone; use the yield printed on the bag or its product information.

Concrete calculator FAQ

How many cubic yards of concrete do I need for a 10 × 12 slab?
It depends on the thickness. At an assumed 4-inch thickness with a 10% waste allowance, a 10 × 12 ft slab works out to about 1.63 cubic yards — the calculator above uses the same math with the thickness and waste allowance you enter.
How much extra concrete should I plan for waste?
There's no single figure that applies everywhere. The calculator defaults to 10%, which is a reasonable planning starting point for a well-prepared subgrade — see the guidance above for when more may be appropriate.
Is concrete measured in cubic feet or cubic yards?
Both, depending on the context. The underlying math naturally produces cubic feet from length, width and thickness in feet; ready-mix is then commonly ordered by the cubic yard, since 27 cubic feet make one cubic yard.
How thick should a concrete slab or pad be?
It depends on the project — what the slab needs to support, the soil and base beneath it, any reinforcement, and local code requirements all play a part. For driveways, foundations or anything load-bearing, check your project's specification or local requirements rather than relying on a general figure.
How many bags of concrete equal a cubic yard?
There's no single universal answer — it depends entirely on the bag size and product yield. Check the cubic-feet yield printed on the specific bag or product you're buying, then divide the cubic-yard result above (converted to cubic feet) by that yield.

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