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Concrete Slab Calculator

Enter the length, width, and thickness of your slab to estimate the concrete needed.

Result

1.48 yd³
40.0 ft³
89

How it is calculated

Length and width are entered in feet and thickness in inches. Volume in cubic feet is ft³ = length × width × (thickness / 12), because the thickness must be converted to feet first. Cubic yards are yd³ = ft³ / 27, since one cubic yard is 3 × 3 × 3 feet. The bag count is bags = ceil(ft³ / 0.45): the tool takes one 60-lb bag to make 0.45 cubic feet and rounds up to whole bags.

The figures are for the exact slab dimensions. No allowance is added for waste, spillage, an uneven sub-base or formwork that bows, and the bag yield is a fixed assumption: other bag sizes or mixes yield different volumes. All three inputs must be above zero. Cubic yards are shown to two decimals, cubic feet to one.

Worked example

A patio is 12 ft long, 10 ft wide and 4 inches thick. Thickness in feet: 4 / 12 = 0.3333. Volume: 12 × 10 × 0.3333 = 40.0 ft³. Divide by 27 for cubic yards: 40 / 27 = 1.48 yd³. For bags, 40 / 0.45 = 88.9, rounded up to 89 bags of 60 lb. Because ready-mix is often ordered with a margin for waste, you might add a few percent to the yardage before you call the supplier.

Try this example →

Common mistakes

  • Entering the thickness in feet or centimetres instead of inches. A 4 inch slab is 4; typing 0.33 (feet) gives a slab a twelfth as thick as you intended.
  • Entering length and width in inches or metres. They are feet, so convert before typing.
  • Ordering exactly the calculated amount. The result has no waste allowance, so spillage, uneven ground and over-dig can leave you short mid-pour.
  • Taking the bag count for any bag. It assumes 60-lb bags yielding 0.45 ft³ each; an 80-lb bag, or a different mix, yields more, so use the bag's own printed yield.
  • Forgetting that thickness varies across a slab. Edges, footings or a thickened section need their own volume added.

About this calculator

Pouring a concrete slab means calculating volume in three dimensions — length times width times thickness — and that raw volume then needs converting into units the concrete industry actually sells in: cubic yards for ready-mix truck orders, or the number of standard 60-lb bags for smaller jobs mixed by hand or with a portable mixer.

This calculator takes a slab's length, width, and thickness and works out both the cubic yardage (and cubic feet) and how many 60-lb bags it would take to fill that volume. It's a practical pre-project estimate for DIY homeowners and contractors pouring a patio, walkway, shed foundation, or driveway, letting them order the right amount of ready-mix or buy the right number of bags before the job starts rather than running short mid-pour.

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Comments (1)

  • Editorial team

    Think about the bag count in weight: for the default 10 by 10 ft, 4 in slab, 75 bags at 60 lb each is 4,500 lb to carry and mix. That is a good moment to compare with a ready-mix quote for 1.23 cubic yards and to ask the supplier whether small loads carry a surcharge. Do you mix by hand, with a mixer, or order a truck?

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