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Concrete Slab Calculator

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

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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