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Concrete Calculator: Slab Volume and Bags

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RenoCalcs Team

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Enter the length, width and thickness of your slab to see how much concrete you need and how many pre-mixed bags to buy. The result includes an allowance for waste and spillage that you can change.

How the concrete calculator works

The calculator multiplies length × width × thickness to get the volume of the slab, adds the waste allowance, and divides by the yield of the bag size you picked. Bag counts are always rounded up to the next whole bag.

  • Volume = length × width × thickness
  • Concrete to order = volume × (1 + waste % ÷ 100)
  • Bags = concrete to order ÷ yield per bag, rounded up

Worked example

A 10 ft × 10 ft patio slab poured 4 inches thick: 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, or 1.23 cubic yards. With 10% extra for waste, plan for 36.7 cubic feet (1.36 cubic yards). An 80 lb bag of pre-mixed concrete yields about 0.60 cubic feet, so you would need 62 bags.

Bag yields used in this calculator

  • 80 lb bag: about 0.60 cubic feet
  • 60 lb bag: about 0.45 cubic feet
  • 40 lb bag: about 0.30 cubic feet

Yields vary slightly between brands and mixes, so check the label on the bag you plan to buy.

Bags or ready-mix?

Above roughly one cubic yard (about 45 bags of 80 lb), mixing by hand gets slow and it becomes hard to keep the pour consistent. For larger slabs, compare the cost of bags with a ready-mix delivery. Many suppliers charge a short-load fee for small orders, so ask before you decide.

Step by step: from measuring to pouring

The calculator handles the math. These are the measurements it needs and how they fit into the job.

Wooden form staked on a gravel base with a tape measure along the length and width
Step 1. Measure the length and width inside the form. For an L-shaped slab, measure each rectangle separately.
Tape measure inside the form measuring slab thickness from the gravel base to the top of the form
Step 2. Measure from the compacted base to the top of the form board. This depth is your slab thickness.
Concrete mix bags next to a wheelbarrow of mixed concrete and a hoe
Step 3. Buy the number of bags the calculator shows, including the waste allowance, and mix one bag at a time.
Gloved hands pulling a straight board across fresh concrete to level the slab
Step 4. Fill the form and pull a straight board across the top in a sawing motion to level the concrete.

Tips for measuring

  • If the ground is uneven, measure the depth in several places and use the average.
  • Split L-shaped or irregular slabs into rectangles, calculate each one and add them up.
  • Order a little extra. Running short in the middle of a pour leaves a weak joint where fresh concrete meets concrete that has started to set.

These results are estimates. The right thickness, base and reinforcement depend on what the slab will carry and on local building codes, so check with a qualified professional for structural work.

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