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Concrete Slab Calculator

Home & ConstructionConcrete and Masonry
Measure in

Extra for spillage, uneven subgrade and form bulge; 5–10% is typical.

Concrete to order —
Cubic metres
—
Cubic feet
—
80 lb bags (0.60 ft³ each)
—
60 lb bags (0.45 ft³ each)
—
Approximate weight (lb)
—

Multiply length by width by thickness, with every dimension in feet, then divide by 27 to get cubic yards. A 10 × 10 ft slab poured 4 inches thick holds 33.33 cubic feet, which is 1.23 cubic yards or 56 bags of 80 lb premix. Add 5–10% for waste before you order.

About this tool

Patios, shed bases, garage floors and hot-tub pads are all rectangular slabs, and the order is the same question each time: how much concrete fills the forms. This calculator takes the slab length, width and thickness in feet and inches or in metres and centimetres, adds a waste allowance you set on the slider, and returns the volume in cubic yards, cubic metres and cubic feet. It also converts that volume into 80 lb and 60 lb premix bags using the yields printed on Quikrete bags, and estimates the wet weight at 150 lb per cubic foot so you can judge trailer loads. It assumes a flat slab of even depth; thickened edges, footings or sloped subgrades need their extra volume added separately.

How to use it

  1. Pick your units

    Choose feet and inches or metres and centimetres; the dimension fields switch to match.

  2. Enter the slab size

    Type the length, width and finished thickness of the pour inside the forms.

  3. Set the waste allowance

    Drag the slider; 5% suits a smooth, compacted base, 10% a rough one.

  4. Read the order

    Use cubic yards for ready-mix and the bag counts for a mixer job.

Examples

10 × 10 ft patio, 4 in thick

Measure in
Feet and inches
Slab length
10
Slab width
10
Thickness
4
Waste allowance
0

Result Concrete to order: 1.23
Cubic metres: 0.94
Cubic feet: 33.33
80 lb bags (0.60 ft³ each): 56
60 lb bags (0.45 ft³ each): 75
Approximate weight (lb): 5,000

  1. Volume = 10 ft × 10 ft × (4 ÷ 12) ft = 33.33 ft³
  2. With 0% waste: 33.33 × 1 = 33.33 ft³
  3. Cubic yards = 33.33 ÷ 27 = 1.23 yd³
  4. 80 lb bags = 33.33 ÷ 0.60 = 55.6 → 56

A typical backyard patio with no waste added: just over a cubic yard, or 56 bags of 80 lb mix.

Same patio, 10% waste

Measure in
Feet and inches
Slab length
10
Slab width
10
Thickness
4
Waste allowance
10

Result Concrete to order: 1.36
Cubic metres: 1.04
Cubic feet: 36.67
80 lb bags (0.60 ft³ each): 62
60 lb bags (0.45 ft³ each): 82
Approximate weight (lb): 5,500

  1. Volume = 10 ft × 10 ft × (4 ÷ 12) ft = 33.33 ft³
  2. With 10% waste: 33.33 × 1.1 = 36.67 ft³
  3. Cubic yards = 36.67 ÷ 27 = 1.36 yd³
  4. 80 lb bags = 36.67 ÷ 0.60 = 61.1 → 62

Adding a 10% allowance for spillage and an uneven base raises the order to 62 bags.

5 × 4 m shed base, 10 cm

Measure in
Metres and cm
Slab length
5
Slab width
4
Thickness
10
Waste allowance
0

Result Concrete to order: 2.62
Cubic metres: 2
Cubic feet: 70.63
80 lb bags (0.60 ft³ each): 118
60 lb bags (0.45 ft³ each): 157
Approximate weight (lb): 10,594

  1. Volume = 5 m × 4 m × 0.1 m = 2 m³ (70.63 ft³)
  2. With 0% waste: 70.63 × 1 = 70.63 ft³
  3. Cubic yards = 70.63 ÷ 27 = 2.62 yd³
  4. 80 lb bags = 70.63 ÷ 0.60 = 117.7 → 118

Twenty square metres at 10 cm is exactly 2 m³, about 2.62 cubic yards.

How it is calculated

V (ft³) = L × W × (T ÷ 12) × (1 + waste ÷ 100); yd³ = V ÷ 27; bags = ⌈V ÷ yield⌉

L, W
slab length and width in feet (metric inputs are converted)
T
slab thickness in inches
waste
extra allowance in percent
yield
0.60 ft³ per 80 lb bag, 0.45 ft³ per 60 lb bag

The slab is treated as a rectangular box. Thickness in inches is divided by 12 so all three dimensions are in feet, and the product is cubic feet. One cubic yard is 3 × 3 × 3 = 27 cubic feet, and one cubic foot is 0.0283168 cubic metres. Bag counts divide the volume by the manufacturer's stated yield and round up, because a part bag still has to be bought. Weight uses 150 lb per cubic foot, the usual figure for normal-weight concrete.

When not to use it

  • Round or L-shaped pads need their area worked out separately first.
  • Slabs with thickened edges or integral footings hold more than a flat box.
  • Structural floors need an engineer's design, not just a volume.

Common mistakes

  • Entering thickness in feet instead of inches turns a 4 in slab into a 4 ft one.
  • Ordering the exact volume leaves you short when the base dips in places.
  • Using a 60 lb bag yield for 80 lb bags undercounts by a quarter.

Frequently asked questions

How thick should a concrete slab be?

Four inches (about 10 cm) is the common thickness for patios, walkways and shed floors. Driveways and garage floors that carry cars are usually 5 to 6 inches. Thinner pours crack readily under point loads, which is why the calculator warns below about 3.5 inches.

How many 80 lb bags make a cubic yard?

An 80 lb bag yields about 0.60 cubic feet, so a cubic yard of 27 cubic feet takes 45 bags. With 60 lb bags at 0.45 cubic feet each, it takes 60 bags.

When does ready-mix beat bagged concrete?

Past roughly one to two cubic yards, mixing 45 or more bags by hand becomes slow and makes it hard to keep a consistent finish. A ready-mix truck delivers it in one pour, though many suppliers charge a short-load fee for small orders.

How much waste should I add?

Five percent covers spillage on a well-graded, compacted base with rigid forms. Ten percent or more is safer when the subgrade is uneven or the forms may bow, since a short pour leaves a cold joint.

Does gravel or rebar change the volume?

A gravel base sits under the slab and is ordered separately, so do not include it in the thickness. Rebar and mesh displace so little volume that they are ignored in the estimate.

How heavy is the concrete in my slab?

Normal-weight concrete is about 150 lb per cubic foot, or roughly 4,000 lb per cubic yard. A 10 × 10 ft slab at 4 inches weighs about 5,000 lb, which matters if you plan to haul bags in a pickup.