PencilSum

Concrete calculator

How much concrete your slab needs, in cubic yards and bags, with the working shown.

Example sizes, change them. The answer updates as you type.

Quikrete: slabs are normally 4 to 6 inches thick depending on load; sidewalks at least 4 inches.

Extra for waste and spills
Ready-mix producers (NRMCA) advise ordering 4% to 10% more than the volume worked out from your plan.
= 1.36 yd³ Full breakdown below
You need
1.36 cubic yards
includes 10% extra1.04 m³
62 80 lb bags
82 60 lb bags
123 40 lb bags

Not yet fact-checkedMethod and sources

Your slab Drawing of your slab: 10 feet long, 10 feet wide and 4 inches thick. The thickness is drawn thicker than true scale. 10 ft 10 ft 4 in
Redraws as you typethickness exaggerated

Order 1.36 cubic yards: that is 62 x 80 pound bags or 82 x 60 pound bags.

How we worked it out

  1. 10 ft × 10 ft × (4 in ÷ 12) = 33.3 ft³
  2. 33.3 ft³ ÷ 27 = 1.23 yd³
  3. 1.23 yd³ + 10% extra = 1.36 yd³
  4. 36.7 ft³ ÷ 0.6 ft³ per 80 lb bag = 62 bags (rounded up)

Everything else this slab needs

  • Gravel base, 4 inches deepour defaultoften forgotten 1.23 yd³
  • Form lumber, 2x4 or 2x6around the edge 40 ft
  • Wood stakesabout one every 3 feet of form, our estimateoften forgotten 14
  • Wire mesh, 5 x 10 foot sheetsoptional, allow extra for overlap 2
  • Control jointsno more than about 10 feet apart; 6-8 feet for 4 inch walks

How thick should a slab be?Draft copy

Quikrete 4 to 6 inches by load; walkways at least 4 inches. No invented per-use table.

Bags or ready-mix?Draft copy

Compare the user's own numbers; link to the cost calculator. No break-even claim (no authoritative source exists).

How much extra to orderDraft copy

ready-mix producers (NRMCA) 4-10% over the plan volume; Sakrete 10% for bagged.

Worked examples

10 x 10 feet patio, 4 inches thick, 10% extra

Length 10 ftWidth 10 ftThickness 4 inExtra for waste and spills 10%

Order 1.36 cubic yards: that is 62 x 80 pound bags or 82 x 60 pound bags.

Concrete to order
1.36 yd³
80 lb bags
62
60 lb bags
82
40 lb bags
123
Cubic metres
1.04 m³
Measured volume (no extra)
33.3 ft³

8 x 10 feet shed base, 4 inches thick, 5% extra

Length 10 ftWidth 8 ftThickness 4 inExtra for waste and spills 5%

Order 1.04 cubic yards: that is 47 x 80 pound bags or 63 x 60 pound bags.

Concrete to order
1.04 yd³
80 lb bags
47
60 lb bags
63
40 lb bags
94
Cubic metres
0.79 m³
Measured volume (no extra)
26.7 ft³

12 x 20 feet parking pad, 5 inches thick, 10% extra

Length 20 ftWidth 12 ftThickness 5 inExtra for waste and spills 10%

Order 4.07 cubic yards: that is 184 x 80 pound bags or 245 x 60 pound bags.

Concrete to order
4.07 yd³
80 lb bags
184
60 lb bags
245
40 lb bags
367
Cubic metres
3.11 m³
Measured volume (no extra)
100.0 ft³

Method and sources

Each calculator uses the standard formula and the manufacturer's own figures, such as bag yields, and rounds up where running short would cost you a trip. Every step is shown. Cost ranges come from named surveys with the date we checked them. Spot something wrong? Tell us and we will fix it and note the change.

  • Bag counts are rounded up to whole bags.
  • Includes the extra you choose for waste and spills.
  • Bag yields from Quikrete's Concrete Mix data sheet.
  • Gravel base shown at 4 inches deep, our default (Quikrete's own slab uses 3 inches).
  • Flat, rectangular slab of even thickness.