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Concrete bag calculator

How many bags of concrete you need for any bag size, and how much they weigh in total.

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

Bag size lb
Standard concrete mix. For setting posts use the fence post calculator.
Extra for waste and spills
Sakrete advises adding 10% to bagged estimates; ready-mix producers (NRMCA) advise 4% to 10%.
= 15 bags Full breakdown below
You need
15 80 lb bags
includes 10% extra0.33 yd³
1,200 lb in total
8.8 ft³ of concrete
0.33 yd³

Not yet fact-checkedMethod and sources

Your slab Drawing of your slab: 6 feet long, 4 feet wide and 4 inches thick. The thickness is drawn thicker than true scale. 6 ft 4 ft 4 in
Redraws as you typethickness exaggerated

Buy 15 x 80 pound bags (1,200 pounds in total).

Every bag size for this job

Bag sizeBags to buyTotal weight
40 lb301,200 lb
50 lb241,200 lb
60 lb201,200 lb
80 lb151,200 lb
90 lb141,260 lb

How we worked it out

  1. 6 ft × 4 ft × (4 in ÷ 12) = 8.0 ft³
  2. 8.0 ft³ + 10% extra = 8.8 ft³ (0.33 yd³, at 27 ft³ per yd³)
  3. 8.8 ft³ ÷ 0.6 ft³ per 80 lb bag = 15 bags (rounded up)
  4. 15 bags × 80 lb = 1,200 lb in total

Bag yield table with the Quikrete source and date; note Sakrete's similar product lists the same 40/60/80 pound yields.Draft copy

"How many bags in a cubic yard?" 27 / yieldDraft copy

90 x 40 pound, 72 x 50 pound, 60 x 60 pound, 45 x 80 pound, 40 x 90 pound (Sakrete confirms 45 x 80 pound).

Hauling noteDraft copy

total weight shown; no vehicle payload claims.

Worked examples

6 x 4 feet landing, 4 inches thick, 80 pound bags, 10% extra

Length 6 ftWidth 4 ftThickness 4 inBag size 80 lbExtra for waste and spills 10%

Buy 15 x 80 pound bags (1,200 pounds in total).

Bags to buy
15
Total weight
1,200 lb
Concrete needed
8.8 ft³
In cubic yards
0.33 yd³

Exactly 1 cubic yard (9 x 9 feet, 4 inches), 80 pound bags, no extra

Length 9 ftWidth 9 ftThickness 4 inBag size 80 lbExtra for waste and spills None

Buy 45 x 80 pound bags (3,600 pounds in total).

Bags to buy
45
Total weight
3,600 lb
Concrete needed
27.0 ft³
In cubic yards
1.00 yd³

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.
  • Yields: 40 pound 0.30, 50 pound 0.375, 60 pound 0.45, 80 pound 0.60, 90 pound 0.675 cubic feet (Quikrete).
  • Total weight is bags x the bag's labelled weight (dry).
  • Includes the extra you choose for waste and spills.