PencilSum

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

Add it to build one shopping list for your whole job.

Checked by JunaydMethod 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).

How many bags of concrete do I need?

To find how many bags of concrete you need, work out the volume in cubic feet, add 10% extra and divide by the bag's yield. An 80 pound bag makes 0.6 cubic feet, so a 6 by 4 foot slab 4 inches thick needs 15 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

How much concrete does each bag size make?

Each bag size makes a set amount of concrete, printed on Quikrete's data sheet. A 40 pound bag makes 0.3 cubic feet, a 60 pound bag 0.45 and an 80 pound bag 0.6. The table below gives every size we use, with the bags that make a cubic yard.

BagCubic feet per bagBags per cubic yard
40 lb concrete mix0.390
50 lb concrete mix0.37572
50 lb fast-setting mix0.37572
60 lb concrete mix0.4560
80 lb concrete mix0.645
90 lb concrete mix0.67540

Sources: Quikrete Concrete Mix No. 1101 data sheet (rev. Oct 2022), Quikrete Fast-Setting Concrete Mix No. 1004-50 data sheet (Nov 2025). Checked .

How much do the bags weigh in total?

Every bag size adds up to about the same weight for the same job, because each pound of mix makes the same amount of concrete. The 6 by 4 foot slab needs 1,200 pounds of mix, as 30 bags of 40 pounds or 15 bags of 80 pounds.

Plan how you will move it. PencilSum gives no vehicle payload figures, so check your vehicle's handbook.

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

Common questions

How many bags of concrete are in a yard?

A cubic yard of concrete takes 45 bags of 80 pound mix, 60 bags of 60 pound mix or 90 bags of 40 pound mix. That is 27 cubic feet divided by each bag's yield on Quikrete's data sheet, before any extra.

How many cubic feet does an 80 pound bag of concrete make?

An 80 pound bag of Quikrete Concrete Mix makes 0.6 cubic feet of concrete, according to its data sheet. That is 45 bags for every cubic yard.

How many bags of concrete are on a pallet?

A pallet of Quikrete Concrete Mix holds 42 bags of 80 pounds, 56 of 60 pounds or 80 of 40 pounds, according to Quikrete. A pallet of 80 pound bags makes 25.2 cubic feet, just under a cubic yard.

How long does Quikrete take to dry?

Quikrete Fast-Setting Concrete Mix sets in 20 to 40 minutes, and you can walk on it after at least 4 hours. Standard Quikrete Concrete Mix gives no walk-on time; wait 24 hours before a post takes strain and cure it for 5 days in warm weather.

What will this concrete cost?

See it priced three ways: bags, ready-mix delivered, or a pro, as honest dated ranges. Your sizes go with you.

Length 6 ftWidth 4 ftThickness 4 inExtra for waste and spills 10%

Price this job

More concrete calculators

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 labeled weight (dry).
  • Includes the extra you choose for waste and spills.