How Many Bags of Concrete Do I Need?
Bag count comes down to one formula: total volume ÷ bag yield. An 80-lb bag yields about 0.60 ft³, a 60-lb bag about 0.45 ft³, and a 40-lb bag about 0.30 ft³. Use the Concrete Bag Calculator to total bags for slabs, post holes and footings.
Ready to run the numbers? Use our Concrete Bag Calculator or the Concrete Calculator to get an instant material estimate.
The bag-yield formula
First find the volume of your pour in cubic feet (length × width × thickness in feet). Then divide by the yield of one bag. Always round up and add a waste allowance — you do not want to run out mid-pour.
Source: 80-lb bag ≈ 0.60 ft³ · 60-lb bag ≈ 0.45 ft³ · 40-lb bag ≈ 0.30 ft³.
A 4×4 ft pad at 4 in thick = 4 × 4 × (4÷12) = 5.33 ft³. With 80-lb bags and 10% waste: 5.33 ÷ 0.60 × 1.1 = 9.8 → 10 bags.
Pick the right bag size
- 80-lb bags — most concrete per bag, fewest trips; best for larger pours.
- 60-lb bags — easier to carry; good for medium jobs and post holes.
- 40-lb bags — lightest; handy for small repairs and topping off.
- Fast-setting mix sets in 20–40 minutes — ideal for fence posts but not for slabs you need to finish.
Counting bags for slabs and post holes
For a slab, use length × width × thickness. For post holes, treat each hole as a cylinder: π × (diameter ÷ 2)² × depth, then multiply by the number of posts. A 4×4 post in an 8 in × 24 in hole needs about 0.7 ft³, or roughly two 80-lb bags.
Once a pour climbs past about 1 cubic yard (45+ eighty-lb bags), delivered ready-mix is usually cheaper and far less work than mixing by hand.
Buying and mixing tips
- Buy all bags from the same batch so the mix and color are consistent.
- Add 5–10% extra for spillage, uneven subgrade and over-excavation.
- Rent a power mixer for anything over about 10 bags — your back will thank you.
- Have water, a hose, and a finishing trowel ready before you open the first bag.
Frequently asked questions
How much concrete does one 80-lb bag make?
About 0.60 cubic feet of finished concrete. A 60-lb bag makes about 0.45 ft³ and a 40-lb bag about 0.30 ft³. Divide your total volume in cubic feet by the bag yield to get the count.
How many bags of concrete do I need for a 4×4 pad?
A 4×4 ft pad at 4 in thick is 5.33 ft³. With 80-lb bags and 10% waste you need about 10 bags. The calculator gives an exact number for any size and thickness.
How many bags of concrete per fence post?
A 4×4 post in an 8 in wide × 24 in deep hole needs about 0.7 ft³, or two 80-lb bags. Use three bags for 6 ft fences or loose, sandy soil.
Calculators for this guide
Put these numbers to work with the free calculators referenced above.