Concrete Bags Calculator
Pouring a concrete slab, post footing, or garden border? Enter your dimensions below to estimate the exact number of 80lb, 60lb, and 40lb bags of concrete mix required for your project.
Results
Bag yield is a volume, not a weight
The number on the bag is what it weighs dry. What matters for planning is what it makes once mixed, and that is always a smaller number than people expect.
cu ft = length × width × (thickness ÷ 12), then × waste factor, then bags = total cu ft ÷ bag yield, rounded up.
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.33 cu ft | 82 |
A cubic yard is 45 bags of 80 lb mix — 3,600 pounds of material to buy, move, and mix one bag at a time. That single fact decides most projects.
Where bags stop making sense
The crossover is around one cubic yard, and it is set by labor rather than by price.
| Pour | Volume | 80 lb bags | Practical route |
|---|---|---|---|
| 4 fence post holes | ~4 cu ft | 8 | Bags, comfortably |
| 3 × 3 ft landing, 4 in | 3.15 cu ft | 6 | Bags |
| 10 × 10 ft slab, 4 in | 35 cu ft | 59 | Borderline; bags only with help |
| 20 × 20 ft slab, 4 in | 140 cu ft | 234 | Ready-mix truck |
The 10 × 10 slab is the honest edge case. Fifty-nine bags is about 4,700 pounds, and a slab has to be poured and finished as one piece before any of it sets — so it is not a job that can be paused. Two people with a mixer can do it; one person with a wheelbarrow should not try.
Ready-mix trucks usually carry a short-load minimum of 1 to 3 yards and charge a fee below it, which is why the crossover sits where it does rather than lower.
Choosing the mix, not just the count
| Mix | Use it for |
|---|---|
| Standard concrete mix | Slabs, footings, steps, general work |
| Fast-setting | Fence and mailbox posts; sets in 20 to 40 minutes |
| High-early-strength | Pours needing traffic within a day or two |
| Crack-resistant / fiber-reinforced | Thin slabs, patios, walkways |
| Sand or mortar mix | Not concrete — no coarse aggregate, not structural |
The last row causes real failures. Mortar mix and sand mix contain no gravel, so they have a fraction of the compressive strength of concrete and should never be substituted for a slab or footing, however similar the bags look on the shelf.
What the bag count does not tell you
- Water, and mixing capacity. Each 80 lb bag needs roughly 3 quarts of water. A wheelbarrow handles two bags at a time; a 3.5 cu ft mixer handles two or three.
- Base preparation. Most slabs want 4 inches of compacted gravel underneath, which is a separate material order.
- Reinforcement. Rebar or wire mesh for anything structural or over 4 inches, plus chairs to hold it at mid-depth.
- Forms. Lumber, stakes, and screws to build the edges, and a release agent or plastic if the forms are to be reused.
- Control joints. Concrete cracks; joints decide where. Cut or tool them at intervals of about 24 to 30 times the slab thickness.
Add a bag or two beyond the calculated count. Running one bag short mid-pour, with the first half already stiffening, is the failure mode this page exists to prevent.
Frequently asked questions
When should I order a delivery truck instead of mixing bags?
If your project volume exceeds 1 cubic yard (which is 27 cubic feet, or forty-five 80 lb bags), mixing concrete by hand becomes physically taxing and hard to pour in a single session before it sets. Under 1 yard, bags are usually cheaper and more flexible.
Can I use normal bag concrete for post holes?
Yes, normal prepackaged concrete mix is ideal. Fast-setting concrete mixes can also be poured directly dry into post holes and then saturated with water, which cuts mixing labor.
How many 80 lb bags of concrete make a cubic yard?
Forty-five. An 80 lb bag yields 0.60 cubic feet, and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45 bags — about 3,600 pounds of mix. In 60 lb bags it takes 60, and in 40 lb bags it takes 82.
How many bags do I need for a 10x10 slab?
A 10 × 10 ft slab at 4 inches thick is 33.3 cubic feet, or 35 with a 5% waste allowance — about 59 bags of 80 lb mix. That is close to the limit of what is reasonable to mix by hand, so it is worth pricing a short-load ready-mix delivery alongside it.