How this calculator works
Concrete is ordered by volume. Each shape is turned into cubic feet — slab: length × width × thickness; footing: length × width × depth; round column: π × radius² × depth; steps: each step is a block one riser taller than the one in front, so the volumes add up as 1 + 2 + 3 … risers. The total is increased by your waste allowance, divided by 27 to get cubic yards, and divided by the bag yields — 0.6 cu ft for an 80 lb bag, 0.45 for 60 lb and 0.3 for 40 lb.
Use the project list to add several pours — a patio, its footing and a few post holes — into one order.
Assumptions and tips
- Bag yields from the Quikrete Concrete Mix data sheet.
- Steps are solid concrete, the top step deep as one tread; for a landing, add a slab line.
- Ground that is not flat or forms that bow need more concrete — 10% waste covers ordinary jobs.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| What are you pouring? | Slab, patio or pad |
| Length | 12 ft |
| Width | 10 ft |
| Thickness | 4 in |
| Waste allowance | 10 % |
| Price per 80 lb bag (optional) | $0 |
| Ready-mix price per cubic yard (optional) | $0 |
| Result | Value |
|---|---|
| Concrete needed | 1.63 cubic yards |
| 80 lb bags | 74 bags |
| 60 lb bags | 98 bags |
| 40 lb bags | 147 bags |
| Volume before waste | 40 cu ft |
| Total weight (≈145 lb/cu ft) | 6,380 lb |
Step by step:
- Volume = 12 × 10 × (4 ÷ 12) = 40 cu ft
- With 10% waste: 44 cu ft = 1.63 yd³
- 80 lb bags = 44 ÷ 0.6 = 73.33 → 74 bags
Concrete for common slabs (4 in thick, 10% waste)
Calculated with this tool.
| Slab | Concrete needed | 80 lb bags |
|---|---|---|
| 4 × 4 ft | 0.22 cubic yards | 10 bags |
| 8 × 8 ft | 0.87 cubic yards | 40 bags |
| 10 × 10 ft | 1.36 cubic yards | 62 bags |
| 12 × 10 ft | 1.63 cubic yards | 74 bags |
| 12 × 12 ft | 1.96 cubic yards | 88 bags |
| 10 × 20 ft | 2.72 cubic yards | 123 bags |
| 16 × 16 ft | 3.48 cubic yards | 157 bags |
| 20 × 20 ft | 5.43 cubic yards | 245 bags |
| 24 × 24 ft | 7.82 cubic yards | 352 bags |
Common mistakes to avoid
- Entering slab thickness in feet instead of inches.
- Ordering ready-mix with no extra — running short mid-pour leaves a cold joint.
- Forgetting the footing or thickened edges under a slab.
Frequently asked questions
How much concrete do I need for a 12 × 10 slab, 4 in thick?
40 cu ft, 1.48 yd³ before waste; with 10% extra, 1.63 yd³ or 74 bags of 80 lb.
How many 80 lb bags make a yard of concrete?
45 bags (27 ÷ 0.6).
When should I order ready-mix instead of bags?
Above about one cubic yard most people order a truck; it is faster and more consistent than mixing 45+ bags by hand.
How do I calculate concrete for post holes?
Choose “Round column / post holes”, enter the hole diameter and depth, and the number of holes.
Sources
Built and reviewed by Anirban Kundu, Creator & Editor · M.Sc Physics, 20 years teaching science