Concrete Calculator — Cubic Yards and Bags (Slab, Post Hole, Footing)
Updated · Free · No sign-up · Runs in your browser
Estimate how much concrete you need for a slab, patio, driveway, round post holes or a wall footing. Enter dimensions in feet and inches.
You get cubic yards for ordering ready-mix, cubic feet and cubic meters, plus the number of 40, 60 and 80 lb premix bags — with an allowance for waste.
Order (with 10% extra)
1.36 yd³
Exact volume
1.23 yd³
33.33 ft³
Metric
1.038 m³
with extra
40 lb bags
123
≈ 0.3 ft³ each
60 lb bags
82
≈ 0.45 ft³ each
80 lb bags
62
≈ 0.6 ft³ each
Cubic yards for a 4-inch slab (no waste)
| Slab size | Cubic yards | 80 lb bags |
|---|---|---|
| 8 × 8 ft | 0.79 | 36 |
| 10 × 10 ft | 1.23 | 56 |
| 12 × 12 ft | 1.78 | 80 |
| 10 × 20 ft | 2.47 | 112 |
| 20 × 20 ft | 4.94 | 223 |
| 24 × 24 ft | 7.11 | 320 |
Worked examples
10 × 10 ft patio, 4 in thick, +10%
- 10 × 10 × 4/12 = 33.33 ft³
- × 1.10 ÷ 27
→ 1.36 yd³ or 62 × 80 lb bags
20 × 24 ft driveway, 4 in, +10%
- 20 × 24 × 0.333 = 160 ft³
→ 6.52 yd³ of ready-mix
8 post holes, 10 in wide, 3 ft deep, +10%
- π × (5/12)² × 3 × 8
→ 14.4 ft³ = 24 × 80 lb or 32 × 60 lb bags
How to calculate concrete in cubic yards
- Convert every dimension to feet (inches ÷ 12).
- Multiply length × width × thickness to get cubic feet.
- Divide by 27 to get cubic yards, then add 5–10% for waste and uneven ground.
How many 80 lb bags of concrete do I need?
A typical 80 lb bag of premix yields about 0.60 cubic feet, a 60 lb bag about 0.45 ft³ and a 40 lb bag about 0.30 ft³. One cubic yard therefore takes roughly 45 bags of 80 lb.
Yields vary by product — check the bag. For more than about one cubic yard, ready-mix delivery is usually easier.
Post hole concrete calculator
A round hole is π × radius² × depth. Measure the hole diameter, not the post; if a post takes up space, the actual concrete is slightly less.
Frequently asked questions
How do I calculate how much concrete I need?
Length × width × thickness in feet gives cubic feet; divide by 27 for cubic yards. A 10 × 10 ft slab 4 inches thick is 33.3 ft³, about 1.23 yd³.
How many bags of concrete are in a yard?
About 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb, using common yields of 0.60, 0.45 and 0.30 ft³ per bag.
How much extra concrete should I order?
Add 5–10% for spillage, uneven subgrade and form movement. The calculator adds 10% by default; set 0% to see the exact volume.
How thick should a concrete slab be?
Patios and walkways are commonly 4 inches; driveways often 4–6 inches. Follow local codes and your project plans.
Can I enter feet and inches?
Yes. Length and width take feet plus inches (12 ft 6 in); thickness and diameter are in inches.
Is my project data stored?
No. The calculator runs in your browser and saves nothing.