Concrete Calculator
Estimate concrete volume and bag counts for common shapes.
Enter all dimensions in feet (e.g., 6 inches = 0.5 ft).
Concrete Estimate
About This Concrete Calculator
A concrete calculator turns your project's dimensions into cubic feet, cubic yards, and bag counts before you buy materials or call a ready-mix plant — no stalled pours, no wasted money. The tool above covers the four shapes behind most residential work: slabs, wall footings, square columns, and round columns such as deck-post footings.
Pick the matching tab, enter every dimension in feet, and click Calculate Volume. You'll get the volume in cubic feet, cubic yards (the unit ready-mix is sold in), and cubic meters, plus the number of 40 lb, 60 lb, and 80 lb bags required — always rounded up.
The Concrete Volume Formula
Every rectangular pour — slab, footing, or column — uses the same formula:
Volume = Length × Width × Thickness
- Length — the longest horizontal dimension.
- Width — the shorter horizontal dimension.
- Thickness (or depth/height) — how deep the concrete runs.
Keep all three dimensions in the same unit. Ready-mix is sold by the cubic yard, so work in feet and convert at the end: Cubic yards = Cubic feet ÷ 27 (one yard is a 3 ft cube — exactly 27 ft³).
Round pours use the cylinder formula Volume = π × (Diameter ÷ 2)² × Height, which the Round Column tab applies automatically.
Worked Example: A 10 ft × 10 ft Patio Slab
Pouring a 10 ft by 10 ft patio slab at the standard 4-inch thickness: first convert, 4 in ÷ 12 ≈ 0.333 ft, then multiply:
10 ft × 10 ft × 0.333 ft = 33.30 ft³
Convert: 33.30 ÷ 27 = 1.23 yd³. The Slab tab returns exactly that — 33.30 ft³, 1.23 yd³, 0.943 m³ — plus 112 forty-pound, 75 sixty-pound, or 56 eighty-pound bags (33.30 ÷ 0.60 ft³ per bag = 55.5, rounded up). With a 5–10% waste allowance, buy 59–62 eighty-pound bags — or simply order 1.5 yards of ready-mix.
Reference Table: Common Slab Sizes
Volumes assume a true 4-inch (⅓ ft) or 6-inch (½ ft) thickness; bag counts use the 0.60 ft³ yield of an 80 lb bag, and the last column adds a 10% waste allowance.
| Slab size | Thickness | Cubic feet | Cubic yards | 80 lb bags | Bags + 10% waste |
|---|---|---|---|---|---|
| 8 ft × 8 ft | 4 in | 21.33 | 0.79 | 36 | 40 |
| 8 ft × 8 ft | 6 in | 32.00 | 1.19 | 54 | 59 |
| 10 ft × 10 ft | 4 in | 33.33 | 1.23 | 56 | 62 |
| 10 ft × 10 ft | 6 in | 50.00 | 1.85 | 84 | 92 |
| 12 ft × 12 ft | 4 in | 48.00 | 1.78 | 80 | 88 |
| 12 ft × 12 ft | 6 in | 72.00 | 2.67 | 120 | 132 |
| 20 ft × 20 ft | 4 in | 133.33 | 4.94 | 223 | 245 |
| 20 ft × 20 ft | 6 in | 200.00 | 7.41 | 334 | 367 |
Plan around the waste column — running out mid-pour forces a weak, visible cold joint.
Round Footings and Post Holes
Deck and fence posts sit in cylindrical holes. A footing 12 inches (1 ft) in diameter and 36 inches (3 ft) deep — a common depth for getting below the frost line — holds:
π × (0.5 ft)² × 3 ft ≈ 2.36 ft³
That is 0.09 yd³ per hole; the Round Column tab confirms 2.36 ft³, or 8 forty-pound, 6 sixty-pound, or 4 eighty-pound bags. A four-post deck needs about 9.42 ft³ in all — 16 eighty-pound bags (21 sixty-pound) before waste.
When to Switch From Bags to Ready-Mix
The rule of thumb: past about 1 cubic yard, order ready-mix. One yard equals 27 ft³ — 90 forty-pound, 60 sixty-pound, or 45 eighty-pound bags. Beyond a yard, bags usually cost more per yard than a truck delivery, and hand-mixing that many bags is slow enough that early batches set before the last are poured. Below a third of a yard — a post hole or small pad — bags mixed in a wheelbarrow are cheapest. Note that many plants add short-load fees below a minimum order, so compare fees for 1–2 yard jobs.
How Thick Should a Concrete Slab Be?
- 4 inches — sidewalks, patios, and shed floors on compacted ground.
- 4–6 inches — residential driveways; use the thicker end where trucks or RVs park.
- 6 inches or more — garage floors and heavy loads, usually reinforced with a thickened edge.
- Footings — sized by local code; the base must sit below the frost line.
Thickness drives cost: a slab poured at 5 inches instead of 4 uses 25% more concrete. On soft ground, add a compacted gravel base — our gravel calculator sizes that layer, and the square footage calculator handles irregular areas before you multiply by thickness.
Concrete Curing Basics
Concrete doesn't dry — it cures through a chemical reaction that needs moisture. Keep new concrete damp for at least the first 7 days (misting, plastic sheeting, wet burlap, or a curing compound); slabs that dry out early lose surface strength. A typical mix reaches about 70% of design strength in 7 days and full strength at 28 days. Keep foot traffic off for 24–48 hours, cars off a new driveway for at least 7 days, and protect fresh concrete from freezing for the first two days. In hot, windy weather, cure even more aggressively.
Frequently Asked Questions
How many bags of concrete do I need?
Divide the volume in cubic feet by the bag yield — 0.30 ft³ (40 lb), 0.45 ft³ (60 lb), or 0.60 ft³ (80 lb) — and round up. A 10 ft × 10 ft slab 4 inches thick holds 33.3 ft³, needing 112 forty-pound, 75 sixty-pound, or 56 eighty-pound bags before waste.
How many cubic feet are in a cubic yard of concrete?
One cubic yard equals 27 cubic feet (3 ft × 3 ft × 3 ft). Divide cubic feet by 27 — for example, 33.3 ft³ ÷ 27 ≈ 1.23 yd³.
Should I order extra concrete?
Yes — add 5–10% for spillage, an uneven subgrade, and form variation. For the 10 ft × 10 ft slab above, that means buying about 62 eighty-pound bags instead of 56.
When should I order ready-mix instead of bags?
A common rule of thumb is to switch at about 1 cubic yard — roughly 45 eighty-pound bags. Beyond that, a truck delivery is usually cheaper per yard and far less labor than hand-mixing.
How thick should a concrete slab be?
Four inches suits sidewalks, patios, and shed floors. Driveways typically use 4–6 inches (5–6 inches where trucks or RVs park). Footings follow local code and must reach below the frost line.
How do I calculate concrete for a round post hole?
Use Volume = π × (diameter ÷ 2)² × depth, with all dimensions in feet. A hole 12 inches (1 ft) across and 36 inches (3 ft) deep holds about 2.36 ft³ — four 80 lb bags per hole.
Can I enter inches or metric units?
Convert dimensions to feet first (6 inches = 0.5 ft; 4 inches ≈ 0.333 ft). The result panel still shows cubic meters alongside cubic feet and cubic yards for reference.