Concrete Volume & Bag Calculator
Estimate concrete for slabs, footings, round columns and steps. Get cubic yards and cubic feet with a waste allowance, the weight of the pour, how many 80, 60 and 50 lb bags you need, and a quick cost comparison against a ready-mix truck.
Written by Suhaib Hassan. Bag yields follow the figures printed on standard premix bags. Our methodology.
It finds the solid volume of the shape in cubic feet, adds your waste allowance, and converts to cubic yards, weight and bags.
Ordering short is expensive and a cold joint ruins a pour. This shows the volume with a buffer and when a truck makes more sense than bags.
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Slab quick guide
Volume and 80 lb bags for common slab sizes, including a 10% waste allowance. Bags are rounded up.
| Slab size | Area | 4 in (yd³) | 4 in (bags) | 6 in (yd³) | 6 in (bags) | Typical use |
|---|---|---|---|---|---|---|
| 10 × 10 ft | 100 ft² | 1.36 | 62 | 2.04 | 92 | Shed base, small hot-tub pad |
| 12 × 12 ft | 144 ft² | 1.96 | 88 | 2.93 | 132 | Patio, gazebo platform |
| 16 × 20 ft | 320 ft² | 4.35 | 196 | 6.52 | 294 | One-car driveway, workshop |
| 20 × 20 ft | 400 ft² | 5.43 | 245 | 8.15 | 367 | Two-car detached garage |
| 24 × 24 ft | 576 ft² | 7.82 | 352 | 11.73 | 528 | Large garage, equipment pad |
How the volume is calculated
All sizes are converted to feet first, then divided by 27 because one cubic yard is 27 cubic feet.
Slab or footing
yd³ = L × W × (T ÷ 12) ÷ 27 × (1 + waste)Round column
ft³ = π × (d ÷ 24)² × hSteps
ft³ = width × (run ÷ 12) × (rise ÷ 12) × n(n + 1) ÷ 2One cubic yard is 27 ft³ or 46,656 in³. Normalweight concrete weighs about 145 lb per cubic foot (roughly 3,900 lb per yd³).
Common compressive strengths
Check local code and your supplier, since required strength depends on climate, use and load.
Worked examples
Illustrative jobs, each checked by hand. The first is the default setup.
20 × 15 ft patio, 4 in
10% waste allowance.
× 1.10 = 110 ft³
110 ÷ 27 = 4.07 yd³
110 ÷ 0.60 = 184 bags (80 lb)
Round post hole
12 in diameter, 4 ft deep, no waste.
π × 0.5² × 4 = 3.14 ft³
3.14 ÷ 0.60 = 5.2 → 6 bags (80 lb)
Four steps
7 in rise, 11 in run, 4 ft wide.
4 ft × (11/12) ft × (7/12) ft × 10
= 21.39 ft³ = 0.79 yd³
How much concrete do I need?
- Measure the length, width and thickness of the pour, or the size of the footing, column or steps.
- Add a waste allowance of about 5% to 10% for uneven ground and spillage, and 10% to 15% for rough subgrades.
- Under about 1 cubic yard, premix bags are practical. Above that, ready-mix is usually easier and gives a more even pour.
- Order ready-mix by the cubic yard and ask about any minimum-load or short-load charge.
This estimate covers the concrete only. It does not include gravel base, forms, reinforcement or finishing, and slabs may need thicker edges.
By Suhaib Hassan · Reviewed 29 September 2026 · How CalculatePilot verifies formulas →
Sources
- ACI CODE-318, Building Code Requirements for Structural Concrete, American Concrete Institute (normalweight concrete unit weight).
- ACI CODE-332, Residential Code Requirements for Structural Concrete, American Concrete Institute.
- Portland Cement Association, Design and Control of Concrete Mixtures.
Frequently asked questions
How many 80 lb bags make a cubic yard?
An 80 lb bag yields about 0.60 cubic feet, so one cubic yard (27 ft³) needs about 45 bags. A 60 lb bag yields about 0.45 ft³ (60 bags per yard) and a 50 lb bag about 0.375 ft³ (72 bags per yard).
How much extra concrete should I order?
Most people add 5% to 10%. Use more for rough or uneven subgrade, deep forms or long trenches. Running short mid-pour can cause a cold joint, which is a weak seam in the slab.
Should I use bags or a ready-mix truck?
Bags suit small jobs such as post holes and small pads. Once a pour passes about a cubic yard, mixing by hand is slow and the mix can vary, so a truck is usually the better choice. Compare the totals above with your local quote.
How much does a cubic yard of concrete weigh?
Normalweight concrete weighs about 145 to 150 pounds per cubic foot, so roughly 3,900 to 4,050 pounds per cubic yard. This tool uses 145 lb/ft³ for the weight estimate.
How thick should a concrete slab be?
A 4 inch slab is common for patios and walkways. Driveways and garage floors are often 5 to 6 inches, and heavy loads need more. Confirm the thickness with your local building code or an engineer.
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