Calculate your materials
What you'll need to buy
- Concrete mix or ready-mix80/60/40 lb bags, or a truck order by the yardShop
- Rebar or wire meshReinforcement to control crackingShop
- Form lumber + stakes2×4 or 2×6 forms to shape the pourShop
- Gravel base4" compacted sub-base under the slabShop
- Concrete float + edgerFinishing the surface and edgesShop
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How the math works
A 10 ft × 10 ft slab at 4" thick = 100 × (4 ÷ 12) = 33.3 cu ft = 1.23 cubic yards. That's about 56 × 80 lb bags, 75 × 60 lb bags, or 112 × 40 lb bags. Above ~1 yard, order ready-mix.
Formula
Reference values
| Item | Value |
|---|---|
| 80 lb bag | = 0.60 cu ft → 45 bags / yd³ |
| 60 lb bag | = 0.45 cu ft → 60 bags / yd³ |
| 40 lb bag | = 0.30 cu ft → 90 bags / yd³ |
| 1 cubic yard | = 27 cu ft ≈ 2 tons (≈ 4,050 lb) |
| Slab thickness | 4" typical (6" for driveways) |
Step by step
- Measure length, width, and thickness; convert thickness in inches to feet (÷12).
- Multiply length × width × thickness for cubic feet, then divide by 27 for cubic yards.
- For bags, divide cubic feet by 0.60 (80 lb), 0.45 (60 lb), or 0.30 (40 lb).
- Add about 10% waste for spillage and uneven subgrade, and round up.
Waste allowance
Always add a waste factor: more for diagonal, herringbone, or irregular layouts and for trenches in rocky soil. It's cheaper to round up than to pay for a second delivery.
Bags vs. bulk
Bagged material is convenient for small jobs, but loose/bulk delivery is almost always cheaper per unit once you pass roughly 1–3 cubic yards. Compare the per-cubic-yard price before deciding.
Frequently asked questions
How many bags of concrete are in a yard?
A cubic yard is 27 cubic feet. An 80 lb bag makes 0.60 cu ft, so a yard takes 45 bags. A 60 lb bag makes 0.45 cu ft (60 bags per yard), and a 40 lb bag makes 0.30 cu ft (90 bags per yard).
How much concrete do I need for a 10x10 slab?
A 10 ft × 10 ft slab at 4" thick is 33.3 cubic feet, or about 1.23 cubic yards. That's roughly 56 bags of 80 lb concrete — at that quantity a ready-mix truck is usually cheaper and easier than mixing bags.
How do I calculate cubic yards of concrete?
Multiply length × width × thickness with every dimension in feet to get cubic feet, then divide by 27. For a slab, the thickness in inches divided by 12 gives the depth in feet (4" = 0.333 ft).
When should I order ready-mix instead of bags?
Mixing bags is fine for footings, post holes, and small pads up to roughly a third of a yard. Past about 1 cubic yard (around 45 × 80 lb bags) the bag cost, effort, and cold-joint risk make a ready-mix truck the better choice.
How much does a yard of concrete weigh?
Cured concrete weighs about 150 lb per cubic foot, so a cubic yard (27 cu ft) is roughly 4,050 lb — just over 2 tons. That weight is why slabs need a compacted, well-drained base.
How thick should a concrete slab be?
Four inches is standard for patios, walkways, and shed floors. Use 6" for driveways and anything carrying vehicles, and pour footings to the depth your local frost line and code require.
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