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Concrete for a slab
Cubic yards, 40/60/80-lb bag counts, and ready-mix cost for any rectangular slab, with waste priced in.
CALCULATOR · CONSTRUCTION · CONCRETE
Concrete Slab Calculator
A 12 × 10 ft slab, 4 inches thick, with 10% waste: that is 40.00 cubic feet of concrete (44.00 with waste), or 1.63 cubic yards. In bags: 74 eighty-pound bags, 98 sixty-pound, or 147 forty-pound. As ready-mix at $150 a cubic yard: $244.44. Enter your slab below — yards, all three bag sizes, and the ready-mix price in one click.
The 60-second rule
Cubic feet = length × width × (thickness ÷ 12). Cubic yards = cubic feet ÷ 27. Bags = round up (cubic feet ÷ yield per bag), with waste added first. At the defaults: 12 × 10 × (4÷12) = 40.00 ft³; with 10% waste, 44.00 ft³ = 1.63 yd³; 44 ÷ 0.6 = 74 eighty-pound bags; at $150/yd³, $244.44 of ready-mix.
The worked example, line by line
Setup: 12 × 10 ft slab, 4 inches thick, 10% waste, $150 per cubic yard.
Volume. 12 × 10 × (4 ÷ 12) = 40.00 cubic feet. Waste: 40 × 1.10 = 44.00 cubic feet. Cubic yards: 44 ÷ 27 = 1.63 yd³ — the number the ready-mix plant wants.
Bags. An 80-lb bag yields 0.6 ft³: 44 ÷ 0.6 = 73.33 → 74 bags. A 60-lb bag yields 0.45 ft³: 44 ÷ 0.45 = 97.78 → 98 bags. A 40-lb bag yields 0.3 ft³: 44 ÷ 0.3 = 146.67 → 147 bags. Always rounded up — you cannot buy a third of a bag.
Ready-mix cost. 1.6296 yd³ × $150 = $244.44. Seventy-four bags at roughly $6 each would run you about $444 in materials plus a long afternoon of mixing — this is why anything over a yard usually goes ready-mix.
Branch — 6-inch driveway slab (12 × 10): 12 × 10 × 0.5 = 60.00 ft³; with waste, 66.00 ft³ = 2.44 yd³; bags 110 / 147 / 220; ready-mix $366.67. Two extra inches of thickness adds 50% to everything — thickness is the multiplier people underestimate.
Branch — 24 × 24 patio, 4 inches: 24 × 24 × (4÷12) = 192.00 ft³; with waste, 211.20 ft³ = 7.82 yd³; bags 353 / 470 / 705; ready-mix $1,173.33. Seven hundred five 40-lb bags — nobody is mixing this by hand.
Branch — 0% waste baseline: 40.00 ft³ = 1.48 yd³; bags 67 / 89 / 134; ready-mix $222.22. The waste factor is the difference between $222.22 and $244.44 — cheap insurance against coming up short mid-pour.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Cubic feet | length × width × (thickness ÷ 12) | 40.00 ft³ at the defaults — the raw volume before waste. |
| Cubic yards (with waste) | ft³ × (1 + waste%) ÷ 27 | 1.63 yd³ — what you order from the plant. |
| Bag counts | ⌈waste-adjusted ft³ ÷ yield per bag⌉ | 74 eighty-lb (0.6 ft³), 98 sixty-lb (0.45), 147 forty-lb (0.3) — always rounded up. |
| Ready-mix cost | cubic yards × price per yard | $244.44 at $150/yd³. |
Who uses this
- The DIY patio builder: a 12 × 10 slab is a weekend project — 1.63 yd³ tells you this sits right on the bags-vs-truck line.
- The shed-foundation pourer: 4-inch slab math with waste included, so the last wheelbarrow doesn't come up empty.
- The driveway estimator: switch thickness to 6 inches and watch the volume jump 50% — $366.67 of concrete for the 12 × 10 example.
- The budget checker: comparing the $244.44 ready-mix quote against 74 bags at the hardware store before committing.
Accuracy & limitations
Bag yields (0.6 / 0.45 / 0.3 ft³) are standard nominal figures — real yield varies by mix and water content, so treat bag counts as minimums. Waste covers spillage, form bulge, and uneven subgrade; on a rough dig, 10% can be light — pros often use 10–15%. Ready-mix plants have minimum loads and short-load fees (often 2–3 yards); a 1.63-yd³ order may carry a surcharge — the price field is where you absorb it. This is a rectangular slab only; see our concrete tube calculator for piers and footings.
Cite this calculatorBright Side Kit. “Concrete Slab Calculator.” brightoolbox.com, 2026.
Bright Side Kit. "Concrete Slab Calculator." https://brightoolbox.com/concrete-slab-calculator/ Accessed October 23, 2026.Frequently asked questions
How much concrete do I need for a 12x10 slab?
At 4 inches thick with 10% waste: 1.63 cubic yards — or 74 eighty-pound bags, 98 sixty-pound bags, or 147 forty-pound bags. As ready-mix at $150 a yard, $244.44.
How many bags of concrete per cubic yard?
A cubic yard is 27 ft³. At a 0.6 ft³ yield, one yard takes 45 eighty-pound bags; 60 sixty-pound bags (0.45 ft³ each); 90 forty-pound bags (0.3 ft³ each).
Should I order ready-mix or mix bags myself?
The rough rule: over 1 cubic yard, ready-mix wins. This slab's 1.63 yd³ costs $244.44 as ready-mix vs about $444 in 80-lb bags — plus hours of mixing. Under a yard, bags are cheaper once the plant's short-load fee lands.
How thick should a concrete slab be?
4 inches for patios, sheds, and walkways; 6 inches for driveways and anything carrying vehicles. Going from 4 to 6 inches adds 50% to the concrete — the 12 × 10 example jumps from 1.63 to 2.44 yd³.
What does the 10% waste cover?
Spillage, forms that bulge under the pour, and subgrade that isn't perfectly level. It is the difference between $222.22 and $244.44 on this slab — cheap insurance against a short load mid-pour.
Can this handle a 24x24 patio?
Yes: 24 × 24 at 4 inches with 10% waste needs 7.82 cubic yards — 353 eighty-pound bags — at a ready-mix cost of $1,173.33. That's a truck job, not a bag job.