Concrete Slab Calculator
To pour a 10 × 10 ft slab at 4 inches thick you need about 1.23 cubic yards of concrete plus 10% waste — roughly 62 bags of 80 lb mix.
Reviewed by The ponderloop teamUpdated 2026-07-11How we calculate
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0 cu yd + 0% waste = 0 cu yd
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How this number is calculated
0 cu yd (no waste added) ÷ 0.0222 cu yd per bag = 0 bags
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- 80 lb concrete mix
- Mixing tub
- Finishing trowel
- Rebar
- Concrete sealer
- Concrete edger
0.0 cu yd — ~0% of a standard truck load
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Methodology & assumptions
Volume is calculated as slab length × width × (thickness in inches ÷ 12), then divided by 27 to convert cubic feet to cubic yards. A waste percentage is added for spillage and uneven subgrade, and the result is divided by the coverage per bag (about 0.0222 cu yd for an 80 lb bag) and rounded up to the next whole bag. Ready-mix delivery typically becomes more economical than bags once a pour exceeds roughly 1 cubic yard.
- ACI 318 concrete practice (waste allowance)
- Manufacturer bag-yield specs (80 lb ≈ 0.60 cu ft)
- Typical US ready-mix and bag price ranges, 2026
Worked examples
| Room | Result |
|---|---|
| 10 × 10 ft slab, 4 in thick, standard waste | 1.23 cu yd + 10% waste ≈ 1.36 cu yd → 62 bags |
| 12 × 14 ft floor slab, 4 in thick, standard waste | 2.07 cu yd + 10% waste ≈ 2.28 cu yd → 103 bags |
| 8 × 10 ft shed slab, 4 in thick, minimal waste | 0.99 cu yd + 5% waste ≈ 1.04 cu yd → 47 bags |
Frequently asked questions
- How much concrete do I need for a slab?
- Multiply the slab length by width in feet, multiply by the thickness in feet (inches divided by 12), then divide by 27 to convert cubic feet to cubic yards. Add 10% for waste, then divide by the coverage per bag (about 0.0222 cu yd for an 80 lb bag) and round up.
- How many bags of concrete are in a cubic yard?
- About 45 bags of 80 lb mix per cubic yard, since each bag yields roughly 0.60 cu ft and a cubic yard is 27 cu ft. A 60 lb bag yields about 0.45 cu ft (around 60 bags per cubic yard), and a 40 lb bag yields about 0.30 cu ft (around 90 bags per cubic yard).
- Is it cheaper to buy bagged concrete or order ready-mix?
- For small pours under about 1 cubic yard, bagged mix is usually more convenient and cost-competitive. Above roughly 1 cubic yard, ready-mix delivered by truck is typically cheaper per yard and far less labor — mixing 45+ bags by hand adds up fast.
- How thick should a concrete slab be?
- A standard interior floor or shed slab is typically 4 inches thick. This calculator defaults to 4 inches, but you can adjust the thickness field for a specific project — thicker slabs (5-6 in.) are used where heavier loads are expected.
- Should I round up or down when figuring out how many bags to buy?
- Always round up to a full bag. Running short mid-pour is a serious problem with concrete since it starts setting once mixed — cold joints from a delayed second batch weaken the slab.