How Many Bags of Concrete Do I Need?
For a 10 × 10 ft slab at 4 in thick (the default thickness) you need about 1.23 cu yd of concrete plus 10% waste — roughly 62 bags of 80 lb concrete mix. This calculator assumes 80 lb bags; see below for 40 lb and 60 lb yields if you are buying a different size.
Reviewed by The ponderloop teamUpdated 2026-07-11How we calculate
Buy 0 bags
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
Shopping list
- 80 lb concrete mix bag
- 60 lb concrete mix bag
- Mixing tub
- Finishing trowel
- Wheelbarrow
- Portable concrete mixer
0.0 cu yd — ~0% of a standard truck load
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Methodology & assumptions
Volume is calculated as 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 over-excavation, and the result is divided by the coverage per bag (about 0.0222 cu yd for an 80 lb bag, by default) and rounded up to the next whole bag. Ready-mix delivery typically becomes the more economical option 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 |
| 4 × 4 ft fence post pad, 4 in thick, minimal waste | 0.20 cu yd + 5% waste ≈ 0.21 cu yd → 10 bags |
| 12 × 12 ft patio, 4 in thick, standard waste | 1.78 cu yd + 10% waste ≈ 1.96 cu yd → 89 bags |
Frequently asked questions
- How many bags of concrete do I need?
- Multiply the length by the width in feet, multiply by the slab or pad thickness (converted from inches to feet), and divide by 27 to get cubic yards. Add waste, then divide by the coverage per bag and round up.
- How much concrete is in a 40 lb, 60 lb, or 80 lb bag?
- A 40 lb bag yields about 0.011 cu yd, a 60 lb bag about 0.0167 cu yd, and an 80 lb bag about 0.0222 cu yd of mixed concrete. This calculator assumes 80 lb bags by default — if you are buying 40 lb or 60 lb bags instead, take the cubic yardage shown and divide by that bag’s yield to get the correct bag count (you’ll need more, smaller bags to hit the same volume).
- How much extra should I buy for waste?
- Budget 10% extra (standard) to cover spillage, uneven subgrade, and over-excavation. Drop to 5% (minimal) only if you are confident in your measurements and pouring on a well-prepared, level base.
- When should I use ready-mix instead of bags?
- Once a pour needs more than about 1 cu yd of concrete, a ready-mix truck or local mini-mix delivery is usually cheaper per yard and dramatically less labor than mixing dozens of bags by hand. Bags make the most sense for small slabs, pads, post holes, and repairs under that threshold.
- What thickness should I use for a slab or pad?
- A typical walkway or patio slab runs 4 in thick; a driveway or anything that will see vehicle loads is usually poured thicker (5-6 in or more) with reinforcement. This calculator defaults to 4 in as a common baseline — check your local code and intended use before finalizing.