Concrete for a 20 × 20 Slab (4 inch)
How much concrete a 20 × 20 ft slab at 4 inches needs — in bags and cubic yards.
A 20 × 20 ft slab at 4 inches needs about 223 × 80 lb bags — ≈ 4.94 cu yd of concrete.
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Material estimate only — confirm quantities against your own measurements and the product packaging before you buy. Round up and keep a little extra for cuts, waste, and mistakes.
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How we calculate it
volume (cu ft) = L × W × (thickness / 12) × (1 + waste) • cubic yards = volume / 27 • bags = round up( volume / bag yield )
Assumptions & sources
- 80 lb bag ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 40 lb ≈ 0.30 cu ft (Quikrete/Sakrete general-purpose Concrete Mix) — source: quikrete.com
- 27 cubic feet = 1 cubic yard (exact geometry)
- Partial bags round UP — a leftover fraction still means buying a whole bag
- Waste allowance covers spillage and uneven subgrade; 5–10% is typical
Formulas reviewed against manufacturer spec sheets and industry standards. Last verified 2026.
How to estimate concrete for a slab
Concrete is figured by volume: multiply length × width × thickness (in feet) to get cubic feet, then divide by 27 for cubic yards. Bags are then a function of the mix’s yield — an 80 lb bag of standard concrete mix makes about 0.60 cubic feet, so it takes roughly 45 bags to fill a cubic yard.
Tips for an accurate order
- Measure thickness in inches: A 4-inch slab is the common default for patios and walkways; 6 inches for driveways and anything bearing vehicle load.
- Bagged vs. ready-mix: Bagged mix is convenient for small pours; past roughly 1 cubic yard, ready-mix delivery is usually cheaper and faster.
- Buy a little extra: A 5–10% waste allowance covers spillage, an uneven base, and slight over-excavation — running short mid-pour is far worse than a spare bag.
Common questions
How many 80 lb bags of concrete are in a cubic yard?
About 45. An 80 lb bag of standard concrete mix yields ~0.60 cubic feet, and a cubic yard is 27 cubic feet (27 ÷ 0.60 ≈ 45).
How much concrete do I need for a 10×10 slab at 4 inches?
About 1.23 cubic yards (33.3 cubic feet) — roughly 56 × 80 lb bags. This calculator does the exact rounding for you, including a waste allowance.
Should I subtract for a 5% waste allowance?
Add it, don’t subtract. Waste allowance increases the order slightly to cover spillage and an uneven base. Set it to 0 if you want the bare theoretical volume.
Does bag yield change with the mix?
Yes — fast-setting, crack-resistant, and high-strength mixes can yield slightly differently. The yields here are for standard Quikrete/Sakrete Concrete Mix; check your bag if you use a specialty product.