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Concrete Slab Calculator
Concrete is bought two ways — by the cubic yard from a truck, or by the bag — and the right answer depends on the pour size. Enter your slab and get both: yards for the order call, bags for the mixer math, and a cost estimate with prices you can set to your local quotes.
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The bag count is why most slabs over a yard get a truck: a 10x10 patio at 4 in thick is 60+ bags and several hours of mixing. Bags earn their keep under about half a yard; above that, ready-mix wins on time even with a short-load fee.
How it works
Cubic yards = cu ft ÷ 27
Bags = cu ft ÷ yield per bag (80 lb = 0.60, 60 lb = 0.45, 40 lb = 0.30 cu ft)
Bag yields are the manufacturers' printed figures (Quikrete and Sakrete publish the same table). The extra percentage is not sloppiness — a gravel base that dips half an inch in spots adds real volume, forms bow, and the last wheelbarrow always spills. Ready-mix plants think in quarter-yard increments and most charge a short-load fee under 4–5 yards; a U-cart trailer (1–2 yards, you tow it) is the middle option nobody tells DIYers about.
How thick should the slab be?
| Project | Thickness | Notes |
|---|---|---|
| Walkway, patio, shed pad | 4 in | The residential default |
| Driveway, garage apron | 5–6 in | Vehicle loads; add rebar or mesh |
| Heavy trucks, RV pad | 6–8 in | Engineering territory — check local code |
Thickness is the cheapest strength there is: going from 4 to 5 inches adds 25% more concrete and far more than 25% more load capacity.
Worked example
A 10×10 ft patio slab, 4 in thick: 10 × 10 × (4/12) = 33.3 cu ft = 1.23 cu yd. With the 10% allowance, order 1.36 cu yd — call it 1.5 yards at the plant. Bagging it instead: 33.3 ÷ 0.60 = 56 bags exact, 62 bags with the allowance. Cost at $150/yd ready-mix is about $204 in material; the 62 bags at $5.50 run $341 — the truck is cheaper and faster at this size.
Frequently asked questions
How much concrete do I need for a 10x10 slab?
At 4 inches thick, 1.23 cubic yards — order 1.5 yards with a waste allowance. In bags, that's 56 × 80 lb bags exact, 62 with the allowance. At 6 inches thick the same pad is 1.85 cu yd (84 bags exact).
How many 80 lb bags of concrete are in a cubic yard?
45 bags. One 80 lb bag yields 0.60 cu ft mixed, and a cubic yard is 27 cu ft: 27 ÷ 0.60 = 45. For 60 lb bags (0.45 cu ft) it's 60 bags per yard; for 40 lb bags (0.30 cu ft), 90.
Is it cheaper to mix bags or order ready-mix?
Per yard, bags usually cost more in material — and far more in labor. Under about half a yard (small footings, a mailbox post cluster, patch work), bags win because trucks add delivery and short-load fees. Over a yard, ready-mix or a U-cart trailer almost always wins on both money and your back.
How much does a yard of concrete cost?
Ready-mix material commonly runs $130–180 per cubic yard delivered in most US markets, before short-load fees (often $50–100 on orders under 4–5 yards) and any pump charges. Bagged concrete works out to roughly $250/yd-equivalent at $5.50 per 80 lb bag. Set the price fields above to your local quotes.
How thick should a concrete driveway be?
5–6 inches for cars and light trucks, over a compacted gravel base, with rebar or welded wire mesh. A 4 in driveway is the classic false economy — it cracks where vehicles turn. Check local code; some jurisdictions spec 6 in at the apron regardless.
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About the author
Harrison Alley is a web publisher and data enthusiast based in Dallas, Texas. He built Urbarama’s remodeling calculators and maintains them.
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