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A concrete calculator estimates how many cubic yards of concrete you need for a slab, plus a 10% overage allowance for spillage, over-excavation, and uneven sub-base. Enter the slab's length and width in feet and the thickness in inches, the calculator returns the exact volume, the order quantity (with overage), and the metric-equivalent figure.
Useful for ordering ready-mix from a concrete plant, sizing bag purchases for small DIY pours, sanity-checking contractor estimates, and any home or yard project involving a poured slab. The 10% overage is the trade-standard buffer; complex pours or pours over rough sub-base may warrant 15-20%.
Key takeaway
Concrete is sold by the cubic yard in the US, and a yard is much bigger than people expect (27 cubic feet, enough to fill a small pickup truck bed). For DIY-scale slabs (say, a 10×10 patio at 4" thick), you'll need ~1.5 cubic yards. For ready-mix delivery, the minimum order is typically 1 yard, and trucks carry 8-10 yards full. Always order extra; running short mid-pour is far worse than having a bit left over.
How it's calculated
The math:
cubic feet = length × width × (thickness in inches ÷ 12)
cubic yards = cubic feet ÷ 27
order amount = cubic yards × 1.10
The ÷ 12 converts inches to feet for the thickness; the ÷ 27 converts
cubic feet to cubic yards (since 3 ft × 3 ft × 3 ft = 27 cubic feet).
Standard slab thicknesses for common applications:
- 2 inches: thin overlay, walkway over existing slab
- 4 inches: walkways, patios, driveways for cars (most common DIY)
- 5-6 inches: driveways for trucks/RVs, small structural slabs
- 6-8 inches: heavy-vehicle parking, foundation slabs
- 8+ inches: industrial slabs, footings (with rebar)
For ready-mix orders, plants typically charge a "short load fee" (usually $50-100) for orders under 3-5 yards, plus delivery. Bag concrete (Quikrete and similar) costs more per yard but works for small DIY pours where 80 lb bags are practical (each 80 lb bag is ~0.6 cubic feet, or 45 bags per cubic yard).
Source: Standard volumetric calculation, length × width × thickness ÷ 27, plus 10% overage
Examples
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12 × 10 ft patio at 4" thick
Order amount (with 10% overage) 1.63
A 12 × 10 ft patio at 4 inches thick needs 1.48 cubic yards exact, or 1.63 cubic yards with the 10% overage. Most ready-mix plants will deliver this with a short-load fee (the order is under their typical 3-5 yard minimum). DIY with bags would be ~67 80-lb bags, borderline practical.
-
20 × 25 ft driveway at 5" thick
Order amount (with 10% overage) 8.49
A 20 × 25 ft driveway at 5 inches thick needs 7.72 cubic yards exact, or 8.49 cubic yards with overage. Above the short-load threshold, so no extra fee. A typical ready-mix truck carries 10 yards, so this fits in one delivery, preferred, because hot weather and slow setting from multiple deliveries can cause cold joints.
Frequently asked questions
How do I calculate cubic yards of concrete?
Multiply length × width × thickness (all in feet, so convert
thickness from inches by dividing by 12), then divide by 27.
Add 10% for overage. So a 10 × 10 ft slab at 4" thick:
10 × 10 × (4÷12) ÷ 27 = 1.234 cubic yards exact. Order
1.234 × 1.10 ≈ 1.36 yards. Always round up to the nearest
quarter-yard or whatever the supplier sells.
How thick should a concrete slab be?
Depends on the load. Walkways and patios: 4 inches is standard. Residential driveways for cars: 4 inches is adequate for normal use. Driveways for trucks/RVs or commercial parking: 5-6 inches with rebar reinforcement. Garage and basement slabs: 4-6 inches typical, depending on local code and soil. Foundation slabs and footings: 8+ inches with structural reinforcement. Always check local building codes for required minimums.
Can I mix my own concrete with bags?
Yes, for small jobs. 80-lb bags of pre-mixed concrete
(Quikrete, Sakrete) cost more per cubic yard than ready-mix
but eliminate truck delivery and minimum-order issues. Each
80-lb bag yields 0.6 cubic feet; a yard takes ~45 bags. For
anything more than 1-2 yards (50-90 bags), the labor of
mixing typically exceeds the cost difference vs. ordering
ready-mix. Always mix and pour bags within ~1 hour to avoid
cold joints.
How long does concrete take to cure?
It's strong enough to walk on after 24 hours, drive on after 7 days, and reaches its full design strength after 28 days. The "cure" continues much longer at slowing rates; concrete actually keeps gaining strength for years under favorable moisture conditions. Keep it wet during the first week, sprinklers or wet burlap, for best long-term strength. Dry concrete cures slower and ends up weaker.