Concrete Calculator
Calculate concrete for slabs, footings and round columns: cubic yards to order for ready-mix, or exactly how many 60 lb / 80 lb bags to buy — waste factor included.
4″ patio · 5–6″ driveway
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You might also need:Cubic Yard Calculator·Asphalt Calculator
How to calculate concrete
column: yards = (π × (dia in ÷ 24)² × height ft) ÷ 27 × waste
A 12 × 10 ft patio at 4 inches is 40 cubic feet — 1.48 yards, or 1.63 yards with 10% waste. That's the awkward zone: too big for comfortable bag mixing (74 × 80 lb bags!), small enough to attract a short-load fee from ready-mix suppliers. Consider a u-cart or scheduling your pour alongside a neighbor's job.
Bag yield reference
| Bag size | Yield | Bags per cubic yard | Bags per cubic foot |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 3.3 |
| 60 lb | 0.45 cu ft | 60 | 2.2 |
| 80 lb | 0.60 cu ft | 45 | 1.7 |
| 90 lb | 0.68 cu ft | 40 | 1.5 |
Bags or ready-mix?
- Under ⅓ yard (about 15 × 80 lb bags): bags, no question — fence posts, small pads, repairs.
- ⅓ to 1 yard: bags if you have strong helpers and a mixer; otherwise price a u-cart trailer from a local supplier.
- Over 1 yard: ready-mix delivery. Your back, your finish quality and usually your wallet will thank you.
- Don't forget the base: most slabs sit on 4 inches of compacted gravel — size it with the gravel calculator.
Frequently asked questions
How many bags of concrete make a yard?+
One cubic yard takes forty-five 80 lb bags or sixty 60 lb bags of pre-mix concrete. Each 80 lb bag yields 0.60 cubic feet; each 60 lb bag yields 0.45 cubic feet. Past about 1 cubic yard, ready-mix delivery is usually cheaper and far less work.
How thick should a concrete slab be?+
Standard residential slabs: 4 inches for patios, walkways and shed bases; 4–5 inches for single-car driveways; 5–6 inches for heavier vehicles or workshops. Footings are typically 8–12 inches deep depending on frost line and local code.
How much does a yard of concrete cost?+
Ready-mix concrete costs roughly $125–$180 per cubic yard delivered, with short-load fees of $50–$100 for orders under 4–5 yards. Bagged concrete works out around $200–$300 per yard equivalent plus your mixing labor.
Should I add extra when ordering concrete?+
Yes — order 5–10% extra. Subgrades are never perfectly level, forms flex, and coming up half a yard short mid-pour is an expensive emergency. This calculator includes a waste factor for exactly that reason.
How long does concrete take to cure?+
Concrete sets in hours but cures over weeks: keep foot traffic off for 24–48 hours, vehicles off for 7 days, and expect full design strength at 28 days. Keep the surface damp for the first week for maximum strength.