Formula
Volume is area times thickness. In feet that is square feet times inches divided by 12, then divided by 27 for cubic yards. In metres it is square metres times millimetres divided by 1,000. Waste multiplies that volume. The order rounds up to the step you set, a quarter yard by default. Bags are the wasted cubic feet divided by the bag yield, rounded up. Ready-mix cost is the rounded order times your price, plus a short-load fee only when the order is under the minimum, plus a delivery fee if you entered one.
yd³ = area × thickness ÷ 12 ÷ 27
order = ceil(yd³ × (1 + waste) ÷ step) × step
bags = ceil(wasted ft³ ÷ yield)
A 6 inch slab is half again the concrete of a 4 inch slab. Reinforcement, base, labor, sealer, forms, and a permit are added from the amounts you type. Past about a cubic yard, mixing bags by hand is a lot of labor. The comparison line appears when both a yard price and a bag price are filled in.
Worked examples
Every dollar figure below is an example price, not a quote.
20 × 20 ft, 4 in, 10 percent, $160/yd³, 8 yd³ minimum, $100 short load, mesh $0.50/ft², labor $4/ft². 4.94 yd³ becomes an order of 5.50 yd³. Concrete is $880 plus $100, mesh $200, labor $1,600, total $2,780, $6.95/ft².
24 × 24 ft, 6 in, same yard price, no short load. Order 11.75 yd³, $1,880 of concrete, $3.26/ft².
10 × 10 ft, 4 in, 80 lb bags at $6. 62 bags, $372, $3.72/ft². The same pad in 60 lb bags is 82 bags.
6 × 4 m, 100 mm, 10 percent, $200/m³, 0.25 m³ steps. 2.75 m³, $550.
Reference
Quikrete Concrete Mix 1101 yields. The 40, 60, and 80 lb figures are the same ones the concrete calculator uses.
| Bag | Yield | Litres |
|---|---|---|
| 40 lb bag | 0.300 ft³ | 8.5 |
| 50 lb bag | 0.375 ft³ | 10.6 |
| 60 lb bag | 0.450 ft³ | 12.7 |
| 80 lb bag | 0.600 ft³ | 17 |
Questions
How much does a 20×20 concrete slab cost?
The volume comes first. A 20 × 20 ft slab at 4 inches is 4.94 cubic yards. At 10 percent waste, rounded up to the next quarter yard, the order is 5.50 cubic yards. With example prices of $160 a yard, an 8-yard minimum, a $100 short-load fee, mesh at $0.50 a square foot, and labor at $4 a square foot, the total is $2,780, or $6.95 a square foot. Those prices are an example, not a quote.
How much is a yard of concrete?
The plant’s quote is the price. Ask about the short-load fee, the minimum load, the delivery or fuel fee, and waiting time. This page does not publish a national average.
How many bags of concrete for a 10×10 slab?
A 10 × 10 ft pad at 4 inches is 33.33 cubic feet. At 10 percent waste that is 36.67 cubic feet. An 80 lb Quikrete bag yields 0.60 cubic feet, so the count rounds up to 62 bags. A 60 lb bag yields 0.45 cubic feet, which is 82 bags.
What is a short-load fee?
A flat charge when the order is under the plant’s minimum load. Enter the minimum and the fee from your quote. The fee applies only when the rounded order is below that minimum. An order that lands exactly on the minimum does not pay it.
Is a 4 or 6 inch slab right for a driveway?
A 6 inch slab is 50 percent more concrete than a 4 inch slab of the same area. Driveways and anything structural follow the local code and a contractor. This page does not say a thickness is adequate.
Bagged or ready-mix?
Bags avoid a truck minimum. Past about a cubic yard, mixing by hand is a lot of labor. Enter both prices and the page shows both totals for this slab.
Sources
- Quikrete Concrete Mix data sheet 1101. A 40 lb bag yields about 0.30 ft³ (8.5 L), a 50 lb bag about 0.375 ft³ (10.6 L), a 60 lb bag about 0.45 ft³ (12.7 L), and an 80 lb bag about 0.60 ft³ (17 L).
- NIST SP 811. One cubic yard is 27 cubic feet. One foot is 0.3048 metres.
- No national price is stored on this page. The dollar amounts in the examples are labelled example prices.