Free Online Calculators
Calculate cement, sand, gravel & water
Concrete volume = length × width × thickness. A 10 × 10 ft slab that is 4 inches thick needs 33.33 cubic feet (1.23 cubic yards); with 10% extra that is 1.36 cubic yards or 62 bags of 80 lb premixed concrete. The calculator also gives cement, sand, gravel and water for mixing your own, in US or metric units.
A slab of 100 square feet (for example 10 × 10 ft) that is 4 inches thick needs about 56 bags of 80 lb premixed concrete, before adding 5 to 10% for waste. The table below covers slab areas from 50 to 500 square feet and thicknesses from 3 to 8 inches.
| Slab area \ Thickness | 3 in | 4 in | 5 in | 6 in | 8 in |
|---|---|---|---|---|---|
| 50 sq ft | 21 | 28 | 35 | 42 | 56 |
| 100 sq ft | 42 | 56 | 70 | 84 | 112 |
| 150 sq ft | 63 | 84 | 105 | 125 | 167 |
| 200 sq ft | 84 | 112 | 139 | 167 | 223 |
| 250 sq ft | 105 | 139 | 174 | 209 | 278 |
| 300 sq ft | 125 | 167 | 209 | 250 | 334 |
| 400 sq ft | 167 | 223 | 278 | 334 | 445 |
| 500 sq ft | 209 | 278 | 348 | 417 | 556 |
Multiply length × width × thickness in the same unit and add 5 to 10% for waste. A slab of 4 m × 3 m and 10 cm thick needs 4 × 3 × 0.1 = 1.2 m³ plus the reserve.
Multiply length × width × thickness in feet and divide by 27. A 10 ft × 10 ft slab that is 4 inches (0.333 ft) thick needs about 33.3 ÷ 27 = 1.23 cubic yards. One cubic meter equals about 1.31 cubic yards.
Divide the volume by the yield printed on the bag and round up. An 80 lb bag of premixed concrete yields about 0.6 cubic feet, so one cubic yard takes about 45 bags. The calculator shows the count for 80, 60 and 40 lb bags.
Multiply length × width × thickness. In US units, measure length and width in feet and the thickness in inches, then calculate cubic feet = length × width × (thickness ÷ 12) and cubic yards = cubic feet ÷ 27. A 10 × 10 ft slab that is 4 inches thick needs 10 × 10 × 0.333 = 33.33 cubic feet, or 1.23 cubic yards. With 10% extra for spillage and uneven ground, plan for 36.67 cubic feet or about 1.36 cubic yards.
Bags of premixed concrete state their yield on the package. A typical 80 lb bag makes about 0.60 cubic feet, a 60 lb bag about 0.45 cubic feet and a 40 lb bag about 0.30 cubic feet. Divide the volume by the yield and round up: 36.67 cubic feet take 62 bags of 80 lb, 82 bags of 60 lb or 123 bags of 40 lb. One cubic yard takes about 45 bags of 80 lb concrete.
Bags are practical for small jobs such as post holes, steps or a small pad. From about one cubic yard, ready-mix delivered by truck is usually the better choice, because mixing 45 or more heavy bags by hand is slow and a large pour is hard to keep consistent. Ready-mix is ordered in cubic yards, so use the cubic yard result including the extra for waste.
If you mix cement, sand and gravel yourself, the calculator gives the quantities by weight for four mixes. The standard mix (C20/25, roughly 3,000 psi) uses about 300 kg of cement, 720 kg of sand, 1,080 kg of gravel and 170 liters of water per cubic meter. Per cubic foot that is about 18.7 lb of cement, 45 lb of sand, 67 lb of gravel and 1.27 gallons of water. Portland cement is sold in 94 lb bags in the US. For the 10 × 10 ft slab with 10% extra, that means about 687 lb of cement (8 bags), 1,648 lb of sand, 2,472 lb of gravel and 47 gallons of water.
The strength classes in the calculator are European: C20/25 means a cylinder strength of 20 MPa. The psi values are rounded equivalents. Local building codes, and for structural work an engineer, decide what is required.
Switch to metric to enter meters. The calculator then shows cubic meters, kilograms and liters, with bags of 25 kg or 40 kg of cement. One cubic meter equals about 1.31 cubic yards or 35.3 cubic feet.
For round columns, tubes or other shapes, the volume calculator gives the volume of a cylinder, cone, pyramid or prism in cubic feet or cubic meters.