Guide
Ordering concrete: what the calculator does not decide
The calculator gives quantities only. The mix, strength class, cover and any reinforcement come from the design and the specification, and delivery terms from your supplier. Whether a slab or footing is thick enough is a structural question that a volume calculator cannot answer.
Strength classes are written differently from market to market. Under the European standard EN 206, a class such as C25/30 gives the characteristic compressive strength in N/mm² measured on a cylinder (25) and on a cube (30); the specimens are a 150 × 300 mm cylinder and a 150 mm cube. Other markets express strength in their own way, so follow your project specification.
For bagged concrete, the yield per bag depends on the product, not on the weight printed on the bag alone. The yield table uses the manufacturers' own data sheets. Quikrete also gives a fresh unit weight of about 140 lb/ft³ for its Concrete Mix and asks for a mixer of at least 1 ft³ per 80 lb bag. For any other product, enter the yield printed on the bag.
For excavation, US Army field manual FM 5-434 (2000) gives average factors from bank (in place) to loose and compacted volume. We offer them as suggestions you can change. They are averages from a manual, not a test of your soil, so test the real soil where the volume matters. The manual has no factor for topsoil, so enter your own for that.
How to measure and order
Measure the finished concrete, not the excavation: the inside faces of the formwork for walls and footings, and the slab area inside the edge forms. Enter each dimension with its own unit; the calculator converts inches, feet, millimeters and meters before it multiplies.
Enter the thickness as it will be poured. An uneven subbase or soft formwork takes more concrete than the nominal size suggests, which is what the waste allowance is for; enter your own percentage from experience with that ground.
For ready-mix, enter the truck mixer capacity your supplier quotes. As one example, the Liebherr HTM 1204 truck mixer has a nominal capacity of 12 m³; capacities vary, so ask. The calculator divides the order by the capacity, rounds up and shows the size of the last load, which helps you decide whether to round the order up.
For bagged concrete, choose the product or enter the yield from the bag. The number of bags is rounded up.
For excavation, enter the size of the pit or trench, the soil and the truck capacity. The loose volume, not the size of the hole, decides how many trips you need.
Units, rounding and common mistakes
Volume is length times width times thickness. In imperial units, work in feet: 4 in is 1/3 ft, so a 10 × 10 ft slab 4 in thick is 33.33 ft³, and dividing by 27 gives 1.2346 yd³. Loads, bags and trips are always rounded up, and only once, at the end.
- Mixing inches and feet: entering a 4 in thickness as 4 ft makes the order twelve times too large.
- Dividing cubic feet by 3 or 9: one cubic yard is 27 cubic feet (3 × 3 × 3).
- Confusing bank and loose volume: trucks carry the loose volume, so the trips use the bulked figure.
- Using the bulking factor for fill: compacted fill can take up less space than in the ground; the compaction factors in the table are averages, not a design value.
- Expecting a price: the calculator gives volumes and counts only; prices come from your supplier.
Questions about concrete and excavation volume
How much concrete do I need for a slab?
Multiply length by width by thickness in the same unit, then add waste. A 10 × 10 ft slab 4 in thick is 10 × 10 × 1/3 = 33.33 ft³, or 1.2346 yd³. In metric, a slab of 15 × 10 m at 0.30 m is 45 m³, and with 5 % waste you would order 47.25 m³. Ask your supplier how they round orders.
How many bags of concrete do I need for a 10 × 10 slab?
At 4 inches thick the slab is 33.33 ft³. Quikrete Concrete Mix in 80 lb bags yields about 0.60 ft³ per bag according to its data sheet, so you need 56 bags before waste. With 60 lb bags at 0.45 ft³ it is 75 bags, and with 40 lb bags at 0.30 ft³ it is 112 bags.
How do I convert cubic feet to cubic yards?
Divide by 27, because a cubic yard is 3 ft × 3 ft × 3 ft. So 33.33 ft³ is 1.2346 yd³. To get cubic meters, multiply cubic yards by 0.764555 or cubic feet by 0.0283168. The calculator shows every result in all three units, so you can order in the unit your supplier uses.
How much waste should I add to a concrete order?
There is no single figure we can source. It depends on the subbase, the formwork, the pump and how the pour is run. The calculator lets you enter your own percentage, and the worked example uses 5 % only as an example. Compare past orders with the volumes actually placed to find a figure that fits your work.
What is the bulking factor of soil?
Excavated soil takes up more space loose than it did in the ground. The bulking factor is the loose volume divided by the bank volume. US Army FM 5-434 gives averages such as 1.11 for sand or gravel, 1.25 for common earth, 1.43 for clay and 1.50 for blasted rock. Real soil varies, so test it when haulage costs matter.