Find the concrete you need in yards or cubic metres, and in bags.
Example Result
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The calculated result will appear here after you press Calculate.
Enter the size of your slab, footing, column or steps, and a waste allowance. You get the volume to order in cubic yards or cubic metres, the number of bags for small jobs, and the weight. Measure the real thickness of the formwork, because a slab that is 10% thicker needs 10% more concrete.
Choose the shape: slab or footing, column or pier, or steps.
Choose metric or US units. US thickness, diameter, rise and tread are in inches.
Enter the sizes, measured at the real depth of the forms.
Set a waste allowance. 10% is common on a flat base.
Press Calculate to get the volume to order, bags and weight.
By O.E. — Partner, chairman of the board and CEO of a metal products manufacturer for 21 years; the company exports to 45 countries.
Drafted with AI assistance; calculations checked against primary sources and tested with the site's own code. About FlowUtil
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A slab 20 ft by 12 ft by 4 in thick:
Inches have to become feet, and cubic feet have to become cubic yards. Skipping the ÷ 27 is the costliest mistake in this sum.
In metric, a slab 6 m by 4 m by 100 mm is 6 × 4 × 0.1 = 2.40 m³, or 2.64 m³ with 10% waste.
Thickness is the number with the least margin, since a small change applies across the whole area. For that 6 m by 4 m slab:
| Average thickness | Volume | Change |
|---|---|---|
| 90 mm | 2.16 m³ | −10% |
| 100 mm | 2.40 m³ | |
| 110 mm | 2.64 m³ | +10% |
| 120 mm | 2.88 m³ | +20% |
Ground that sits 10 mm low on average uses up the whole 10% waste allowance. Level the sub-base first, then measure the depth at several points and use the average.
For a round column: V = π × (D ÷ 2)² × H.
A pier 12 in across and 4 ft deep holds 3.14 ft³, or 0.116 yd³. With 10% waste that is 0.128 yd³. For a hole dug in soil, the sides are rarely clean, so allow more. The excavation calculator gives the soil to remove.
This calculator treats the flight as a solid stack, with each step resting on the ground:
V = ½ × n × (n + 1) × rise × tread × width
Four steps with a 7 in rise, 11 in tread and 4 ft width need 21.4 ft³, or 0.79 yd³. A flight with a sloped underside uses less, so this figure is on the high side. Check your rise and tread with the stair calculator before you build the forms.
Bagged mix suits small pours. Yield is printed on the bag, and these are typical for pre-mixed concrete:
| Bag size | Yield | Bags per yd³ |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
The 12 in pier above takes 6 bags of 80 lb. The patio takes 147, so a ready-mix truck makes more sense. Many suppliers charge extra for small loads, so ask before you order. The cement bag coverage chart covers more sizes.
If you batch on site by volume, ratios such as 1:2:4 (cement : sand : gravel) are common for general work. A site estimating rule multiplies wet volume by about 1.54 to get the dry materials, since water and paste fill the gaps between the grains. It is an approximation, not a standard. For 2.4 m³ at 1:2:4 that gives about 760 kg of cement (16 bags of 50 kg), 1,690 kg of sand and 3,170 kg of gravel.
Treat this as a shopping estimate only. Thickness, reinforcement and mix for a structural slab or footing come from your drawings and local building code. Ready-mix is ordered by strength, and the plant works out the proportions.
Normal concrete weighs about 2,400 kg/m³, or 150 lb/ft³. That makes one cubic yard about 4,050 lb. The 2.4 m³ slab weighs about 5,760 kg before it hardens. Forms and bracing have to carry that load wet, and for tall walls the concrete wall calculator covers form pressure.
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