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Calculating how much concrete you need for a project is simply a matter of math. You just need to know some basic geometry formulas and then plug your measurements into the formula. The only tools you need for this part of the project are a measuring tape, calculator, pencil and paper.
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You can take the measurements in centimetres or metres (inches, feet or yards), but all of them will have to be converted to the same type of measurement to get an accurate count. You are dealing with volume, and a large amount of it. Because of this, you want your final measurement to be in cubic metres (feet or cubic yards). To convert inches to feet, divide the inches by 12. To convert inches to yards, divide the inches by 36. To convert feet to yards, divide the number of feet by 3. Finally, if you want to convert cubic feet into cubic yards, divide the number of cubic feet by 27.
Slabs and Footers
For a concrete slab, you need to know three measurements: thickness, length and width. Make sure all of the measurements are in the same units and then multiply them together. This works best for a rectangular slab or a footer. If you have an odd-shaped slab, then you should break the shape into smaller rectangular shapes. Then add the totals of the smaller shapes together to get the concrete needed for the entire slab. For example, you need a slab of concrete that is 3.6m (12 feet) wide, 3m (10 feet) long and 10cm (4 inches) thick. The math is: 3.6m x 3m (12ft. x 10ft.) x (10cm/30cm per m) (4 in./12 in. per ft.) ft. = 12 x 10 x .333 = 40 cubic feet.
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The area of rectangular columns can be calculated the same way as a slab, but since most columns are circular, you need to use a different formula: hπr2. In this formula, h is the height of the column, π is 3.14, r is the radius of the column circle (half of the circle's diameter). For example, you need concrete for a column that is 12 feet high and is circular with a diameter of 1 foot. The math is: 12ft. x 3.14 x (1ft./2)2 = 12 ft. x 3.14 x (.5ft.)2 = 12ft. x 3.14 x .25 ft.2 = 9.42 cubic feet.
Steps are actually a series of smaller concrete slabs stacked on each other. This is the way you should calculate the volume. You need the ultimate height of the stairs, the ultimate depth, the height of each stair and width of the stairs. Consider each step a slab, calculate its volume and then add it to the volume of the other stairs. For example, you need concrete for three stairs that are 20cm (8 inches) high and 20cm (8 inches) deep and the total set of stairs will be 90cm (1yard) deep and 60cm (2 feet) across. Since there are three steps, we'll make three measurements. The first step is 1 yd x 2ft. x 8 in. The second step is (1 yd -- 8 in.) x 2ft. x 8 in. The third step is (1 yd -- 16 in.) x 2ft. x 8 in. The first thing to do is to convert everything to the same units. In this case we'll use feet. So the equation is: (3ft. x 2ft. x .67ft.) + (.78ft. x 2ft. x .67ft.) + (.56ft. x 2ft. x .67ft.) = 4.02 cubic ft. + 1.05 cubic ft. + .75 cubic ft. = 5.82 cubic ft.
When dealing with concrete, you need to allow extra for spillage or variations that might occur in the surfaces. Many people simply add 10 per cent to their final numbers. Simply multiply your final concrete needs by 1.1 to add the 10 per cent overage.
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