Work out how much concrete a slab, footing or round column needs in yd³, m³ and ft³, plus the number of premix bags.
A 10 ft by 10 ft slab that is 4 in thick holds 10 × 10 × (4 ÷ 12) = 33.33 ft³ of concrete. Divide by 27 to get cubic yards: 33.33 ÷ 27 = 1.235 yd³. The page adds 10% for waste by default, which makes 36.67 ft³ or 1.358 yd³, and that is the amount to order. In metres the same idea applies: a 3 m by 3 m slab that is 10 cm thick is 3 × 3 × 0.1 = 0.9 m³, or 0.99 m³ with 10% waste. Length and width are typed in feet or metres, and the thickness in inches or centimetres, so there is no need to convert by hand.
A strip footing is a long box, so its volume is length × width × depth. A footing 30 ft long, 16 in wide and 8 in deep holds 30 × 1.3333 × 0.6667 = 26.67 ft³. A round column or cylinder uses the circle area: volume = π × (diameter ÷ 2)² × height. A column 12 in across and 10 ft tall holds π × 0.5² × 10 = 7.854 ft³, and four of them with 10% waste need 34.56 ft³, or 1.28 yd³. Use the number of identical pieces box for several columns, pads or slabs of the same size. The page does not work out stairs, sloped slabs, or shapes with holes: split those into simple parts and add the results by hand.
Waste is the extra you order so that you do not run short. It covers spillage, ground that is lower than planned, and forms that are not perfectly square. The default is 10%, and you can set it from 0 to 100%. At 0% the page shows the exact volume from your dimensions. The page does not include reinforcement, a thicker edge to a slab, or a pour that is wider than the forms, so add those to the size or to the waste. Volumes show 3 decimals, or 4 to 6 for amounts under 1, so that a small job does not round to zero.
Bag counts divide the volume with waste by a typical yield per bag and round up: 80 lb about 0.6 ft³, 60 lb about 0.45 ft³ and 40 lb about 0.3 ft³. For the 10 ft slab above, 36.67 ÷ 0.6 = 61.1, so 62 bags of 80 lb, or 82 bags of 60 lb. In metric units the page uses 0.012 m³ for a 25 kg bag, scaled by weight for 20 kg and 40 kg bags. These are typical yields, check the bag: the number printed on your product is the one to use, and it differs between brands and mixes. Bag counts are for premix you add water to. Ready-mix delivered by truck is ordered by volume, in yd³ or m³.
Multiply length × width × thickness, with all three in the same unit, then divide by 27 for cubic yards if you work in feet. A 10 ft by 10 ft slab 4 in thick is 33.33 ft³, which is 1.235 yd³, and 1.358 yd³ with 10% waste.
About 45 bags, using a yield of 0.6 ft³ per bag: 27 ÷ 0.6 = 45. For 60 lb bags at 0.45 ft³ it is 60 bags. These are typical yields, so check the number on your bag.
3 × 3 × 0.1 = 0.9 m³. With the default 10% waste you would order 0.99 m³, which is 1.295 yd³ or 34.96 ft³. Choose Metres and centimetres to type those sizes directly.
The page defaults to 10%, which makes a 33.33 ft³ job 36.67 ft³. Use more if the ground is uneven or the forms are rough, and 0% to see the exact volume of your dimensions.
Use π × (diameter ÷ 2)² × height. A column 12 in across and 10 ft tall holds π × 0.5² × 10 = 7.854 ft³. Choose Round column, type the diameter in inches and the height in feet.
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