Ordering concrete feels simple until the truck shows up. I once watched a weekend patio job stall because the pour came up half a yard short. The crew stood around while the first section started to set. The math behind a concrete order is not hard, but you have to do it in the right order.
Here is the method I use on every job. I work out the volume by hand first, then I check it with a free concrete calculator that handles slabs, round columns, and custom shapes. The page is written in Spanish, but it switches between meters, feet, and inches, and it adds a waste margin and a bag count for you. A second opinion costs nothing, and it catches the slips I make when I rush.
*Start with the basic formula
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Volume is length times width times depth. That works for any box shape, like a slab, a step, or a footing. The only real trap is units. If you measure length and width in feet but depth in inches, your answer will be wrong by a lot.
Pick one unit and convert everything to it before you multiply. In the US, I turn inches into feet by dividing by 12. Then I divide the cubic feet by 27 to get cubic yards, because that is how most suppliers sell concrete. In metric countries, you multiply meters and order cubic meters.
*Slabs, patios, and driveways
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Let’s say you want a patio that is 12 feet long, 10 feet wide, and 4 inches thick. Four inches is about 0.33 feet. So 12 times 10 times 0.33 gives you 40 cubic feet. Divide by 27 and you get about 1.48 cubic yards.
Here is a shortcut I like for rough planning. One cubic yard covers about 81 square feet at 4 inches thick. If your patio is 120 square feet, you can see right away that you need a bit under two yards. It is not exact, but it keeps you from ordering way too much or way too little.
*Round columns and post holes
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For a round shape, the formula is 3.14 times the radius squared times the height. The radius is half the diameter. Say you have a column that is 12 inches wide and 8 feet tall. The radius is 0.5 feet, so 3.14 times 0.25 times 8 gives you about 6.3 cubic feet.
Four of those columns need about 25 cubic feet, or just under one cubic yard. The most common mistake here is using the diameter instead of the radius. That makes your answer four times too big, and you will notice it on the invoice.
*Footings and trenches
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A footing is just a long box, so the basic formula works. Picture a footing that is 40 feet long, 2 feet wide, and 1 foot deep. That is 40 times 2 times 1, or 80 cubic feet. Divide by 27 and you land at about 2.96 cubic yards.
I would round that up to 3 and then add extra. Hand-dug trenches are never perfectly even. The soil crumbles at the edges, and the bottom dips in spots. Every little dip eats concrete.
*Add a waste margin
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Always order more than the math says. Five to ten percent extra is a good range for most jobs. Uneven ground, bulging forms, and spills all use up concrete you did not plan for.
Take the patio from earlier. Ten percent extra turns 1.48 yards into about 1.63 yards. I would order 1.75 and be done. Running short is much worse than having a little left over, because new concrete poured against concrete that has started to set makes a weak seam called a cold joint. That seam is often the first place a crack shows up.
*Bags or ready-mix?
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An 80 pound bag of premix makes about 0.6 cubic feet. Our 40 cubic foot patio would need about 67 bags. That is more than 5,000 pounds of material to lift, mix, and pour before it starts to set. Check the yield printed on your bag, because brands differ a little.
For small repairs, a few post holes, or a spot a truck cannot reach, bags make sense. For anything over a yard or two, ready-mix is usually easier and gives a more even result. Many suppliers charge a short load fee on small orders, so call and ask before you decide. Prices and minimums change from place to place.
A quick checklist before you order
Measure twice, and write down every number.
Convert all measurements to one unit.
Multiply to get the volume of each section, then add the sections together.
Add 5 to 10 percent for waste.
Ask your supplier about minimums, delivery fees, and the mix strength you need.
Concrete math looks scary for about five minutes. After that, it is just boxes and circles. Do the numbers by hand once, check them with a tool, and you will walk onto your next pour with a lot more confidence.
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