Concrete Yardage Calculator: Estimate Volume for Any Project

The guy at the concrete plant knows my voice now, which is not something I’m proud of, because it means I’ve called him mid-pour too many times with the same exact problem, the one where I was sure I calculated everything right and the truck is empty and the forms still have a foot of unfilled space and everyone is standing around looking at me like I’m the reason their Saturday is about to get longer, which I guess I am.

Concrete yardage isn’t hard to calculate, but it’s weirdly easy to miscalculate in ways that cost actual money and real time and sometimes the structural integrity of your project if you end up with a cold joint in the wrong place, and I’ve been on both sides of that equation enough times to know that the five minutes you spend double checking your numbers is worth more than the hour you’ll spend waiting for a second truck that costs $200 just to show up with a partial load.

Basically.

This isn’t going to be a textbook chapter about cubic measurements, it’s going to be what I’ve actually learned from pouring slabs and footings and walls and columns over the years, with real numbers and real mistakes and the stuff that nobody tells you when you watch a three minute YouTube video about concrete estimating and think you’ve got it figured out.

Concrete is sold by the cubic yard, and here’s why that matters

A cubic yard is 27 cubic feet, which is a cube three feet on each side, and ready-mix trucks carry between 9 and 10 yards typically, and the minimum order at most plants is somewhere between 1 and 5 yards depending on how busy they are and how much they like your voice on the phone, and if you order under the minimum they tack on a short load fee that makes budgeting impossible if you didn’t know it was coming. And one cubic yard of concrete covers about 81 square feet at 4 inches thick, and that’s the quick mental math shortcut I use when someone tells me their patio dimensions and I want to know if we’re talking one truck or two before I even break out the calculator, and honestly it’s right about 90 percent of the time which is close enough for the napkin estimate phase.

Slabs, which are what most people are actually pouring

For a rectangular slab, the formula is length times width times thickness in feet, then divide by 27, and the only hard part is remembering that 4 inches is 0.333 feet and not 0.4 feet because that single decimal place error adds up to a lot of missing concrete on a big slab, and I’ve watched it happen twice on jobs where the homeowner did the math and the contractor trusted it without checking.

A 12 foot by 16 foot patio slab at 4 inches thick: 4 inches is 0.333 feet, multiply 12 times 16 times 0.333 and you get 63.9 cubic feet, divide by 27 and you need 2.37 cubic yards of concrete, then add 10 percent waste for spillage and uneven ground and the fact that your forms are probably not perfectly level, and 2.37 times 1.10 gives you 2.61 cubic yards, and I’d order 2.75 to be safe because rounding down on concrete is the financial equivalent of playing chicken with a cement truck and you always lose.

Nope.

A 10 foot by 10 foot shed slab at the same 4 inches thick: that’s 100 square feet times 0.333 feet equals 33.3 cubic feet, divide by 27 is 1.23 cubic yards, plus 10 percent waste is 1.35 yards, and since most plants have a 1 yard minimum you’re barely over it, and you might actually be better off mixing bags for this one if you don’t have other small pours to combine into a single order because the delivery fee alone might cost more than the concrete.

Footings, and why they’re different

Footings are long narrow trenches and you use the same rectangular prism formula but the waste factor is different because the concrete is contained in a trench rather than spread across open ground, and you’re generally losing less to spillage so 5 percent waste is usually enough unless your trench is really rough or you’re pouring in rain or something.

A footing 30 feet long and 2 feet wide and 1 foot deep: 30 times 2 times 1 is 60 cubic feet, divide by 27 is 2.22 yards, add 5 percent waste and you’re at 2.33 yards, round up to 2.5 and call it good because footing shortages are harder to fix than slab shortages since the concrete disappears into the ground and you can’t really see where the level is until it comes back up.

And if your footing has a wider base like a spread footing for a foundation wall, measure the average width and use that, because trying to calculate the exact volume of a trapezoidal trench cross section is the kind of precision that doesn’t survive contact with the shovel operator who dug the trench in the first place, you know?

Walls, which are basically vertical slabs with higher stakes

A wall 50 feet long and 8 feet high and 8 inches thick: 8 inches is 0.667 feet, multiply 50 times 8 times 0.667 and you get 266.8 cubic feet, divide by 27 is about 9.88 cubic yards, and this is already a full truck load if you’re ordering from a standard 10 yard truck, so you might want to check if the plant dispatches 11 or 12 yard loads before you add waste.

Add 7 percent waste for walls because form bulges and pump loss and the fact that wall forms always leak a little at the bottom no matter how carefully you stake them, and 9.88 times 1.07 gives you 10.57 yards, and that’s pushing the limit of a single truck so maybe split it into two pours or confirm the truck can actually carry that much, and if you’re using a pump truck the pump lines hold about half a yard of concrete that never reaches the forms and you need to account for that too, which is the kind of detail that nobody mentions until you’re looking at a pump hose that’s still full of concrete when the truck is empty.

Seriously.

Columns, round ones specifically

Round columns use the cylinder formula: pi times radius squared times height, all in feet, and the most common mistake is using diameter instead of radius because you measured across the column form and forgot to divide by two, and I am personally guilty of ordering twice the concrete I needed for a set of deck footings because I used 12 inches as the radius instead of 6 inches, and my wife still brings this up at parties, etc.

A 12 inch diameter column that’s 10 feet tall: radius is 6 inches which is 0.5 feet, pi is 3.14, square the radius to get 0.25, multiply 3.14 times 0.25 times 10 and you get 7.85 cubic feet per column, divide by 27 and each column is 0.29 yards, times three identical columns is 0.87 yards, add 10 percent waste and you’re at 0.96 yards, and at that volume you’re under the 1 yard minimum at most plants so you might want to combine this order with something else or just buy bags and mix it yourself.

The waste factor table, because guessing is expensive

If your dirt is soft or the weather is windy or you’re pouring on a slope, bump all these numbers by about 5 percent, and if you’re pouring in winter with heated concrete and blankets and all that, add another couple percent because cold weather pours just seem to consume more material for reasons I’ve never fully understood. I mean, maybe it’s the forms contracting or the concrete stiffening faster, but whatever it is, I’ve noticed that December pours always run short compared to July pours with the exact same dimensions, and I’m not the only one who’s noticed this, and adjusting for it has saved me from that cold joint panic more than once.

Real numbers that don’t match the mental math most people do

A standard 8 foot by 10 foot shed slab at 4 inches thick needs about 1 cubic yard with waste included, and a 20 foot by 30 foot garage slab at 5 inches thick uses 20 times 30 times 0.417 which is 250.2 cubic feet divided by 27 equals 9.27 yards, plus 10 percent waste is 10.2 yards, and that’s a full truck, and if your garage has a thickened edge for the perimeter you need to add that as a separate footing calculation because it’s deeper than the slab itself and the volume add can be significant on a big garage.

A single 6 foot by 6 foot stepping stone at 2 inches thick: 2 inches is 0.167 feet, so 6 times 6 times 0.167 is 6 cubic feet, divide by 27 is 0.22 yards, and you’d never order a truck for that, you’d buy about 14 bags of 60 pound concrete mix and do it in a wheelbarrow, and things like that, you get the idea.

The stuff that goes wrong when you estimate

Converting inches to feet wrong is the number one mistake, and 4 inches is 0.333 feet not 0.4 and not 0.33 on a big enough pour that rounding matters, and I’ve seen someone order for a 2000 square foot slab thinking 4 inches was 0.4 feet and they were short by something like 4 yards, which is not a small mistake. And forgetting the waste factor entirely is number two, and the waste factor goes on after you calculate volume not before, because if you add it before you’re basically adding waste to numbers that already include waste which doubles up the safety margin and you end up with way too much concrete. So rounding too early is number three, and you should keep every decimal until the final answer and then round up to the nearest quarter yard, and I’ve seen people round at every step and end up with an answer that’s a full yard different from the correct one on a big pour. But assuming every plant delivers partial yards without a fee is number four, because most don’t, and the short load fee for under 4 or 5 yards can be $200 or more, and if you’re pouring a small patio that only needs 3 yards you should call ahead and ask what the minimum is before you give them your credit card.

Multiple shapes, how to add them together

Calculate each shape separately and add the volumes, then apply one waste factor to the total, and don’t apply waste to each piece individually because then you’re stacking waste on waste and you’ll order 15 percent too much on a multi shape project.

If your slab is 3.5 yards and your footings are 1.2 yards, the total is 4.7 yards, and with 10 percent waste that’s 5.17 yards, so order 5.25 yards, and the leftover quarter yard is your birdbath base or your stepping stone or just the price of not having a cold joint in your foundation.

Honestly after all the pours I’ve done I’d rather explain why there’s a small pile of leftover concrete on the site than explain why there’s a seam running through the middle of someone’s garage floor, and if you’ve ever had to have that conversation with a homeowner who just paid you ten thousand dollars you know exactly what I’m talking about, and the extra eighty bucks for another half yard is the cheapest insurance you’ll ever buy on a concrete job.