How to Calculate Concrete Yardage: Easy Formula for Slabs, Footings & More

I once watched a guy pour a driveway and come up a yard and a half short because he eyeballed the measurements instead of doing the actual math like a reasonable person. The finishing crew stood around for 45 minutes waiting on a reload while the first half of the pour was setting up, and the cold joint where old met new was visible for years afterward, a permanent seam running right down the middle of his driveway like a scar that never healed. That mistake cost him about $300 in extra delivery fees plus a driveway that never looked right for as long as he owned the house.

Brutal. Just brutal. And completely avoidable.

The math itself isn't hard. Honestly it's grade-school level arithmetic that you could do on the back of a napkin while eating lunch. But it's easy to mess up if you rush through it or forget one conversion step and end up ordering the wrong amount and paying for it twice, once in extra delivery fees and once in the cold joint you'll stare at forever. Here's the formula that covers basically every rectangular shape you'd ever pour, whether it's a slab or a footing or a wall or columns or whatever else you're building:

Volume in cubic feet = Length (ft) x Width (ft) x Thickness (ft)

Then divide by 27 to get cubic yards. That's it. That's the whole formula and there's nothing else to it. Twenty-seven is the magic number because a cubic yard is 3 feet by 3 feet by 3 feet, which works out to exactly 27 cubic feet, and concrete in the US is always sold by the cubic yard so that's the unit you need to land on every single time.

And honestly?

But here's the part that trips people up more than anything else, the thing that generates wrong orders and angry phone calls and wasted money: most plans give thickness in inches, not feet, and you absolutely have to divide inches by 12 first before you plug anything into the formula or your numbers will be wildly wrong. A 4-inch slab is 0.333 feet and a 6-inch slab is 0.5 feet. I know it sounds dumb and obvious, but I guarantee you someone somewhere is ordering concrete right now using inches directly in the formula and wondering why the number came out so absurdly huge and the dispatcher is probably laughing, or sighing, or whatever they do when they get those calls. This happens all the time, way more than you'd think, and the dispatchers probably have a collection of stories about it.

So yeah.

Plus all the little extras, you get the idea.

Slabs (driveways, patios, floors)

Formula: (L x W x T) divided by 27. That's literally the whole thing, nothing fancy.

Say you're pouring a 20 x 30 foot patio at 4 inches thick, which is a pretty common weekend warrior project size. Thickness converts to 4 divided by 12 which is 0.333 feet. Volume is 20 x 30 x 0.333 which gives you 199.8 cubic feet on the nose. Divide by 27 and you get 7.4 yards. With 10 percent waste added, 7.4 times 1.10, you're at 8.14 yards. Round up to 8.25 or 8.5 yards and make the call. Done.

That simple.

Not complicated at all. Three steps and you're finished, and stuff.

Plus all the little extras, you get the idea.

Footings

Same formula, you just measure the actual trench dimensions instead of a flat area. Footings go in the ground where nobody sees them anyway. If your footing trench is 40 feet long and 2 feet wide and 1 foot deep, the volume is 40 x 2 x 1 which is 80 cubic feet and then divide by 27 to get 2.96 yards. With 5 percent waste that's 3.11 yards, round up to 3.25 and order it and you're good.

One thing about trenches that nobody mentions in the YouTube videos or the how-to guides: they're almost always wider at the top than the bottom because dirt collapses and shovels aren't precision instruments and the ground never cooperates with your plans. Use the average width when you calculate, not the width at the top or the bottom individually. Overestimating slightly beats underestimating every single time and it's not even a debate, trust me, I've been on both sides of that equation and learned the hard way which one sucks more.

Walls (retaining or foundation)

Walls are basically vertical slabs when you think about it for more than two seconds, same math just rotated 90 degrees. Length times height times thickness, all converted to feet because everything has to be in feet before you divide by 27, that's the golden rule.

A retaining wall that's 30 feet long and 4 feet high and 8 inches thick: thickness converts to 8 divided by 12 which is 0.667 feet. Volume is 30 x 4 x 0.667 which comes out to 80.04 cubic feet, and divide by 27 to get 2.96 yards. With 10 percent waste you're at 3.26 yards so order 3.5 and sleep better at night knowing you've got a cushion.

Columns

Round column formula: pi times radius squared times height, then divide by 27, and it sounds fancier than it actually is in practice. A column that's 12 inches in diameter and 10 feet tall: radius is 6 inches which converts to 0.5 feet. So it's 3.1416 times 0.25 times 10 which gives you 7.854 cubic feet and divide by 27 to get 0.29 yards. With 10 percent waste that's 0.32 yards per column. For multiple columns just multiply by however many you're pouring that day and round up a little.

Square columns are even simpler, same formula as a slab: length x width x height, divide by 27, and you're done. Nothing to it, honestly.

The waste factor is cheap insurance

I've watched contractors skip the waste factor entirely and end up four-tenths of a yard short, which doesn't sound like much until you're standing there watching the last of the concrete slide out of the chute with three feet of form still empty and the crew looking at you like "now what?" A short load costs $150 to $200 minimum and if the plant can't get you more concrete fast enough you're looking at a cold joint that weakens the whole structure permanently and ruins the appearance. Adding 5 to 10 percent is nothing compared to that kind of headache, it's basically free peace of mind wrapped in math.

For a 10-yard pour, 10 percent waste adds one extra yard to your order and at $150 a yard that's $150 extra on the invoice total, which honestly isn't that bad in the grand scheme of a concrete project that's already costing thousands. A short-load fee often costs more than that and the cold joint could permanently weaken the slab and look terrible, so the math is pretty straightforward when you actually run the numbers. Flat even ground with an experienced crew who knows what they're doing? Five percent is probably fine and you'll likely have a little left over for stepping stones or a birdbath base or whatever else you dream up. Rocky soil or steep slopes or your first time ever pouring concrete and you're nervous about everything? Go with ten percent and don't think twice about it because the alternative is way worse and way more expensive.

How to order like someone who's done this before

Calculate your base volume using the formulas above and write it down somewhere you won't lose it. Multiply by 1.05 or 1.10 for waste depending on your situation and how risk averse you're feeling that day. Round up to the nearest quarter yard because 8.14 becomes 8.25, not 8.00, and the plant will happily sell you quarter-yard increments without batting an eye. Call the plant and confirm the mix type, usually 3000 PSI for most homeowner stuff like patios and walkways and shed pads and that sort of thing, the slump which is normally 4 to 5 inches for flatwork, and any admixtures you might need for your specific job and local conditions. And definitely have a plan for leftover concrete because there's almost always a little extra and you don't want to be standing there with a wheelbarrow full of setting concrete wondering frantically where to dump it before it turns into a 400-pound paperweight.

If your slab isn't a perfect rectangle, which almost none of them are in real life, break it down into smaller rectangles and calculate each one separately and add them all together at the end. An L-shaped patio? Split it into two rectangles and do the math on each and sum them up. Curved edges you can approximate with rectangles or use the average width approach, whichever makes more sense for the shape you're dealing with. Overestimate a tiny bit because extra concrete becomes garden stones or a birdbath base or path pavers or planter bases or whatever creative thing you come up with on the spot. Not enough concrete becomes a full blown disaster that you'll be telling stories about for years and showing people the cold joint as evidence.

Ignore rebar for volume calculations entirely and don't even think about it. It displaces almost nothing and the math isn't worth the headache, or the decimal places, or the time you'd spend worrying about it, or anything else for that matter. And don't include the gravel base in your concrete numbers because the concrete fills the form above the gravel layer, not the gravel itself, those are two completely separate material orders and mixing them up will give you a wildly wrong number.

There are apps that do all this math for you on your phone in seconds and they're convenient when they work correctly. I've used them and I've also watched them give completely wrong answers because someone typed inches instead of feet into one field and didn't notice until the truck was halfway to the house and it was too late to fix. Understand the formula yourself well enough to spot when an app is feeding you garbage numbers that don't pass the smell test. Double-check everything before you dial the plant because the dispatcher on the other end of the phone can't fix a math error after the truck has already left the yard and is on its way to your house with six yards of concrete you might not actually need, might not have room for, and definitely can't return or send back or anything like that.

Measure twice, add your waste factor, round up to the nearest quarter yard, call the plant, confirm the details. That's the whole system from start to finish and it works every time. And if you end up with a little extra concrete at the end of the day, you've got yourself some free stepping stones or the base for that birdbath you've been meaning to put in for the last three years and never got around to. Beats staring at a cold joint for the next decade, that's for sure. Or dealing with extra fees, or angry calls to the plant, or any of that other nonsense. Not even close.