Concrete Yardage Calculator: Volume Formulas for Slabs, Footings & Columns

I’ve seen too many DIY patios end up short by half a yard because someone forgot to account for waste and stood there looking at the gap between the truck and the forms like the concrete fairy was going to magically fill it in from thin air while the driver tapped his watch and asked where he should dump the rest of the load, or worse, they ordered a full extra yard and had to figure out what to do with a pile of leftover concrete hardening in their driveway while their spouse gave them that look that needs no translation, and the trick is knowing the right volume formula and adding a realistic waste factor and not letting optimism override basic math, which is honestly what gets most of us in trouble, and I know this because I’ve been that person in the driveway. Not bad right, but let’s break it down anyway because the difference between a clean pour and an expensive disaster is usually about five minutes of math that you’d rather skip but really shouldn’t. Concrete is sold by the cubic yard and one cubic yard equals 27 cubic feet which is just 3 feet times 3 feet times 3 feet and once you internalize that conversion everything else becomes simple multiplication with a few adjustments for different shapes and the occasional curveball that the ground throws at you when you start digging and discover that the earth is not as flat as your eyes told you it was, you know?

Basically the core formula goes like this: Cubic yards equals Length in feet times Width in feet times Thickness in feet all divided by 27, and every single concrete project on the planet starts with some version of that calculation whether you’re pouring a patio or a foundation or a birdbath base that you promised your partner you would finish by last spring and still haven’t gotten around to, and all the variations in between, you get the idea, etc., you know what I mean, all the common pours and stuff.

Slabs and Patios

For a flat Slab it’s straightforward enough that even I can do it before my first coffee and I am not a morning person at all which is why I double check every measurement at least three times and still manage to second guess myself halfway through the pour when the truck is already churning and there’s no going back. Say you’re pouring a 20 foot by 30 foot Patio that’s 4 inches thick and you need to convert those 4 inches to feet first which gives you 4 divided by 12 equals 0.333 feet. Then the volume is 20 times 30 times 0.333 which gives you 199.8 cubic feet and dividing by 27 gets you 7.4 cubic yards.

Now add a waste factor and please don’t skip this part because I have skipped it exactly once and the result was a very awkward conversation with a Concrete truck driver who had better places to be on a Friday afternoon and was not interested in my creative explanations about why we came up short. For Slabs I usually go with 5 to 7 percent because you might lose a little to spillage or an uneven base or that one corner where the form settled overnight when you weren’t looking and the math you did yesterday no longer quite matches the hole you dug, and 7 percent of 7.4 is about 0.52 yards on this patio, and order 8 yards, and call it good, and honestly you sleep better knowing there’s some wiggle room instead of hoping everything goes perfectly, which it never does.

Footings

Footings are often rectangular or shaped like a trapezoid that gets wider at the bottom which makes measuring them feel more like an art project than a math problem and I’ve seen people measure the top width only and forget that the bottom is 30 percent wider and that little mistake alone can throw your entire order off by enough to matter when the truck shows up and the Concrete starts flowing and you realize the hole is bigger than your numbers said it would be. For a rectangular Footing that’s 2 feet wide and 1 foot deep and 10 feet long you multiply 2 times 1 times 10 which gives you 20 cubic feet and dividing by 27 gives you 0.74 yards, which is honestly a tiny order and you’re probably mixing bags for that one unless you can combine it with other pours, but I get it, sometimes you just want to run the numbers before deciding.

But if the Footing is wider at the bottom like most of them are you need to use the average width because the wide base eats up way more material than the narrow top suggests and that’s the kind of detail that separates a clean pour from a panic phone call. If it’s 2 feet at top and 3 feet at bottom the average is 2.5 feet, you get the idea.

Honestly that alone can change your yardage by 20 percent and nobody wants to be the person explaining to their partner why there’s a Concrete truck parked in front of the house with half a load still in the drum and nowhere to put it except maybe the neighbor’s flower bed which is defintely not going to make you popular on the block.

Walls

Concrete Walls are basically vertical Slabs and the math is the same as the flat stuff except you’re measuring up instead of across and forgetting to subtract window or door openings is the kind of mistake that makes you feel like an amateur even if you’ve been doing this for years and should know better by now. For a 30 foot long Wall that’s 8 feet high and 8 inches thick you first convert the thickness to feet which is 8 divided by 12 giving you 0.667 feet and then you multiply 30 times 8 times 0.667 which gives you 160 cubic feet and dividing by 27 gets you 5.93 yards, so about 6 yards. But openings matter more than you’d think for something that’s basically just empty space where Concrete isn’t going and a 3 foot by 4 foot window opening at 0.667 feet thick works out to 3 times 4 times 0.667 which is 8 cubic feet and that saves you roughly 0.3 yards which is not huge but it adds up when you’ve got multiple windows and a door and suddenly you’re half a yard short of what the simple formula told you to order.

Columns

Columns are the shape that confuses everyone because now you’re dealing with circles instead of rectangles and the formula switches from length times width to pi times radius squared times height and I have seen people stare at that for way too long trying to remember whether radius is half the diameter or twice the diameter before finally just guessing and hoping for the best which never works out the way they want it to, I mean honestly it’s the kind of thing you learn in eighth grade math and then immediately forget until you’re standing in front of a Column form with a tape measure and a calculator and a vague sense of dread.

For a Column with a 1 foot diameter and a 10 foot height the radius is 0.5 feet and the area is 3.14 times 0.5 squared which equals 0.785 square feet and then you multiply by the height of 10 feet to get 7.85 cubic feet which divided by 27 gives you 0.29 yards. Add 10 percent waste because Columns are narrow and tricky to pour without spillage and you’re looking at about 0.32 yards per Column and for a porch with 12 of them that’s roughly 3.8 yards total which is enough to make you wonder why you didn’t just build a deck instead.

Waste Factor Stuff

Most contractors I know add 5 percent for simple Slabs and 10 percent for anything complex like Footings with Rebar or Columns where the Concrete has to squeeze into tight spaces and something always splashes out no matter how careful you are with the chute, and a 10 percent waste on a 10-yard pour gives you an extra yard which is usually enough to cover mistakes without wasting money and leaving you with a hardened pile that you’ll be chipping at with a sledgehammer next weekend, and nobody wants that, so add the waste factor and don’t overthink it.

Practical Stuff Nobody Tells You

Always convert inches to feet before doing any math because mixing units is the number one way people mess up their Concrete order and I keep a little cheat sheet taped to the inside of my toolbox that says 4 inches equals 0.333 feet and 6 inches equals 0.5 feet and 8 inches equals 0.667 feet and honestly I still look at it every single time because my brain refuses to memorize these numbers no matter how many times I pour Concrete which is either a sign of humility or early memory loss and I’m not sure which is worse at this point. Measure twice and use a laser measure if you have one because misreading the tape by a couple inches on a large Slab can throw your final number off by half a yard and that’s the difference between a clean Saturday pour and a disaster that stretches into Sunday.

Round up to the nearest quarter yard because Concrete trucks deliver in quarter yard increments and ordering 7.93 yards just means they’ll round to 8 anyway so you might as well do the rounding yourself and feel like you’re in control of something and things like that, I guess, and I’d rather have half a yard left over than be scrambling for more Concrete on a Friday afternoon when the plant closes in 20 minutes and the dispatcher stopped answering their phone an hour ago and you’re standing there with a half filled form and a sinking feeling in your stomach that this is going to be an expensive weekend.