If there’s one question I get more than any other on concrete jobs, it’s this one: “How much concrete do I need?” Sounds simple. But I’ve seen guys mess this up enough times to know it isn’t. Under-order and your pour stops cold, you get a cold joint, and you’ve got a structural weak spot that’s going to give you grief down the road. Over-order and you’re paying $125-150 per yard for concrete that ends up in a pile in the corner of the yard.
I’ve been pouring concrete for over 20 years, from small fence post footings in the backyard to full garage floors. The math is the same every time, but the details matter. Let me walk you through how I calculate concrete for different project types, how to convert between cubic yards and bags, and the mistakes that show up on job sites more often than they should.
The Formula You Need to Know
Everything starts with volume. Concrete is measured in cubic yards, and the formula is the same no matter what you’re pouring:
Length (ft) x Width (ft) x Thickness (ft) / 27 = Cubic Yards
The divide-by-27 converts cubic feet to cubic yards (there are 27 cubic feet in one cubic yard). The one thing that trips people up every time is thickness. You have to convert inches to feet before you multiply. Four inches of thickness is not 4, it’s 0.333 (4 / 12 = 0.333).
That single mistake, leaving thickness in inches instead of converting to feet, will blow your estimate completely. I’ve seen contractors order 12 times more concrete than they needed because of it. You only make that mistake once.
How Much Concrete for a Slab
Slabs are the most common pour I see on residential jobs, whether it’s a garage floor, a back patio, a shed foundation, or a utility pad.
Standard residential slab thickness is 4 inches. Go to 5 or 6 inches if you’ve got weak soil, freeze-thaw cycles, or anything heavier than a regular vehicle driving on it.
Real Example: 20x24 Garage Slab
- Length: 20 ft, Width: 24 ft, Thickness: 4 inches (0.333 ft)
- Square footage: 20 x 24 = 480 sq ft
- Cubic feet: 480 x 0.333 = 159.84 cubic feet
- Cubic yards: 159.84 / 27 = 5.92 yards
- Add 10% waste: 5.92 x 1.10 = 6.5 yards
I’d order 6.5 to 7 yards for that job. The 10% waste factor isn’t optional. It accounts for spillage, slight grade variations, concrete left in the drum, and the fact that your sub-base is never perfectly level no matter how careful you are.
On bigger jobs over 6 yards, I’ll sometimes bring the waste factor down to 8%, but I never go below that.
Quick Reference: Cubic Yards by Slab Size (4” Thick)
| Slab Size | Square Footage | Yards (no waste) | Yards (10% waste) |
|---|---|---|---|
| 10 x 10 | 100 sq ft | 1.23 | 1.35 |
| 12 x 16 | 192 sq ft | 2.37 | 2.6 |
| 16 x 20 | 320 sq ft | 3.95 | 4.35 |
| 20 x 24 | 480 sq ft | 5.92 | 6.5 |
| 24 x 30 | 720 sq ft | 8.89 | 9.8 |
Use our concrete calculator to get an instant calculation for your specific dimensions and thickness.
How Much Concrete for Footings
Footings are trickier because they run continuously around a perimeter or in isolated spots under posts and columns. The math is the same, but you need to measure each section carefully.
Continuous footings formula: Length (ft) x Width (ft) x Depth (ft) / 27 = Cubic Yards
Real Example: House Foundation Footing
Say you’re pouring a continuous footing around a 24x30 addition foundation. The footing is 12 inches wide and 8 inches deep.
- Total linear feet of footing: (24 + 30) x 2 = 108 linear feet
- Width: 12 inches = 1.0 ft
- Depth: 8 inches = 0.667 ft
- Volume: 108 x 1.0 x 0.667 = 72 cubic feet
- Cubic yards: 72 / 27 = 2.67 yards
- With 10% waste: 2.93 yards (order 3 yards)
One thing to watch on footings: if the soil has been disturbed or the trench walls aren’t perfectly clean, you’ll fill more volume than your math says. I bump my waste factor to 12-15% on footing pours for that reason.
How Much Concrete for Fence Posts and Column Footings
Individual post footings are often calculated separately. The approach depends on whether you’re using a cylindrical tube form (Sonotube) or a square form.
Cylinder formula: 3.14 x (Radius in ft)^2 x Depth (ft) / 27 = Cubic Yards
For a Sonotube that’s 10 inches in diameter and 3 feet deep:
- Radius: 5 inches = 0.417 ft
- Volume: 3.14 x (0.417 x 0.417) x 3 = 1.63 cubic feet
- Per post: 1.63 / 27 = 0.06 cubic yards
For a single post that’s a tiny amount, but multiply it by 20 fence posts and you’re looking at 1.2 yards, which is worth calculating precisely.
For post footings and small pours like this, bags are usually the right call. I’ll cover that in the next section.
How Much Concrete for Walls
Concrete walls (retaining walls, foundation walls, basement walls) are calculated the same way as anything else, but you have to account for the full volume of the form, not just the face you see.
Wall formula: Length (ft) x Thickness (ft) x Height (ft) / 27 = Cubic Yards
A 30-foot retaining wall that’s 8 inches thick and 4 feet tall:
- 30 x 0.667 x 4 = 80 cubic feet
- 80 / 27 = 2.96 yards
- With 10% waste: 3.25 yards
On formed walls, I get a bit more conservative with waste because concrete doesn’t go sideways. But form blowouts happen, especially with walls poured too fast. Fill forms in lifts and don’t rush it.
Cubic Yards vs. Bags: The Honest Comparison
This comes up on every small project. The short answer: if you need more than 1 yard, call a ready-mix truck. If you need less than half a yard, bags are usually more practical. There’s a gray zone in between where it depends on truck access, labor available, and timeline.
Bag Coverage (60-lb bags)
One 60-lb bag of premix concrete covers about 0.45 cubic feet. That means:
- 1 cubic yard (27 cubic feet): needs about 60 bags
- Half a yard (13.5 cubic feet): needs about 30 bags
At roughly $4.50-5.50 per bag, here’s the cost comparison for a 2-yard job:
- Bags: 120 bags x $5 = $600 in materials alone, plus a full day of mixing
- Ready-mix: 2 yards x $135/yard = $270 delivered, poured in an hour
The bags option is almost always more expensive and always more labor-intensive. I stopped mixing bags for anything over a half yard about 15 years ago. The truck just makes more sense.
The one exception is when you have no truck access. Narrow side yards, gated properties, tight urban lots. In those situations, bags and a portable mixer or wheelbarrow relay are your options. Factor in the extra labor cost when you’re pricing the job.
For a full cost breakdown on ready-mix pricing in your area, use the concrete cost per yard calculator.
Common Mistakes That Cost Real Money
I’ve watched contractors make these same errors over and over. Learn them here instead of on the job.
Mistake 1: Not Converting Inches to Feet
Covered above but worth repeating. If you’re calculating a 4-inch slab and you plug in “4” for thickness instead of “0.333,” your answer is 12 times too high. Double-check every time.
Mistake 2: Skipping the Waste Factor
No pour is perfectly exact. The sub-base has low spots. Some concrete stays in the drum. A form shifts slightly. You spill some during the pour. The 10% waste factor covers all of this. Skip it and you’ll be scrambling for extra concrete mid-pour, which is about the worst situation you can be in on a Saturday morning.
Mistake 3: Not Accounting for Slope
A patio or driveway that drains properly needs about a 1/4-inch drop per foot of run. That means one side of your slab is thicker than the other. On a 15-foot-wide driveway, you’ve got a 3.75-inch slope, which changes your average thickness slightly. For short slabs it’s minor. For long driveways, it adds up.
Mistake 4: Measuring the Wrong Thing
I’ve seen people measure a fence line instead of where the posts actually go. Or measure a wall’s face dimension instead of the total perimeter. Sketch it out before you start calculating. It takes two minutes and saves a lot of headaches.
Mistake 5: Not Calling the Plant Early Enough
In peak season, spring through summer, ready-mix plants book up fast. Call 3-5 days ahead minimum. Confirm the morning of your pour. Nothing derails a project like a canceled concrete delivery when your forms are set and your crew is standing there.
Real-World Scenario: Backyard Shed Pad with Footings
A homeowner called me last fall wanting a 12x16 shed pad with perimeter footings rather than just a slab on grade. The footing was 12 inches wide, 12 inches deep, and ran around the full perimeter. The slab inside was 4 inches thick.
Footing calculation:
- Perimeter: (12 + 16) x 2 = 56 linear feet
- Volume: 56 x 1.0 x 1.0 = 56 cubic feet
- Cubic yards: 56 / 27 = 2.07 yards
Slab calculation (net interior area, inside the footings):
- Interior dimensions: approximately 10.5 x 14.5 (accounting for 12-inch footing on each side)
- Square footage: 10.5 x 14.5 = 152.25 sq ft
- Cubic feet: 152.25 x 0.333 = 50.7 cubic feet
- Cubic yards: 50.7 / 27 = 1.88 yards
Total: 2.07 + 1.88 = 3.95 yards, plus 10% waste = 4.35 yards
We ordered 4.5 yards. Poured clean, had about a quarter yard left over that went toward a small equipment pad nearby. Perfect.
How to Check Your Math
Even after 20 years, I verify my hand calculations with a calculator before I order. Our concrete calculator handles slabs, footings, columns, and walls. Put your numbers in, compare to your hand calc, and if they match, you’re good to go.
For more on what concrete actually costs once you’ve figured out how much you need, see our detailed breakdown of concrete patio cost per square foot with labor and materials. And if you want to go deeper on slab math specifically, the guide on how to calculate concrete for a patio walks through patio shapes including L-shapes and curves.
If your project is a driveway rather than a slab or footing, the same formulas apply but with a few driveway-specific factors to watch for. See how much concrete do I need for a driveway for thickness recommendations, apron calculations, and the waste factors unique to driveway pours. For a full look at what drives slab cost once you have your volume, the concrete slab cost calculator guide breaks down pricing by thickness, reinforcement, and finish type.
Frequently Asked Questions
How much concrete do I need for a 10x10 slab?
At 4 inches thick: 10 x 10 x 0.333 / 27 = 1.23 yards. Add 10% waste and you’re at 1.35 yards. Order 1.5 yards to be safe.
How many bags of concrete equal a yard?
At 60 lbs per bag (0.45 cubic feet): you need about 60 bags per cubic yard. At 80 lbs per bag (0.60 cubic feet): about 45 bags per yard.
How thick should a concrete slab be?
4 inches is standard for residential patios and shed pads. 5-6 inches for driveways, garages with heavy vehicles, or weak soil. Never under 3.5 inches for anything structural.
Do I really need to add 10% waste?
Yes. Every professional does. The cost of running short mid-pour is far greater than the cost of a little extra material. This is not the place to try to save money.
How many cubic yards in a concrete truck?
Standard ready-mix trucks carry 8-10 cubic yards. Some companies have smaller trucks (4-6 yards) for tighter sites. Most plants have a minimum delivery, often around 1-2 yards, with a short-load surcharge below that minimum.
Whether you’re pouring a simple pad or a full foundation, accurate concrete estimation is one of those skills that separates the professionals from the guys who are always scrambling at the job site. Measure carefully, use the formulas above, always add your waste factor, and verify with a calculator before you call the plant.
“Measure twice, cut once” applies to concrete just as much as it does to lumber.
Brad is a licensed contractor with 20+ years of experience in residential remodeling and concrete work. He’s the founder of EstimationPro.AI, which helps contractors produce accurate estimates in minutes instead of hours.
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