Concrete Calculator
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Find out how many cubic yards of concrete you need for a slab, footing, or pour from length, width, and thickness — plus the equivalent bag count for DIY jobs.
Quick answer: A 10×10 ft slab poured 4 in. thick needs about 1.23 cubic yards of concrete before waste, or 1.36 cubic yards with the standard 10% overage allowance — the figure to give a ready-mix supplier. That same slab needs about 62 bags of 80 lb premix if you're mixing it yourself, per Quikrete's published 0.6 cu ft per bag yield.
Concrete Needed (Ready-Mix Order)
Includes waste allowance — the figure for your ready-mix supplier
Volume before waste:
📐 Formula
Volume (cu ft) = length × width × (thickness in inches ÷ 12). Cubic yards = volume (cu ft) ÷ 27, since 1 cubic yard equals 27 cubic feet. Volume with waste = volume × (1 + waste %). Bags needed = ceil(volume with waste in cu ft ÷ cu ft yielded per bag).
How to Use the Concrete Calculator
Enter slab length and width
Enter your slab, footing, or pour's length and width in feet so the calculator can work out the total area.
Enter slab thickness in inches
Enter your slab thickness in inches — a typical patio or sidewalk is 4 inches thick, while driveways and footings need more. The calculator converts inches to feet automatically.
Set your waste/overage percentage
Accept the 10% default or adjust it to match your project — a simple, well-prepped slab needs less overage, an irregular pour or uneven sub-base needs more.
Review cubic yards and bag-count equivalent
Review the cubic yards needed for a ready-mix order, plus the equivalent number of premix bags if you're mixing the job yourself.
Why Concrete Is Ordered in Cubic Yards
Ready-mix concrete suppliers in the US sell and deliver concrete by the cubic yard, not by area or bag count — it's the standard unit for a delivery truck's load, and it's what you'll be asked for the moment you call a supplier to schedule a pour. Converting a slab's dimensions into cubic yards is simple geometry once you know the conversion factor: a cubic yard is a cube measuring 3 ft on every side, so it holds 3 × 3 × 3 = 27 cubic feet. That's why this calculator's formula always finishes by dividing total cubic feet by 27 — it's the fixed bridge between the cubic-feet volume math (length × width × thickness) and the cubic-yard figure a ready-mix supplier actually needs to hear.
Thickness is the one dimension that trips people up, since it's conventionally specified in inches — a "4 inch slab" — while length and width are almost always given in feet. Mixing units without converting is a common source of a wildly wrong volume estimate, so this calculator always divides your entered thickness by 12 before multiplying, converting it to feet first so every dimension going into the formula is consistent.
How Many Bags of Concrete Equal a Cubic Yard?
For smaller jobs — a fence post, a small pad, a repair — bagged premix concrete is often more practical than ordering a ready-mix delivery. According to Quikrete's published yield chart, one of the standard references major concrete-mix manufacturers publish for exactly this purpose, a bag's mixed yield scales directly with its dry weight: an 80 lb bag yields about 0.6 cubic feet once mixed with water, a 60 lb bag yields about 0.45 cubic feet, and a 40 lb bag yields about 0.3 cubic feet. Dividing 27 cubic feet (one cubic yard) by each of those figures gives a clean per-bag count: 45 bags of 80 lb premix, 60 bags of 60 lb premix, or 90 bags of 40 lb premix per cubic yard.
This calculator always rounds its bag count up to the next whole bag, never down — a partial bag isn't something you can buy or reliably mix, and rounding down would leave a real pour short of material. For any job pushing toward 1-2 cubic yards or more, the bag count climbs into the dozens fast, which is usually the point where ready-mix delivery becomes the more practical and often cheaper option once you account for the physical labor of mixing that many bags by hand.
Why Add a Waste/Overage Allowance for Concrete
No real pour uses exactly the calculated volume. Some concrete is always lost to spillage during the pour, some sinks into a sub-base that isn't perfectly compacted or level, and small measurement errors compound across a slab's full area. Concrete industry guidance commonly recommends adding 5-10% overage to your calculated volume for exactly these reasons — a similar order of magnitude to the waste factors this site's other Renovation material calculators use for paving and brick, though concrete's own overage guidance is specific to concrete pours rather than simply borrowed from those figures. This calculator defaults to 10%, the upper end of that commonly cited range, because running short mid-pour on a ready-mix delivery is a genuinely serious problem — a truck that's already emptied its load can't easily bring you another partial batch mid-job, and a cold joint where an old and new pour meet is a real weak point in the finished slab.
You can lower the waste percentage for a simple, well-prepped rectangular slab with a compacted, level sub-base once you're confident in your own measurements, or raise it for an irregular shape, a pour with lots of formwork, or a sub-base you haven't fully compacted yet.
Worked Example: A 10 ft × 10 ft Slab, 4 Inches Thick
Take a slab 10 ft long and 10 ft wide, poured 4 inches thick, using the 10% default waste/overage allowance:
Slab area = 10 ft × 10 ft = 100 sq ft.
Volume (cu ft) = 100 sq ft × (4 in. ÷ 12) = 33.33 cu ft.
Cubic yards (no waste) = 33.33 ÷ 27 = 1.23 cu yd — matching the Quick Answer above.
Volume with 10% waste = 33.33 × 1.10 = 36.67 cu ft, or 1.36 cu yd.
Bags needed (80 lb, 0.6 cu ft each) = ceil(36.67 ÷ 0.6) = ceil(61.11) = 62 bags.
A larger or smaller pour scales the same way — double the area or the thickness and the cubic-yard figure roughly doubles too, since volume is driven directly by all three dimensions multiplied together.
Recommended Slab Thickness by Project Type
Slab thickness isn't one-size-fits-all — it depends entirely on what the concrete needs to carry. Here's typical US residential guidance for common project types, useful as a planning reference before you settle on the thickness to enter above.
| Project Type | Typical Thickness | Notes |
|---|---|---|
| Sidewalk / Walkway | 4 in. | Light foot traffic only |
| Patio | 4 in. | Foot traffic, outdoor furniture |
| Residential Driveway (passenger vehicles) | 4–6 in. | 6 in. recommended if trucks or heavier vehicles will park |
| Garage Floor | 5–6 in. | Supports vehicle weight, often reinforced |
| Footing (typical residential) | 8–12+ in. | Varies by structural load & local frost depth — confirm with a structural engineer or local code |
Concrete Needed by Common Slab Size
For a quick planning reference before you measure your own project precisely, here's how cubic yardage and bag count scale across a few common slab sizes, using this calculator's default 10% waste allowance and 80 lb bags.
| Slab Size | Thickness | Cu Yd (No Waste) | Cu Yd (10% Waste) | 80 lb Bags |
|---|---|---|---|---|
| 10 × 10 ft | 4 in. | 1.23 | 1.36 | 62 |
| 12 × 12 ft | 4 in. | 1.78 | 1.96 | 88 |
| 20 × 20 ft | 4 in. | 4.94 | 5.43 | 245 |
| 20 × 24 ft driveway | 6 in. | 8.89 | 9.78 | 440 |
| 50 ft footing (1 × 1 ft) | 12 in. | 1.85 | 2.04 | 92 |
Notice how fast the bag count climbs for anything beyond a small pad — a 20×24 ft driveway needs 440 bags of 80 lb premix, which makes the labor case for ready-mix delivery obvious well before the cubic yardage itself gets large. For a small footing or fence-post job, though, bagged premix stays the simpler, more practical choice.
DIY Bagged Concrete vs. Ready-Mix Delivery
This calculator estimates material quantities, not labor, mix design, or finishing quality — pouring and finishing concrete well (getting it level, properly cured, and crack-resistant) is a genuine skill, and a rushed or poorly mixed pour shows every flaw once it hardens. For a small, low-stakes job — a mailbox post footing, a small stepping-pad, a repair patch — bagged premix and a wheelbarrow are a realistic weekend DIY project. For anything larger, structural, or highly visible — a driveway, a garage floor, a load-bearing footing — a ready-mix delivery and, for structural work, a licensed contractor are worth the added cost, since an improperly mixed or poured large slab is difficult and expensive to fix after it cures.
Once you've worked out your concrete needs, it's worth planning the rest of a materials project too. If you're not certain of your project's exact dimensions before pricing concrete, our square footage calculator can help you get an accurate area first. If your project also includes a brick or block wall, our brick calculator works out bricks and mortar bags the same way this tool works out concrete and premix bags. For an adjoining paver patio or walkway, our paving calculator covers paver count and bedding sand, and if the finished slab gets tiled, our tile calculator works out tile, thin-set mortar, and grout. Browse every tool in this category on our renovation calculators hub.
Frequently Asked Questions
Multiply your slab's length, width, and thickness (converted from inches to feet) to get cubic feet, then divide by 27 since 1 cubic yard equals 27 cubic feet — the standard unit ready-mix suppliers use for ordering. A 10×10 ft slab poured 4 in. thick works out to (10 × 10 × 4⁄12) ÷ 27 ≈ 1.23 cubic yards before any waste allowance. Always add a waste/overage percentage on top of this base figure before calling your supplier, since ready-mix trucks deliver in fixed increments and running short mid-pour is a real problem.
An 80 lb bag of pre-mixed concrete yields about 0.6 cubic feet once mixed, per Quikrete's published yield chart. Since 1 cubic yard equals 27 cubic feet, that works out to roughly 45 bags of 80 lb premix per cubic yard (27 ÷ 0.6). A 60 lb bag yields about 0.45 cu ft (60 bags per cubic yard), and a 40 lb bag yields about 0.3 cu ft (90 bags per cubic yard). Always round bag counts up, since a fraction of a bag isn't something you can buy or reliably mix.
Concrete industry guidance commonly recommends adding 5-10% overage to your calculated volume when ordering, to cover spillage, uneven sub-base or form work, and minor measurement error — the same order of magnitude as the waste factors used elsewhere in this site's Renovation category for paving and brick. This calculator defaults to 10%, the upper end of that commonly cited range, since running short mid-pour on a ready-mix delivery is a far bigger problem than having a small amount left over. You can lower it for a simple, well-prepped rectangular slab once you're confident in your measurements.
Slab thickness depends on what it needs to support. A standard sidewalk, walkway, or patio is typically poured 4 in. thick, a residential driveway for passenger vehicles is usually 4-6 in. (6 in. recommended where heavier vehicles or trucks will park), and a garage floor is typically 5-6 in. Footings vary far more, commonly 8-12 in. or deeper depending on the structure's load and local frost-depth requirements, and should be sized by a structural engineer or your local building code rather than a generic calculator. Always confirm the correct thickness for your specific project before pouring.
For small jobs, bagged premix is usually more practical — mixing it yourself with a wheelbarrow or mixer for a fence post, small pad, or repair under roughly 1-2 cubic yards. Beyond that, ready-mix delivery becomes more cost-effective and far less labor-intensive, since ordering by the cubic yard and having a truck pour it directly saves the time and physical effort of mixing dozens of individual bags by hand. This calculator shows both figures so you can compare — the bag count for a DIY job, or the cubic yardage to give a ready-mix supplier for a larger pour.
Sources & Methodology
This calculator's core geometry — volume divided by 27 cubic feet per cubic yard — is a fixed unit-conversion fact, not a figure that needed sourcing. Its bag-yield figures (0.6 cu ft for an 80 lb bag, 0.45 cu ft for a 60 lb bag, 0.3 cu ft for a 40 lb bag) come from Quikrete's published yield chart, a major US concrete-mix manufacturer's own reference. The 5-10% waste/overage guidance and the recommended slab-thickness ranges by project type reflect commonly cited concrete-industry practice from the National Ready Mixed Concrete Association (NRMCA) and standard residential-construction reference material. Actual quantities and required thickness vary by mix design, sub-base condition, and structural requirements; verify with your concrete supplier and local building code before ordering or pouring.