A Free Tool · Slabs & Post Holes · Standard Bag Yields
How much concrete do you actually need?
Enter the dimensions of a slab, footing, or round post hole and get the volume in
cubic feet and cubic yards, plus how many 40, 60, or 80 lb bags it takes. The numbers
use the standard QUIKRETE and Sakrete bag yields (an 80 lb bag makes about 0.60 cubic
feet) and include a 10% overage so you don't run short mid-pour.
Read this first
These are estimates based on standard bag yields, not a substitute for your own
measurements or your supplier's specs. Always buy a little extra — running out
halfway through a pour is far worse than having a partial bag left over. For anything
larger than about a cubic yard, bagged mix gets impractical and ready-mix delivered by
the cubic yard is usually the better choice. Check the yield printed on the actual bag
you buy; it can vary slightly by product and mix.
The calculator
Estimate concrete and bags
Pick a shape, enter your measurements (thickness and diameter in inches; length, width, and height in feet — the way you actually measure), and you'll get the total volume plus bags needed at each bag size, with overage applied.
4″ is typical for patios & walkways; 5–6″ for driveways.
A post in the hole displaces some concrete, so you may need slightly less.
How many slabs of these dimensions you're pouring.
5–10% is commonly recommended for spillage and uneven subgrade.
Volume (cubic feet)
Volume (cubic yards)
Overage applied
80 lb bags (0.60 cu ft each)
60 lb bags (0.45 cu ft each)
40 lb bags (0.30 cu ft each)
The math, honestly
How the bag count is figured
It's all volume. For a slab or square footing, the volume in cubic
feet is length × width × thickness, with everything in feet
— so a thickness entered in inches is divided by 12 first. For a
round column or post hole, treat it as a cylinder:
π × (diameter/2)² × height, again in feet.
To convert to cubic yards — the unit ready-mix is sold in
— divide cubic feet by 27 (a cubic yard is 3 ft × 3 ft
× 3 ft). To convert to bags, divide by the bag's yield and
round up: an 80 lb bag yields 0.60 cu ft, a 60 lb bag 0.45,
and a 40 lb bag 0.30. That's why a cubic yard takes 45 of the 80 lb bags,
60 of the 60 lb, or 90 of the 40 lb.
Why the overage matters: real subgrade is never perfectly flat,
forms bow, and some mix is always lost to spillage. Adding 5–10% protects you
from the worst outcome — running out mid-pour and leaving a cold joint where the
next batch meets concrete that's already setting up.
Bag yields and bags per cubic yard
The standard yields for bagged concrete mix, and how many bags it takes to make one
full cubic yard (27 cubic feet). These are the QUIKRETE and Sakrete figures the
calculator uses — always confirm against the bag in your hands.
Bag size
Yieldmixed concrete
Bags per cu yd27 cu ft ÷ yield
80 lb
0.60 cu ft
45
60 lb
0.45 cu ft
60
40 lb
0.30 cu ft
90
Yields are for standard concrete mix and assume normal water content; high-strength,
fast-setting, and crack-resistant blends can differ slightly. Bags-per-yard figures are
27 cu ft divided by the yield, rounded to whole bags.
Common slab sizes
Worked examples for a few common slabs, computed with the same formula and a 10%
overage — so these match what the calculator gives you. Cubic yards are shown both
raw and with overage; bag counts are 80 lb bags with overage included.
Slab
Cubic yardsraw
Cubic yards+10% overage
80 lb bagswith overage
10 × 10 @ 4″
1.23
1.36
62
12 × 12 @ 4″
1.78
1.96
89
20 × 20 @ 4″
4.94
5.43
245
10 × 10 @ 6″
1.85
2.04
92
Even the smallest slab here is over a cubic yard — about 62 bags of 80 lb mix.
Hand-mixing that many bags is a long day; at this scale, pricing out ready-mix delivery
is worth doing.
Reading the result well
A volume number is only useful if you act on it sensibly. Four things worth knowing
before you buy.
Always add 5–10% overage
No subgrade is perfectly level, forms flex under the weight of wet concrete, and some mix is always lost to spillage and what sticks to the mixer and wheelbarrow. The overage covers all of that. This calculator defaults to 10%; for a small, well-formed pour you might drop it to 5%, but never to zero.
Match thickness to the job
Four inches is standard for patios, walkways, shed floors, and other foot-traffic surfaces. Driveways and anything that carries vehicles usually want 5 to 6 inches, often with rebar or wire mesh. Thickness scales volume directly, so going from 4″ to 6″ on the same footprint adds 50% more concrete — and 50% more bags.
Know the bagged-vs-ready-mix line
Bagged mix is the right tool for post holes, small pads, steps, and repairs — jobs under about one cubic yard. Past roughly a cubic yard (around 60 bags of 80 lb mix) the labor, cost, and difficulty of keeping a consistent mix tip in favor of ready-mix concrete delivered by the cubic yard. Note that ready-mix suppliers often have a minimum load, so very small jobs still favor bags.
Buy whole bags — and don't under-buy
You can't buy a fraction of a bag, so the calculator rounds every bag count up. Keep the extra on hand until the pour is done; a leftover bag is cheap insurance. Running short mid-pour forces you to stop while fresh concrete sets, creating a cold joint that weakens the finished slab. When in doubt, round up again.
Where to buy
Got your numbers? Here's where to pick up what you need:
The terms behind the calculator, in plain English. These are background definitions,
not engineering specifications — follow your local building code and any
engineered plan for the real thing.
Cubic yard
The standard unit for ordering bulk and ready-mix concrete — a cube three feet on each side, equal to 27 cubic feet. When a pour gets large, you stop thinking in bags and start thinking in cubic yards, because that's how trucks deliver it.
Cubic foot
A cube one foot on each side. It's the working unit for bag math, because bag yields are stated in cubic feet. Divide a cubic-foot total by 27 to get cubic yards.
Bag yield
How much mixed concrete one bag produces, by volume. The standard QUIKRETE/Sakrete figures are 0.60 cu ft for an 80 lb bag, 0.45 for a 60 lb bag, and 0.30 for a 40 lb bag. Yield is what matters for counting bags — not the bag's weight directly.
Ready-mix
Concrete batched at a plant and delivered wet by truck, sold by the cubic yard. It's the practical choice above about one cubic yard, where hand-mixing bags becomes slow and inconsistent. Suppliers usually have a minimum load and may charge for short loads.
Slab
A flat, horizontal pour — a patio, walkway, shed floor, or driveway. Its volume is simply length × width × thickness. Slab thickness is usually given in inches even though length and width are in feet.
Footing
A wider, often buried base that spreads a structure's load onto the soil — under a foundation wall, deck post, or column. Square and rectangular footings use the same length × width × thickness math as a slab; round pier footings use the cylinder formula.
Overage (waste factor)
The extra concrete bought above the calculated volume — typically 5 to 10% — to cover spillage, uneven or over-excavated subgrade, and measurement error. It's cheap insurance against the much costlier problem of running short during a pour.
PSI (mix strength)
Pounds per square inch, the compressive strength of cured concrete. Common bagged mixes cure to around 4,000 PSI in 28 days, which is plenty for typical residential slabs and footings. PSI doesn't change the volume math, but it's why you might choose a high-strength mix for a driveway over a standard one for a garden border.
Frequently asked
About 45. One cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.60 cubic feet once mixed — 27 ÷ 0.60 = 45 bags. A 60 lb bag yields about 0.45 cu ft, so 60 bags make a yard, and a 40 lb bag yields about 0.30 cu ft, so 90 bags make a yard. These are the standard QUIKRETE and Sakrete yields; always check the figure printed on the bag you buy.
A 10 ft × 10 ft slab poured 4 inches thick is 10 × 10 × (4/12) = 33.3 cubic feet, about 1.23 cubic yards. Adding a 10% overage brings it to roughly 36.7 cubic feet, or about 1.36 cubic yards — that's about 62 bags of 80 lb mix, 82 of 60 lb, or 123 of 40 lb. At well over a cubic yard, this is right at the edge of where ready-mix delivery usually beats mixing bags by hand. Try it in the calculator.
Four inches is the typical thickness for patios, walkways, and shed floors. For driveways and anything that carries vehicles, 5 to 6 inches is more common, sometimes with reinforcement. Thickness drives volume directly: going from 4″ to 6″ on the same slab adds 50% more concrete. Always follow your local building code and any engineered plan, since requirements vary by use and soil.
Bagged mix is convenient for small jobs — post holes, small pads, repairs, and slabs under roughly one cubic yard. Above about a cubic yard, which is around 60 bags of 80 lb mix, hand-mixing becomes slow, expensive, and hard to keep consistent, and ready-mix concrete delivered by the cubic yard is usually the better call. Many suppliers have a minimum load, so very small pours still favor bags.
Add about 5 to 10% over your calculated volume. This covers spillage, an uneven or over-excavated subgrade, slab edges that slump, and the fact that measurements are rarely exact. This calculator uses 10% by default, and you can change it. Running short mid-pour is far worse than a leftover partial bag, because a cold joint where fresh concrete meets already-setting concrete can weaken the finished slab.
Treat the hole as a cylinder. The volume in cubic feet is π × radius² × height, with everything in feet. For a 12-inch-diameter hole 3 feet deep, the radius is 0.5 ft, so the volume is 3.1416 × 0.5 × 0.5 × 3 = about 2.36 cubic feet, roughly 4 bags of 80 lb mix. Multiply by the number of holes, then add your overage. A post sitting in the hole displaces some concrete, so you'll usually need a little less than the raw cylinder figure. The calculator has a round-hole mode.
There are 27 cubic feet in one cubic yard. A cubic yard is a cube 3 feet on each side, and 3 × 3 × 3 = 27. Ready-mix concrete is sold by the cubic yard, so converting your cubic-foot total to cubic yards (divide by 27) tells you how much to order for a larger pour.