How Much Concrete Do I Need? Bags, Yards & Cost

Few questions cause more last-minute panic on a build day than this one: how much concrete do I need? Order too little and your pour stops halfway, leaving a cold joint and a weak slab. Order too much and you have paid for concrete that ends up as a useless lump in the corner of your yard. Getting the number right the first time is the difference between a clean, professional job and an expensive, stressful one.

The good news is that the math is simple once you understand it, and you can check it in seconds with our concrete calculator. This guide walks through exactly how to calculate the concrete you need for any project — slabs, footings, sidewalks, driveways, and post holes — and answers every related question: how many bags you need, how many bags are in a yard, what a yard of concrete costs, and how much it weighs.

How much concrete do I need? The short answer

Concrete is sold and measured by volume, not by weight or area. In the United States the standard unit is the cubic yard (often just called a "yard"); in the UK and most of the world it is the cubic metre. Every concrete calculation comes down to finding the volume of the space you want to fill.

The universal formula is:

Volume = Length × Width × Depth

The only trick is keeping your units consistent and then converting the result into cubic yards (or cubic metres) so you can order the right amount. Let's break that down step by step.

How to calculate cubic yards of concrete

A cubic yard is a cube measuring 3 feet on every side, which equals 27 cubic feet. So to find cubic yards, you calculate the volume in cubic feet and divide by 27.

Here is the process for a typical concrete slab:

1. Measure length and width in feet. Say your patio slab is 12 feet by 10 feet.
2. Convert the thickness to feet. Most slabs are poured 4 inches thick. Since there are 12 inches in a foot, 4 inches = 4 ÷ 12 = 0.333 feet.
3. Multiply all three together. 12 × 10 × 0.333 = 40 cubic feet.
4. Divide by 27 to get cubic yards. 40 ÷ 27 = 1.48 cubic yards.

That's it — your 12 × 10 ft patio at 4 inches thick needs about 1.5 cubic yards of concrete. If you would rather not do the arithmetic, enter the same numbers into the concrete calculator and it returns the volume in cubic yards, cubic feet, and bags instantly, along with a cost estimate.

If you are working in metric, the logic is identical but cleaner: measure everything in metres and multiply. A 3.6 m × 3 m slab at 0.1 m (10 cm) thick is 3.6 × 3 × 0.1 = 1.08 cubic metres.

A quick note on measuring odd shapes

Not every pour is a neat rectangle. For an L-shaped patio, split it into two rectangles, calculate each separately, and add the results. For a circular pad (such as a base for a hot tub), the area is π × radius², then multiply by the depth. If you need help finding the area of an irregular space before you start, our square footage calculator handles the area step, and you simply multiply by your thickness.

How many bags of concrete do I need?

For small jobs, mixing your own from pre-mixed bags is cheaper and more convenient than ordering a ready-mix truck. The question then becomes: how many bags of concrete do I need? That depends on the bag size, because each bag yields a fixed volume of mixed concrete:

• A 40 lb bag yields about 0.30 cubic feet.
• A 60 lb bag yields about 0.45 cubic feet.
• An 80 lb bag yields about 0.60 cubic feet.

To find the number of bags, take your total volume in cubic feet and divide by the yield of your chosen bag. Using our patio example (40 cubic feet):

• 40 ÷ 0.60 = 67 × 80 lb bags
• 40 ÷ 0.45 = 89 × 60 lb bags
• 40 ÷ 0.30 = 134 × 40 lb bags

You can immediately see why bagged concrete only makes sense for small projects — 67 heavy bags for a modest patio is a lot of mixing. As a rule of thumb, anything above roughly half a cubic yard (about 22 × 80 lb bags) is usually cheaper and far less back-breaking to order as ready-mix. The concrete calculator shows the bag count for all three common bag sizes at once so you can compare.

How many bags of concrete in a yard?

This is one of the most-searched concrete questions, and the answer follows directly from the bag yields above. Since one cubic yard is 27 cubic feet:

• 80 lb bags in a yard: 27 ÷ 0.60 = 45 bags
• 60 lb bags in a yard: 27 ÷ 0.45 = 60 bags
• 40 lb bags in a yard: 27 ÷ 0.30 = 90 bags

So if a supplier or a calculator tells you a job needs "1 yard of concrete," that is the equivalent of 45 of the large 80 lb bags. Seeing it in those terms is usually the moment people decide to call the ready-mix company instead.

How much does a yard of concrete cost?

Cost is the other half of planning a pour. Prices vary by region, supplier, and the concrete mix (strength), but these are realistic 2026 ballpark figures for budgeting:

• Ready-mix delivered: roughly $130 to $170 per cubic yard for a standard mix. Most companies have a minimum order (often 1 cubic yard) and may add a "short load" fee for small deliveries plus a delivery charge.
• Bagged concrete: an 80 lb bag costs around $5 to $8, so a yard's worth (45 bags) lands at roughly $225 to $360 — more expensive per yard than ready-mix once you pass the small-job threshold, before you even count your labor.

For our 1.5-yard patio, ready-mix would run somewhere around $200 to $260 in concrete plus delivery — comfortably cheaper than buying and mixing 67 bags by hand. The concrete calculator lets you plug in your local price per yard to get a tailored estimate. Remember that the concrete itself is only one line in the budget: forms, rebar, a gravel base, and finishing tools all add up.

How much does concrete weigh? (concrete weight per yard)

Weight matters for two reasons: knowing whether your vehicle or trailer can carry bagged concrete, and understanding the load your slab places on the ground beneath it. The standard figure is:

• One cubic yard of cured concrete weighs about 3,600 to 4,050 lbs — call it roughly 2 tons per yard.
• One cubic foot weighs about 150 lbs.
• One cubic metre weighs about 2,400 kg (2.4 tonnes).

That heft is exactly why a solid base is non-negotiable — which brings us to the layer most beginners forget.

Don't forget the gravel base under the concrete

A concrete slab should almost never be poured directly onto bare soil. A compacted layer of gravel or crushed stone — typically 4 inches thick — gives the slab a stable, well-draining foundation that resists cracking, frost heave, and settling. Skipping it is the single most common cause of a slab failing within a few years.

That gravel is a separate material order, calculated the same way (length × width × depth), but bought by weight in tons rather than by the yard. Our gravel calculator works out exactly how much crushed stone you need for the base and roughly what it will cost, so you can order both materials in one trip. As a quick reference, one cubic yard of gravel weighs about 1.4 tons and covers about 100 square feet at 3 inches deep.

Concrete needed for common projects

Here is a quick reference for typical jobs at standard thicknesses. Always confirm with your own measurements, but these give you a feel for the scale:

• Patio or shed slab (10 × 10 ft, 4 in): about 1.2 cubic yards.
• One-car driveway section (10 × 20 ft, 4 in): about 2.5 cubic yards.
• Sidewalk (3 ft × 30 ft, 4 in): about 1.1 cubic yards.
• Footing (1 ft wide × 1 ft deep × 40 ft long): about 1.5 cubic yards.
• Fence post holes (one 12 in diameter × 2 ft deep hole): about 1.6 cubic feet each, or roughly three 80 lb bags per post.

Driveways and any slab that will carry vehicle weight are usually poured thicker — 5 to 6 inches rather than 4 — so adjust your depth accordingly. Increasing a 4-inch slab to 6 inches adds 50% more concrete, which is a large and easy-to-miss jump in both volume and cost.

How to avoid running short (the 10% rule)

Real-world pours never match the math perfectly. Forms bulge slightly, the sub-base is rarely perfectly level, and some concrete is always lost to spillage and the bottom of the mixer. Professionals account for this by ordering about 5 to 10% more than the calculated volume.

For our 1.5-yard patio, adding 10% means ordering 1.65 yards — and because ready-mix is sold in quarter-yard increments, you would round up to 1.75 yards. That small cushion is cheap insurance against the nightmare scenario of the truck running dry three feet from the edge. When you use the concrete calculator, take its figure and add that 5 to 10% buffer before you place the order.

Step-by-step: calculating your project from scratch

1. Measure the length, width, and thickness of every section you are pouring. Note thickness in inches.
2. Convert thickness to feet (inches ÷ 12) so all three dimensions share the same unit.
3. Multiply length × width × thickness to get cubic feet, then divide by 27 for cubic yards.
4. Add up all sections if your project has more than one shape.
5. Add 5 to 10% for waste and over-dig.
6. Decide bags vs ready-mix: under ~½ yard, bags are fine; above that, order a truck.
7. Calculate the gravel base separately with the gravel calculator.

Do all of that in under a minute by entering your numbers into the concrete calculator, which handles the unit conversions, the divide-by-27, and the bag counts for you.

Concrete vs cement vs mortar: knowing what you're buying

One of the most common sources of confusion at the building-supplies counter is the difference between concrete, cement and mortar. Buying the wrong product can wreck a project, so it pays to understand exactly what each one is before you estimate quantities.

Cement is the grey powder (usually Portland cement) that acts as the binder. On its own it is almost never used structurally. Concrete is the finished structural material: cement plus a coarse aggregate (gravel or crushed stone), a fine aggregate (sand) and water. Mortar is cement plus sand and water with no coarse aggregate, used to bond bricks and blocks rather than to form slabs. If your project involves pouring a slab, footing, post base or path, you want concrete. If you are laying bricks, you want mortar.

Material	Contains coarse aggregate?	Typical use	Sold as
Cement	No	Binder ingredient only	Bags of powder
Concrete	Yes (gravel/stone)	Slabs, footings, posts, paths	Pre-mixed bags or ready-mix truck
Mortar	No (sand only)	Bricklaying, blockwork, pointing	Pre-mixed bags
Screed	No (sharp sand)	Smooth floor topping	Pre-mixed bags or site-batched

When you have a quantity figured out, you can sanity-check the volume against a concrete calculator before placing an order, which helps avoid the classic mistake of ordering mortar when a slab actually needs concrete.

Choosing the right concrete strength for your project

Not all concrete is equal. Strength is measured in megapascals (MPa) or pounds per square inch (psi), and choosing the correct grade matters as much as getting the volume right. A driveway poured with a mix that is too weak will crack within a couple of winters; an over-specified mix wastes money.

In the UK, concrete is typically specified by a "C" grade such as C20 or C25, where the number is the characteristic compressive strength in MPa at 28 days. In the US the same idea is expressed in psi (roughly: 1 MPa is about 145 psi).

UK grade	Approx. US strength	Best for
C10 / GEN1	~1,450 psi	Blinding, trench fill, non-structural fill
C20 / GEN3	~2,900 psi	Garage floors, internal slabs, shed bases
C25 / ST2	~3,600 psi	Footings, foundations, driveways
C30 / ST3	~4,350 psi	Heavy-duty driveways, paths bearing vehicles
C35 / C40	~5,100–5,800 psi	Commercial slabs, structural beams

A useful rule of thumb: if a vehicle will ever sit on or drive across the concrete, step up at least one grade from what you think you need. Freeze-thaw cycling in colder regions is also a factor; an air-entrained mix resists spalling far better on exposed surfaces.

Hand-mixing versus ready-mix: when each makes sense

Once you know your volume, the next decision is how to get the concrete on site. There are three realistic routes, and the crossover points are worth knowing before you commit.

• Bagged, hand-mixed: Practical up to roughly 0.5 cubic yards (about 0.4 cubic metres). Beyond that you are mixing dozens of bags by hand, which is exhausting and risks cold joints between batches.
• Bagged with a powered mixer: Sensible from about 0.25 up to 1 cubic yard. A hired or owned drum mixer lets one or two people keep a continuous pour going.
• Ready-mix truck: Almost always the right choice above 1 cubic yard. Trucks typically carry up to about 8 cubic yards, and the concrete arrives at a consistent, certified strength.

Ready-mix suppliers usually impose a minimum charge (often the cost of 1 to 3 cubic yards even if you order less) and may add a "short load" surcharge. They also charge waiting time, so have your formwork, barrows, helpers and a clear access route ready before the truck arrives. A typical truck wants to discharge within 30 to 45 minutes.

Estimating awkward and non-rectangular shapes

Most guides assume a simple rectangular slab, but real projects involve circles, triangles, footings and steps. The trick is to break any shape into simple components, calculate each volume, then add them together.

1. Circular slab or pad: Volume = pi x radius x radius x thickness. For a 3 ft diameter pad 4 inches thick: radius 1.5 ft, thickness 0.333 ft, so 3.14159 x 1.5 x 1.5 x 0.333 = about 2.35 cubic feet.
2. Cylindrical post hole or pier: Same formula. A 12-inch diameter hole 3 ft deep: radius 0.5 ft, so 3.14159 x 0.5 x 0.5 x 3 = 2.36 cubic feet per hole. Multiply by the number of holes.
3. Triangular section: Volume = 0.5 x base x height x thickness.
4. Stairs: Treat each step as a rectangular block, calculate each, and sum. Don't forget the supporting slab beneath.

For tapered footings or sloped sites, average the two end depths to get a representative thickness, then treat the section as a rectangular prism. This slightly over-estimates, which is exactly what you want given that you should never run short mid-pour.

Reinforcement and how it affects your order

Steel reinforcement (rebar) or welded mesh does not change the concrete volume in any meaningful way for estimation purposes; the steel displaces a negligible amount. What it does change is the minimum slab thickness you can pour and the cover you must maintain.

Rebar and mesh need a minimum "cover" of concrete around them to prevent rusting and spalling, typically 40 to 75 mm (1.5 to 3 inches) depending on exposure. This is why a reinforced driveway is rarely thinner than 100 mm (4 inches): you simply cannot fit the steel plus adequate cover in anything thinner. If your design calls for reinforcement, double-check that your planned thickness leaves room for proper cover top and bottom, or the steel will do more harm than good.

Curing: the step that protects all your effort

Estimating and pouring the right amount of concrete is only half the job. Concrete gains strength through a chemical reaction called hydration, not by drying out, and that reaction needs moisture and time. Concrete that dries too fast at the surface ends up weaker and prone to dusting and cracking.

• First 24–48 hours: Keep the surface protected from rain, direct sun and frost. Foot traffic should wait at least a day; vehicle traffic at least a week.
• Days 1–7: Keep it damp. Cover with plastic sheeting or hessian and mist it, or apply a curing compound. This is when the majority of strength develops.
• 28 days: Concrete reaches its rated design strength (the figure behind C25, C30 and so on) at 28 days, though it continues to gain strength slowly for months afterwards.

In hot weather, pour early in the morning and cure aggressively. In cold weather, protect against frost for the first few days; freezing fresh concrete can permanently destroy its strength. These conditions don't change how much you order, but they decide whether the concrete you ordered actually performs.

Worked example: a complete patio order

Suppose you are pouring a patio 12 ft long by 10 ft wide at 4 inches thick, with a 100 mm gravel sub-base beneath it. Here is the full estimation, the way a contractor would do it.

1. Convert thickness to feet: 4 inches = 0.333 ft.
2. Concrete volume: 12 x 10 x 0.333 = 40 cubic feet.
3. Convert to cubic yards: 40 / 27 = 1.48 cubic yards.
4. Add 10% waste allowance: 1.48 x 1.10 = 1.63 cubic yards. Round up to order 1.75 cubic yards.
5. Gravel base (separate order): 100 mm is about 0.328 ft, so 12 x 10 x 0.328 = 39.4 cubic feet, or about 1.46 cubic yards of crushed stone.

Because 1.63 cubic yards sits comfortably above the hand-mixing threshold, this is a ready-mix job. Ordering 1.75 yards covers the waste factor without leaving a wasteful surplus, and confirming the figure against an online estimator before you phone the supplier removes the guesswork.

Common costing mistakes that inflate your bill

Even with the right volume, several avoidable mistakes routinely push project costs up. Watch for these before you order.

• Forgetting the minimum delivery charge: Ordering 0.6 yards from a supplier with a 1-yard minimum means you pay for a full yard anyway. It can be cheaper to slightly enlarge the project than to waste the difference.
• Ignoring pump hire on tight sites: If the truck cannot reach the pour, you may need a concrete pump, which can add several hundred pounds or dollars. Factor access in early.
• Under-ordering then topping up: A second small delivery costs disproportionately more and creates a cold joint. The 10% over-order rule almost always saves money overall.
• Mismatched sub-base: Skipping or skimping on the gravel base leads to settlement cracks that cost far more to fix than the base would have cost to lay.

Treat the concrete estimate, the sub-base estimate, the delivery minimums and the curing plan as a single connected order. Getting all four right is what separates a slab that lasts decades from one that fails in a season.

Metric and imperial: working across both measurement systems

Concrete estimation regularly forces you to cross between metric and imperial units, especially on UK projects where the slab dimensions arrive in metres but bag yields are quoted in litres, and on US projects where everything is in feet, inches and cubic yards. Mixing the two without care is one of the most common causes of a wildly wrong order, so it pays to keep a small set of conversions to hand.

From	To	Multiply by
Cubic feet	Cubic yards	divide by 27
Cubic metres	Cubic yards	1.308
Cubic yards	Cubic metres	0.765
Inches	Feet	divide by 12
Millimetres	Metres	divide by 1,000
Litres (mixed concrete)	Cubic metres	divide by 1,000

A safe workflow is to pick one system at the very start and convert every dimension into it before you do any multiplication. If your plans are metric, convert the thickness from millimetres to metres first (100 mm becomes 0.1 m), then multiply length by width by thickness to get cubic metres directly. If you are working imperially, convert every inch measurement to a decimal fraction of a foot before multiplying. Never multiply a length in feet by a thickness in inches; the answer will be twelve times too large and the mistake is surprisingly easy to make when you are tired on site.

A final practical tip: a standard ready-mix order in the UK is placed in cubic metres, while a US order is placed in cubic yards, and the two are close enough in size (a cubic metre is about 1.3 cubic yards) that a careless swap still produces a plausible-looking but wrong figure. Always state the unit out loud when you phone the supplier, and confirm it back, because a one-word slip here is an expensive load of surplus concrete sitting in your driveway.

Frequently asked questions

How many 80 lb bags of concrete make a yard?

It takes 45 bags of 80 lb concrete to make one cubic yard, because one yard is 27 cubic feet and each 80 lb bag yields about 0.60 cubic feet (27 ÷ 0.60 = 45).

How do I calculate how much concrete I need for a slab?

Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 for cubic yards. A 10 × 10 ft slab at 4 inches (0.333 ft) is 10 × 10 × 0.333 = 33.3 cubic feet ÷ 27 = about 1.23 cubic yards.

How much does a yard of concrete cost in 2026?

Ready-mix concrete typically costs about $130 to $170 per cubic yard delivered, plus possible short-load and delivery fees. Buying the equivalent in 80 lb bags costs more once you pass roughly half a yard.

How much does a cubic yard of concrete weigh?

A cubic yard of cured concrete weighs roughly 3,600 to 4,050 lbs — about 2 tons. One cubic foot weighs about 150 lbs.

Should I order extra concrete?

Yes. Order about 5 to 10% more than your calculated volume to cover spillage, uneven sub-base, and form bulge. Running short mid-pour creates a weak cold joint, so a small surplus is worth it.

Do I need gravel under a concrete slab?

In almost all cases, yes. A compacted 4-inch gravel or crushed-stone base provides drainage and a stable foundation that helps prevent cracking and settling. Calculate it with the gravel calculator.