🧱 Concrete Calculator: How Much Concrete Do I Need?
By ToolNimba Construction Team · Updated 2026-06-20
Enter the slab dimensions to see how much concrete you need.
Pouring a slab, footing or pad and not sure how much concrete to order? This calculator turns your length, width and thickness into the volume you need, shown in cubic yards (how ready-mix is sold in the US) and cubic meters. It also estimates how many pre-mixed bags it would take in the common 40, 60 and 80 lb sizes, and adds a waste allowance so you do not come up short halfway through the pour.
What is the Concrete Calculator?
Concrete volume is simply length times width times thickness, with every dimension in the same unit. The catch is that thickness is usually given in inches while length and width are in feet, so a four inch slab is really one third of a foot deep, not four feet. Get the units consistent first and the rest is straightforward multiplication. This tool does that conversion for you and lets you enter thickness in inches or in the same unit as the slab.
Not every pour is a rectangle. Round columns, sonotubes and circular footings use the area of a circle, so the volume is pi times the radius squared times the depth. A set of steps is really a stack of rectangular blocks, and an irregular patio is best broken into several rectangles that you total. The principle is identical for all of them: find the cross sectional area, multiply by the depth or length, then convert cubic feet into yards or meters. For a round column the formula is pi times radius squared times height, divided by 27 to reach cubic yards.
Ready-mix concrete is ordered by the cubic yard in the United States and by the cubic meter almost everywhere else, so the calculator reports both. One cubic yard is 27 cubic feet, and one cubic meter is about 35.31 cubic feet, so a cubic meter is roughly 1.31 cubic yards. A single cubic yard spread four inches deep covers about 81 square feet, which is a handy way to sanity check a slab order against the floor area you can see. For small jobs you may prefer bagged concrete that you mix on site. A standard 80 lb bag of pre-mixed concrete yields about 0.6 cubic feet once mixed, a 60 lb bag about 0.45 cubic feet, and a 40 lb bag about 0.30 cubic feet, so it takes around 45 of the 80 lb bags to fill a single cubic yard.
Always order a little more than the bare calculation. Subgrades are rarely perfectly level, forms bow slightly, some concrete sticks to the chute or wheelbarrow, and spillage happens. A waste allowance of 5 to 10 percent is normal, and many crews round up to the next quarter or half yard when ordering ready-mix because a second short delivery is expensive and a cold joint between two pours is a weak point. This calculator applies an adjustable waste percentage (10 percent by default) and rounds bag counts up to whole bags.
Strength and cost go hand in hand. Residential concrete is usually specified by its 28 day compressive strength in pounds per square inch, with 3,000 to 4,000 PSI covering most slabs, driveways and footings and 4,500 PSI or higher used for heavy duty pavements. A common mix proportion for that range is roughly 1 part cement, 2 parts sand and 3 parts gravel with a water to cement ratio near 0.5. In 2026 ready-mix runs roughly 120 to 200 dollars per cubic yard delivered, with a national average near 150 dollars, so multiplying your volume by a local per yard price gives a quick budget. Short load fees, pump rental and a long delivery distance can add meaningfully on top of the base price.
Concrete keeps gaining strength as it cures, not just as it dries, and the chemical reaction needs moisture and time. Most mixes reach about 70 percent of their design strength in seven days and full rated strength near 28 days, which is why slabs are kept damp and protected during the first week. Knowing the volume up front lets you plan crew size, the number of wheelbarrows, and whether you need a pump or a chute, all of which matter once the truck arrives and the clock starts.
When to use it
- Working out how many cubic yards of ready-mix to order for a driveway, patio, shed base or garage floor.
- Estimating how many 40, 60 or 80 lb bags to buy for a small DIY pour such as fence post footings or a paver base.
- Sizing a round pour such as a sonotube column, deck footing or circular pad using the radius and depth.
- Putting a budget on a job by multiplying the volume by a local ready-mix price per cubic yard.
- Converting between cubic yards and cubic meters when a supplier quotes in different units than your plan.
- Adding a sensible waste margin so a pour is not stopped short by running out of concrete.
How to use the Concrete Calculator
- Choose whether your length and width are in feet or meters using the unit toggle.
- Enter the slab length and width, or for a round pour enter the diameter or radius and depth.
- Enter the thickness and pick whether it is in inches or in the same unit as the slab.
- Adjust the waste allowance if you want more or less than the default 10 percent.
- Read off the volume in cubic yards and cubic meters, the number of bags for each common size, and the estimated cost.
Formula & method
Worked examples
A patio slab 10 ft long, 10 ft wide and 4 inches thick, with a 10 percent waste allowance.
- Convert thickness to feet: 4 ÷ 12 = 0.3333 ft
- Base volume = 10 × 10 × 0.3333 = 33.33 ft³
- Add 10% waste: 33.33 × 1.10 = 36.67 ft³
- Cubic yards = 36.67 ÷ 27 = 1.36 yd³
- Cubic meters = 36.67 × 0.0283168 = 1.04 m³
- 80 lb bags = 36.67 ÷ 0.60 = 61.1, round up to 62 bags
Result: About 1.36 yd³ (1.04 m³), or 62 of the 80 lb bags, including 10% waste.
A garage floor 12 ft × 12 ft poured 6 inches thick, with no extra waste added.
- Convert thickness to feet: 6 ÷ 12 = 0.5 ft
- Volume = 12 × 12 × 0.5 = 72 ft³
- Cubic yards = 72 ÷ 27 = 2.67 yd³
- Cubic meters = 72 × 0.0283168 = 2.04 m³
- 80 lb bags = 72 ÷ 0.60 = 120 bags
Result: About 2.67 yd³ (2.04 m³), or 120 of the 80 lb bags before any waste allowance.
A round sonotube footing 12 inches in diameter poured 4 ft deep, with a budget at 150 dollars per cubic yard.
- Radius = diameter ÷ 2 = 12 in ÷ 2 = 6 in = 0.5 ft
- Volume = pi × 0.5² × 4 = 3.1416 × 0.25 × 4 = 3.14 ft³
- Cubic yards = 3.14 ÷ 27 = 0.116 yd³
- Cost = 0.116 × 150 = about 17 dollars of concrete
- 60 lb bags = 3.14 ÷ 0.45 = 7.0, round up to 7 bags
Result: About 0.12 yd³ or 7 of the 60 lb bags per footing, roughly 17 dollars of ready-mix at 150 per yard.
Approximate yield and bags per cubic yard for common pre-mixed concrete bag sizes
| Bag size | Yield (mixed) | Bags per cubic yard | Bags per cubic meter |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | 118 |
| 60 lb | 0.45 ft³ | 60 | 79 |
| 80 lb | 0.60 ft³ | 45 | 59 |
Cubic yards of concrete for a square slab at common thicknesses (before waste)
| Slab size | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 8 ft × 8 ft | 0.79 yd³ | 0.99 yd³ | 1.19 yd³ |
| 10 ft × 10 ft | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ |
| 12 ft × 12 ft | 1.78 yd³ | 2.22 yd³ | 2.67 yd³ |
| 20 ft × 20 ft | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ |
Typical concrete strength, common uses and approximate area covered per cubic yard
| Strength (PSI) | Common use | Coverage per yard |
|---|---|---|
| 2,500 to 3,000 | Footings, sidewalks, paths | 81 sq ft at 4 in deep |
| 3,000 to 3,500 | Patios, residential slabs | 65 sq ft at 5 in deep |
| 3,500 to 4,000 | Driveways, garage floors | 54 sq ft at 6 in deep |
| 4,500 and up | Heavy duty pavement, structural | Varies by design |
Common mistakes to avoid
- Leaving thickness in inches. If length and width are in feet, thickness must also be in feet before you multiply. A 4 inch slab is 0.333 ft, not 4 ft. Forgetting to divide by 12 inflates the volume twelve-fold. This tool converts inches for you, so just pick the right thickness unit.
- Ordering exactly the calculated amount. Uneven subgrade, bowing forms and spillage mean the real volume is always a little more than the math. Skipping a waste allowance is the most common reason a pour runs short, and a second small delivery is both costly and creates a weak cold joint.
- Treating cubic yards and cubic feet as the same. There are 27 cubic feet in a cubic yard, not three. Mixing them up underorders ready-mix by a factor of 27, so always confirm which unit your supplier is quoting.
- Assuming bag yield equals dry bag volume. The 0.6 cubic foot figure for an 80 lb bag is the volume of mixed concrete after adding water, not the size of the dry bag. Use the mixed yield when counting bags or you will buy too few.
- Using diameter instead of radius for round pours. The circle area formula needs the radius, which is half the diameter. A 12 inch sonotube has a 6 inch radius. Plugging the full diameter into pi times radius squared overstates the volume four times over.
- Forgetting short load and delivery fees in the budget. The per yard price is only part of the cost. Many plants charge a short load fee under about one cubic yard, plus delivery, pump rental or a long haul surcharge. Add these on top of the volume times price figure.
Glossary
- Cubic yard
- A volume of 27 cubic feet (3 ft × 3 ft × 3 ft). The standard unit ready-mix concrete is sold in across the US.
- Cubic meter
- A volume of one meter cubed, about 35.31 cubic feet or 1.31 cubic yards. The standard metric unit for concrete.
- Ready-mix
- Concrete batched at a plant and delivered by truck, ordered by the cubic yard or cubic meter.
- Yield
- The volume of mixed concrete a bag produces once water is added, for example about 0.6 cubic feet for an 80 lb bag.
- Waste allowance
- An extra percentage added to the calculated volume to cover spillage, uneven ground and form movement.
- Slab
- A flat, horizontal pour of concrete such as a floor, patio, driveway or pad.
- PSI
- Pounds per square inch, the measure of concrete compressive strength at 28 days. Most residential pours are 3,000 to 4,000 PSI.
- Sonotube
- A cardboard tube form used to cast round concrete columns and footings, sized by its inside diameter.
Frequently asked questions
How much concrete do I need?
Multiply the slab length, width and thickness in the same unit to get the volume. For a 10 by 10 ft slab 4 inches thick that is 10 × 10 × 0.333 = 33.3 cubic feet, which is about 1.23 cubic yards. This calculator does the conversion and adds a waste allowance automatically once you enter the three dimensions.
How many 80 lb bags of concrete are in a cubic yard?
An 80 lb bag of pre-mixed concrete yields about 0.6 cubic feet once mixed, and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 is about 45 bags per cubic yard. A 60 lb bag yields about 0.45 cubic feet (60 per yard) and a 40 lb bag about 0.30 cubic feet (90 per yard).
How much does a yard of concrete cost in 2026?
Ready-mix concrete typically runs about 120 to 200 dollars per cubic yard delivered, with a national average near 150 dollars. A standard 3,000 PSI mix sits at the lower end, while 4,500 to 5,000 PSI or fiber-reinforced mixes cost more. Expect extra short load, delivery and pump fees on small orders.
How much area does a yard of concrete cover?
One cubic yard spread 4 inches deep covers about 81 square feet. At 5 inches it covers roughly 65 square feet, and at 6 inches about 54 square feet. Deeper pours cover less area per yard, so coverage drops as your slab gets thicker.
How do I calculate concrete for a round column or footing?
Use the area of a circle: volume equals pi times the radius squared times the depth. The radius is half the diameter, so a 12 inch sonotube has a 6 inch (0.5 ft) radius. A 12 inch tube 4 ft deep holds about 3.14 cubic feet, or roughly 0.12 cubic yards. Divide cubic feet by 27 for yards.
Should I order concrete in bags or ready-mix?
For small jobs of under roughly half a cubic yard, bags you mix on site are usually cheaper and more convenient. Beyond that, mixing dozens of bags by hand becomes impractical and ready-mix delivered by truck is faster and more consistent. The crossover depends on your access, labor and local prices.
How much extra concrete should I order for waste?
A waste allowance of 5 to 10 percent is typical to cover spillage, uneven subgrade and form movement. Many crews also round up to the next quarter or half cubic yard when ordering ready-mix, because running short forces an expensive second delivery and a weak cold joint between pours.
What PSI concrete do I need for a driveway or slab?
Most residential driveways and garage floors use 3,500 to 4,000 PSI concrete, while patios and interior slabs are commonly 3,000 to 3,500 PSI, and footings or sidewalks around 2,500 to 3,000 PSI. Cold climates with freeze and thaw cycles favor the higher strengths plus air entrainment. Always follow local code.
How do I convert cubic yards to cubic meters?
Multiply cubic yards by 0.7646 to get cubic meters, or multiply cubic meters by 1.308 to get cubic yards. For example, 2 cubic yards is about 1.53 cubic meters. This tool shows both figures at once so you can match whatever unit your supplier uses.
Does this calculator account for rebar, gravel base or footings?
No. It calculates the plain rectangular or round volume of the pour itself. Steel reinforcement displaces a negligible amount of concrete, but a separate gravel base, thickened edges or footings are extra pours you should measure and add on their own. For an irregular shape, split it into rectangles and total the results.