How Much Concrete Do I Need for a Slab? 10×10, 12×12, 20×20 and More

To calculate how much concrete you need for a rectangular slab, multiply length × width × thickness. In US customary units, convert the result from cubic feet to cubic yards by dividing by 27. In metric units, use metres for length and width and convert slab thickness from millimetres to metres before multiplying.

That formula is the easy part. The useful estimate comes from choosing the correct finished thickness, measuring the prepared base rather than trusting a nominal drawing, separating thickened edges or footings, adding a reasoned allowance and then converting the volume into the way concrete is actually bought: cubic yards, cubic metres or complete bags.

The Concrete Slab Formula

A slab is a rectangular volume. If all three dimensions are expressed in compatible units, the geometry is simply length multiplied by width multiplied by thickness.

US customary
Cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
Cubic yards = cubic feet ÷ 27

Metric
Thickness (m) = thickness (mm) ÷ 1000
Cubic metres = length (m) × width (m) × thickness (m)

Planning quantity = geometric volume × (1 + allowance ÷ 100)

Thickness is the dimension people most often underestimate

Length and width are easy to see on a plan. Thickness is hidden once the concrete is placed, but it changes the volume in direct proportion.

A 6 in slab contains 50% more concrete than a 4 in slab with the same length and width.

Measure several points on the prepared base when depth can vary, especially after excavation or compaction.

Quick Table: Concrete for Common Slab Sizes at 4 Inches Thick

The table below is intentionally simple: rectangular slabs, 4 in uniform thickness and no thickened edge. The final column shows a 10% planning allowance because that is a common search scenario, not because every slab automatically needs 10%.

10×10, 12×12 and 20×20 Slabs at 4, 5 and 6 Inches

When someone asks how many yards are needed for a 10 × 10 or 20 × 20 slab, the missing word is usually thickness. Increasing thickness by one or two inches can change the order more than a small change in length or width.

One footprint, three very different orders

At the same footprint, volume rises in direct proportion to thickness.

A 20 × 20 ft slab grows from about 4.94 yd³ at 4 in to 7.41 yd³ at 6 in before allowance.

Do not choose the thickness from the cheapest column. Use the thickness required by the project, then calculate the quantity.

Metric Slab Examples: 100, 125 and 150 mm

For metric calculations, convert millimetres to metres before multiplying. A 100 mm slab is 0.10 m thick, 125 mm is 0.125 m and 150 mm is 0.15 m. Keeping the thickness in millimetres while the other dimensions are in metres is one of the fastest ways to make a concrete order wildly wrong.

Should You Add 5%, 10% or More Concrete?

The geometric volume describes a perfect box. The prepared site is rarely a perfect box. Small low spots, over-excavation, forms that bow or move, concrete left in handling equipment and normal placing losses can all increase the real amount used.

The Concrete Society notes that an overage is often included when ordering and gives general contractor guidance of about 5% for formed elements and 10–15% for unformed elements. That is useful context, but it is not a rule to paste onto every slab. A carefully formed slab over a level prepared base is different from concrete placed into irregular excavation.

Measure the Prepared Base, Not Just the Plan

If the slab is intended to be 4 in or 100 mm thick, the concrete volume assumes the top surface and the prepared base stay that distance apart. A low spot adds concrete. A hump reduces thickness. Neither is visible in a length × width calculation.

• Measure inside the forms after they are braced.
• Check length and width at more than one location when the form is not perfectly square.
• Measure depth from the planned finished surface to the compacted base at a grid of points.
• Mark thickened edges, steps, beams, pads and footings as separate zones.
• Recheck after heavy rain, final compaction or other work that may have changed the base.
• Do not assume reinforcement, pipes or inserts will compensate for missing concrete volume.

A slab can be rectangular on top and irregular underneath

Use the design thickness for the slab itself, then identify local areas that are intentionally or accidentally deeper.

Calculate planned thickened zones separately so they are not hidden inside a vague waste percentage.

If the base varies substantially, a few depth measurements or a simple level survey can be more valuable than adding another 5% by guesswork.

Do Thickened Edges and Footings Count Separately?

Yes. If the slab has a deeper perimeter, integral beam, strip footing, equipment pad or local thickening, a uniform slab calculation can understate the order. Treat each extra zone as an additional volume and avoid double-counting the portion already included in the main slab thickness.

Bagged Concrete or Ready-Mix?

Small pads and repairs may be convenient with bagged concrete. As volume grows, the number of bags, mixing cycles, water measurements, lifting and placing time grow surprisingly fast. Ready-mix or ready-mixed concrete moves the batching to a plant or volumetric supplier, but introduces delivery access, minimum-load charges, discharge time and mix-specification questions.

The cheapest-looking unit is not always the easiest pour

Count complete bags from the mixed yield printed on the product, not from bag weight alone.

For ready-mix, ask how the supplier rounds order volume and what small-load or waiting charges apply.

Before the truck leaves the plant, make sure the site can actually receive and place the concrete at the planned rate.

Example using 80 lb and 60 lb bags

QUIKRETE Concrete Mix lists approximate yields of 0.60 ft³ for an 80 lb bag and 0.45 ft³ for a 60 lb bag. Those figures are product-specific examples. Always use the yield for the exact mix you are buying.

Metric bag example

The Concrete Society recommends using the volume yield printed on the bag for dry-blended concrete. If a particular 25 kg bag states a mixed yield of 0.010 m³, or 10 L, a 3 × 3 m slab at 100 mm thickness contains 0.90 m³. With a 10% allowance the planning volume is 0.99 m³, so that product would require 99 bags.

How Many Bags Are in One Cubic Yard of Concrete?

One cubic yard contains 27 ft³. Using the example yields above, one cubic yard is theoretically 45 bags at 0.60 ft³ per bag or 60 bags at 0.45 ft³ per bag. That is before any project allowance and assumes the current product really provides those yields.

How Much Does One Cubic Yard or One Cubic Metre Cover?

Coverage is simply volume divided by thickness. It is a useful reverse check when a supplier quote is already expressed in cubic yards or cubic metres.

Ready-Mix Ordering: Volume Is Only the First Question

Once the quantity is known, the supplier still needs enough information to provide a suitable concrete. In the UK, ready-mixed concrete may be specified under BS 8500 and BS EN 206 terminology; in the US, Canada, Australia and other markets the specification language and adopted standards differ. Tell the supplier what the concrete is for and provide the project specification rather than guessing a strength or mix from an internet table.

• Calculated order volume and the allowance included in it
• What the slab will be used for
• The concrete specification from the drawings, engineer, local guidance or approved project details
• Required delivery time and expected placing sequence
• Whether the truck can reach the forms or a pump, conveyor, buggy or wheelbarrow route is needed
• Site restrictions such as narrow roads, weak driveways, overhead lines, gates or soft ground
• How the supplier rounds volume and whether minimum-load, waiting or returned-concrete charges apply
• A backup plan if weather, access or site preparation changes before the pour

A correct volume can still arrive at the wrong side of the building

Measure the route from the vehicle position to the farthest part of the slab.

Confirm whether the chute can reach or whether another placing method is required.

Prepare enough people, tools and finishing capacity for the planned delivery rate before the concrete arrives.

Does Reinforcement Change the Concrete Quantity?

Rebar or reinforcing mesh occupies some volume, but for an ordinary small material estimate it is usually safer to calculate the full concrete geometry rather than trying to recover a few litres or cubic feet by subtracting every bar. In heavily reinforced work, the project takeoff should follow the structural drawings and supplier or engineer guidance.

Reinforcement quantity is a separate calculation. Bar size, spacing, cover, laps and the number of mats come from the structural design; concrete volume alone cannot tell you what reinforcement is required.

Worked Example: 12×12 ft Slab at 4 Inches

The calculation says about 1.96 yd³ with the example 10% allowance. That does not automatically mean you should phone a supplier and say 'bring exactly 1.96'. Ask how the supplier sells and rounds concrete, then reconcile that commercial quantity with your site measurement and project specification.

Worked Example: 4×5 m Slab at 100 mm

This example also shows why bagged concrete becomes a logistics question long before the arithmetic becomes difficult. Two hundred and twenty 25 kg bags would represent 5.5 tonnes of dry product before water is added. Whether that is sensible depends on access, labour, equipment, delivery options and the particular product.

What About L-Shaped Slabs, Paths and Several Pads?

Do not force an irregular slab into one average rectangle if the sections can be measured separately. Divide the layout into rectangles, calculate each volume and add them. Use the same approach when thickness changes between zones.

That deliberate check illustrates a useful site habit: do not trust a number because it looks plausible. Recalculate the total in a second way—by adding section areas first or by adding section volumes—and make sure both methods agree.

Common Concrete Slab Estimating Mistakes

Before You Order: A Five-Minute Slab Check

1. Confirm the slab length and width inside the finished forms.
2. Confirm the required thickness from the approved project information.
3. Check base elevation and depth at several locations.
4. Calculate thickened edges, footings, steps and pads separately.
5. Choose an allowance that reflects measured site uncertainty.
6. Convert the result into cubic yards, cubic metres or the exact bag yield.
7. Confirm the concrete specification with the supplier or project documents.
8. Check supplier rounding, minimum order and delivery charges.
9. Confirm vehicle access and how concrete will reach every part of the slab.
10. Have labour, tools, forms and finishing equipment ready before delivery.

Frequently Asked Questions

How much concrete do I need for a 10×10 slab at 4 inches thick?

A 10 × 10 ft slab at 4 in thick contains about 33.33 ft³, or 1.23 yd³, before allowance. Adding 10% gives about 36.67 ft³ or 1.36 yd³.

How much concrete do I need for a 12×12 slab at 4 inches thick?

A 12 × 12 ft slab at 4 in thick contains 48 ft³, or about 1.78 yd³. With a 10% planning allowance, the quantity is about 52.8 ft³ or 1.96 yd³.

How much concrete do I need for a 20×20 slab at 4 inches thick?

A 20 × 20 ft slab at 4 in thick contains about 133.33 ft³, or 4.94 yd³. With a 10% planning allowance, it becomes about 146.67 ft³ or 5.43 yd³.

How much concrete for a 10×10 slab at 6 inches thick?

A 10 × 10 ft slab at 6 in thick contains 50 ft³, or about 1.85 yd³. With a 10% allowance, the planning quantity is about 2.04 yd³.

How many 80 lb bags of concrete are needed for a 10×10 slab at 4 inches?

If the selected 80 lb concrete mix yields 0.60 ft³ per bag, the geometric 33.33 ft³ volume requires about 56 bags. With a 10% allowance, 36.67 ft³ requires about 62 bags. Check the exact yield on the current product label.

Is 10% extra concrete always necessary?

No. Ten percent is a common planning example, not a universal rule. A well-formed slab over a carefully measured base may justify less, while irregular excavation can justify more. Measure the real geometry first and choose an allowance for the uncertainty that remains.

Should I order exactly the calculated cubic yards?

Usually the calculated volume is the starting point, not the final phone order. Add the justified allowance, then ask the local supplier how volume is rounded, what minimum orders apply and whether the project dimensions or concrete specification require another check.

Can I use 4 inches as the thickness for every patio, shed slab or driveway?

No. Four inches is used in this guide because it is a common quantity-search example, not because it is correct for every project. Slab thickness depends on use, loads, support conditions, base preparation, local requirements and the structural design where one is required.

How do I calculate concrete for a slab in metres?

Multiply slab length in metres by width in metres by thickness in metres. Convert millimetres to metres first: 100 mm = 0.10 m, 125 mm = 0.125 m and 150 mm = 0.15 m. The result is cubic metres.

How many square feet does one yard of concrete cover at 4 inches?

One cubic yard contains 27 ft³. At a uniform 4 in thickness, or one-third of a foot, it covers 81 ft² before allowance or irregular depth is considered.

When should I use ready-mix instead of bags?

There is no universal crossover volume. Compare bag count, mixer output, labour, continuous placing time, site access, supplier minimums and delivery charges. A bag calculation can be mathematically possible long after it stops being practical for one continuous slab pour.

Official and Manufacturer References

The calculation method is basic geometry, but product yield, concrete specification and ordering practice should be checked against current supplier information and the requirements that apply to the project. The following references are useful starting points.

Final Concrete Order Check

• Length and width were measured from the actual forms or final layout.
• Thickness comes from the project requirement rather than a generic online table.
• Millimetres, inches, feet and metres were converted consistently.
• Thickened edges, beams, footings and steps were calculated separately.
• The prepared base was checked for meaningful depth variation.
• The allowance has a reason and is not hiding unknown geometry.
• Bag counts use the exact mixed yield of the selected product.
• Ready-mix volume is reconciled with supplier ordering increments and minimums.
• Concrete specification, reinforcement and joints are checked separately from volume.
• Truck or mixer access, placing route, labour and finishing capacity are ready for the pour.

A dependable answer to “how much concrete do I need for a slab?” is not just length × width × a familiar thickness. It is the actual footprint multiplied by the required thickness, corrected for every deliberate deeper zone, checked against the prepared base and then converted into a practical order with a controlled allowance. Do that, and the final number is easy to explain—to yourself, to the supplier and, most importantly, to the empty corner of the form that you do not want to meet after the truck has gone.