How much concrete do I need?

You need enough concrete to fill every part of the planned pour, plus a separately chosen allowance. For a rectangular slab, multiply its length by its width by its concrete thickness, using compatible units.

For example, a 12-foot by 10-foot slab at 4 inches thick contains 40 cubic feet, or about 1.48 cubic yards. A hypothetical 10% allowance increases that to 44 cubic feet, or about 1.63 cubic yards; this percentage is an example, not a requirement for every project.

Volume is only one planning decision. Use the thickness, footing dimensions, reinforcement and concrete specification required by your project design. The dimensions in this article illustrate quantity calculations; they do not specify a suitable structural design.

Concrete versus cement: order the right material

Cement is the binder ingredient; concrete is the combined material used for a pour. The American Cement Association’s explanation of cement and concrete describes cement paste binding sand and coarse aggregate together.

If you searched for a cement calculator to figure cement for a patio, check whether you actually need premixed concrete. A concrete estimator can calculate finished volume and premix bags, but it cannot determine separate cement, sand and stone quantities for an engineered mix design.

Measure your slab, pad or footing

If your shed maker approves a stone pad instead of a concrete foundation, use the gravel shed pad planner for that distinct assembly. A material calculation does not decide which foundation the shed requires.

Concrete slab and pad measurements

Measure the inside of the forms: length, width and the vertical thickness of concrete. Include only the concrete layer, rather than the combined depth of concrete, gravel base and excavation.

A concrete pad calculator uses the same rectangular-volume method as a slab. For a floor with different depths, divide it into separate sections instead of assuming one thickness covers the whole pour.

If you need to order aggregate beneath the slab, calculate that layer separately with the gravel calculator. Adding its depth to the concrete thickness would overstate your concrete order.

Before choosing the base aggregate, compare clean stone with graded material in the gravel types and base-material guide. Match the order to the project specification rather than assuming every stone grade performs the same job.

Rectangular footings and repeated components

A concrete footing calculator needs the footing’s length, width and depth. Multiply by the number of identical footings only when all their dimensions match.

For a continuous footing, use the actual concrete run and account for corners or intersections. Adding overlapping rectangles without subtracting their shared space counts the same concrete twice.

Round footings, columns and circular pads

Measure the inside diameter and concrete depth or height. A cylinder’s radius is half its diameter, so entering the diameter as the radius makes the volume four times too large.

Where a post occupies part of the hole, subtract the embedded post’s volume if the estimate is intended to represent only the surrounding concrete. Match the calculation to the actual geometry.

How to calculate concrete in cubic yards

Rectangular slab formula

When length and width are in feet and thickness is in inches, use:

Cubic yards = length × width × thickness ÷ 324
Length and width: feet. Thickness: inches.

The divisor includes both conversions: 12 inches per foot and 27 cubic feet per cubic yard. If all three dimensions are already in feet, multiply them and divide by 27 instead.

Known area and thickness

If you already know the area in square feet, multiply it by thickness in inches and divide by 324. Square footage alone cannot tell you how much concrete to buy.

For broader volume conversions, use the cubic yard calculator. Keep area, linear distance and volume separate when copying measurements between tools.

Circular and hollow-cylinder formulas

Solid cylinder: cubic feet = π × radius² × height
Hollow cylinder: cubic feet = π × (outer radius² − inner radius²) × height
Use feet for every dimension, then divide cubic feet by 27.

A hollow-cylinder calculation applies when a central opening remains empty. The inner diameter must be smaller than the outer diameter; a solid round pad has no opening to subtract.

Metric concrete volume

Multiply length, width and thickness in meters to obtain cubic meters. If thickness is in millimeters, divide it by 1,000 before multiplying.

A 3-meter by 2-meter pad at 100 millimeters thick contains 0.60 cubic meters. When comparing US and metric quotations, 1 cubic yard equals exactly 0.764554857984 cubic meters.

How to estimate concrete yards without hiding the allowance

Keep the measured volume and allowance-adjusted volume visible as separate numbers. An allowance helps plan an order; it does not correct inaccurate measurements.

Choose it after checking actual form dimensions, depth variation and the contractor’s ordering approach. A measured low area or thickened edge should be added as concrete geometry, rather than concealed in a generic percentage.

Adjusted volume = measured volume × (1 + allowance ÷ 100)

Why one extra inch matters

For a 12-foot by 10-foot slab, each extra inch adds 10 cubic feet, or about 0.37 cubic yard. Increasing an assumed depth from 4 inches to 5 inches increases this slab’s volume by 25%.

Check several points across the prepared surface when estimating a concrete slab. Where the top and bottom surfaces form a consistent linear taper, their average thickness can represent the volume; irregular pockets need separate measurement.

Concrete slab coverage per cubic yard

One cubic yard contains 27 cubic feet. Divide that volume by the slab thickness in feet to find its theoretical coverage.

These figures assume a uniform layer and exclude thickened edges or separate footings. They compare coverage; they do not recommend a thickness for your slab.

Concrete bag calculator: use the labeled yield

The number of premix bags depends on the mixed volume each bag produces. Divide the required cubic feet by that yield and round upward to a whole bag.

Whole bags = round up(required cubic feet ÷ yield per bag in cubic feet)

Read the product name, bag weight, mixed yield and yield unit together. An 80-pound bag describes dry packaged weight, not 80 pounds of finished concrete or a fixed yield for every product.

QUIKRETE Concrete Mix bag yields

The QUIKRETE Concrete Mix No. 1101 data sheet gives the following approximate yields. Use these only for the stated product and confirm the bag you purchase.

How many bags of concrete are in a yard?

At a labeled yield of 0.60 cubic foot, one cubic yard requires 45 bags before allowance. At 0.45 cubic foot, it requires 60 bags; at 0.30 cubic foot, it requires 90 bags.

A bag concrete estimator should also show the remaining volume after rounding. If the exact calculation calls for 73.33 bags, buying 74 covers the calculated quantity; that small rounding surplus is not a substitute for a separately chosen allowance.

How many cubic yards does one 80 lb bag make?

For a product yielding 0.60 cubic foot, divide 0.60 by 27. The result is about 0.02222 cubic yard per bag.

Keep full precision during calculations, then round the final order. Rounding each bag to 0.02 cubic yard first would distort the total.

Using a QUIKRETE calculator or another brand’s yield

The QUIKRETE product data sheet rounds results to whole bags and notes that its slab yields exclude waste and uneven substrate. Compare those assumptions with your own measurements before ordering.

A QUIKRETE concrete calculator preset should identify the exact mix. Mortar, resurfacer and post-setting products have different applications; a bag’s brand alone does not identify its yield.

Worked concrete quantity examples

Example 1: a 12 × 10 ft slab at 4 inches

The slab’s measured volume is 12 × 10 × 4 ÷ 324 = 1.48148 cubic yards, or 40 cubic feet. Adding an illustrative 10% allowance gives 44 cubic feet, or 1.62963 cubic yards.

The Sakrete High-Strength Concrete Mix product page lists a 0.45-cubic-foot yield for its 60-pound bag. These are two specific ordering options, not a claim that one brand always yields more than another.

Example 2: six identical rectangular footings

Suppose each footing measures 2 feet long, 2 feet wide and 1 foot deep. Each contains 4 cubic feet, so six contain 24 cubic feet, or 0.88889 cubic yard.

At a yield of 0.60 cubic foot per bag, the measured volume takes 40 bags. An illustrative 10% allowance raises the volume to 26.4 cubic feet and the order to 44 bags.

Example 3: four round pours

For four solid cylinders, each 12 inches in diameter and 24 inches deep, the radius is 0.5 foot and the height is 2 feet. Their combined volume is 4 × π × 0.5² × 2 = 6.28319 cubic feet.

That is about 0.23271 cubic yard, requiring 11 bags at a 0.60-cubic-foot yield before allowance. These dimensions illustrate the math, not a suitable foundation size for a particular load.

Example 4: adding a thickened edge correctly

Start with the full slab at its ordinary thickness. Then add only the extra depth below that slab thickness along the edge.

If the ordinary slab is 4 inches thick and an edge is 12 inches deep overall, the additional edge depth is 8 inches. Use a non-overlapping edge area so corners are counted once.

Ready-mix versus premix bags

Choose the ordering method after seeing the volume and bag count. A smaller pour may be manageable with bags, while a large bag count makes mixing capacity, handling and pour continuity important.

For the adjusted slab example above, 74 dry 80-pound bags total 5,920 pounds of packaged material before mixing water. That handling requirement belongs in the decision, alongside the supplier’s ready-mix delivery terms.

Concrete weight calculator: volume needs a density

Weight equals volume multiplied by the specified density. Separate finished-concrete weight from the dry bag weight you may need to transport.

For a product-specific example, QUIKRETE No. 1101 lists an approximate unit weight of 140 pounds per cubic foot in its data sheet. At that density, 27 cubic feet weighs approximately 3,780 pounds; use your supplier’s density for a different mix.

Concrete weight in pounds = cubic feet × density in lb/cu ft
This is a material estimate, not a vehicle payload or structural-load approval.

Quantity is not the installed project cost

Multiplying cubic yards by a local concrete price gives a material subtotal. Delivery, minimum-load charges, excavation, forms, reinforcement, pumping, finishing and labor may need separate lines.

Ask each supplier or contractor to identify what the quote includes. A material calculator for a concrete slab should not imply that the concrete volume alone represents the completed project budget.

Curing: plan beyond the pour

Curing manages moisture and temperature after placement so concrete can develop its intended properties. The NRMCA guide to curing in-place concrete explains that this starts after placement and finishing and commonly continues for 3–7 days, subject to the specification.

Choose the curing method for the mix, weather and planned surface treatment. Follow the project requirements and product instructions rather than treating a dry-looking surface as permission to load it.

Concrete-order checklist

• Confirm inside-form dimensions and concrete-only depth.
• Include footings, thickened edges and extra sections without overlap.
• Save the measured volume and allowance-adjusted volume separately.
• For bags, record the exact product, bag size and mixed yield.
• For ready-mix, confirm the mix specification, order increments and delivery access.
• Check placement equipment, crew availability and timing.
• Confirm curing requirements before the pour begins.

Concrete calculator questions

How do I figure concrete for a slab?

Multiply length in feet by width in feet by thickness in inches, then divide by 324 for cubic yards. Calculate separate depths and thickened sections individually, then add the volumes.

How many bags of concrete do I need?

Divide your required cubic feet by the chosen product’s mixed yield per bag. Round upward once at the end, after applying any separate allowance.

Can I use the same calculation for a floor and a pad?

Yes, when each is a rectangular solid. A concrete floor calculator must still account for changes in depth, openings and thickened areas; using the same formula does not make the designs interchangeable.

Can an online concrete calculator choose my slab thickness?

It calculates the quantity for a thickness you supply. Loads, soil conditions, reinforcement and project requirements determine the appropriate design.

How much does a yard of concrete weigh?

Multiply 27 cubic feet by the mix’s density in pounds per cubic foot. For example, an explicitly assumed 140 lb/cu ft gives 3,780 pounds; another density gives a different result.

How do I work out concrete for several pours?

Calculate each pour separately, including its geometry and count, then total the volumes if they use the same mix and ordering schedule. Avoid combining unlike shapes or different delivery dates into one unexplained number.

Is a concrete yardage calculator the same as a cubic yard converter?

The unit conversion is shared, but concrete planning also needs pour geometry, bag yield and ordering assumptions. A concrete volume calculator estimates the pour; a unit converter only changes how that volume is expressed.

Does adding more water increase the usable bag yield?

Use the product’s specified mixing instructions and labeled yield. Do not change the water quantity to compensate for a shortage; recalculate the material order instead.

Product yields and primary sources. Worked examples are original calculations. Confirm your project dimensions and the label of the product you buy.

Turn the quantity into a supplier-budget comparison with the concrete price per yard calculator.