For a rectangular slab, calculate volume first: length × width × thickness, with all dimensions in the same unit. Then divide that volume by the published yield of the concrete mix you plan to use and round up to a whole bag. A 10 ft × 6 ft slab that is 4 in thick is about 20 ft³. With a mix yielding 0.60 ft³ per 80 lb bag, the purchasing estimate is 20 ÷ 0.60 = 33.33, or 34 bags.
That calculation is for bagged concrete mix, not a bag of pure cement. Concrete mix is a preblended combination of cement and aggregates. Portland cement is only one ingredient and must be proportioned with sand, gravel, water and any specified admixtures. Choosing the right calculation starts with identifying which material your project actually requires.
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Concrete mix and cement are different materials
People often use “cement” to mean the entire gray material placed in a form, but the terms are not interchangeable:
- Cement is the binding ingredient. Portland cement is a common type.
- Concrete is cement combined with water and aggregates such as sand and gravel. Some mixes also contain supplementary cementitious materials, fibers or chemical admixtures.
- Bagged concrete mix is preblended cement and aggregates. You add the specified amount of water and place the resulting concrete.
The American Cement Association says cement typically makes up 10% to 15% of a concrete mix by volume. That percentage explains cement’s role, but it does not tell you how many bags to buy. Bag count depends on the finished volume, the product’s aggregate proportions and the manufacturer’s published yield.
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If a product label says it is concrete mix, use its yield per bag for the quantity calculation. Do not divide your slab volume by the amount of cement contained in a bag, and do not call a bag of concrete mix a bag of cement.
Start with the project dimensions
Use one unit throughout
Measure the length, width and finished thickness of the form. Convert every dimension to feet before calculating cubic feet. Convert inches to feet by dividing by 12:
- 4 in = 4 ÷ 12 = 0.333 ft (approximately 0.33 ft)
- 6 in = 6 ÷ 12 = 0.5 ft
- 8 in = 8 ÷ 12 = 0.667 ft (approximately)
Writing the conversion beside each measurement prevents a common error: multiplying feet by inches produces a number with no useful volume meaning.
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For a rectangular form, use:
Volume (ft³) = length (ft) × width (ft) × thickness (ft)
For a slab measuring 10 ft long, 6 ft wide and 4 in thick:
- Convert thickness: 4 ÷ 12 = 0.333 ft.
- Multiply: 10 × 6 × 0.333 ≈ 20 ft³.
The same method works for rectangular footings, steps, walkways and equipment pads. If a project has multiple rectangular sections, calculate each section separately and add the volumes.
Irregular shapes
Break a complex form into shapes whose volume you can calculate, then add the results. For example, divide an L-shaped slab into two rectangles. For a triangular section, calculate its plan area as base × height ÷ 2, then multiply by thickness. For a round pier or pad, calculate plan area as π × radius², then multiply by depth. Keep all dimensions in feet so the result is cubic feet.
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When sections have different thicknesses, do not average the thickness by eye. Calculate each thickness zone independently; a thin area and a deepened edge can require substantially different quantities.
Convert cubic feet to cubic yards when needed
Concrete suppliers commonly quote ready-mixed concrete by the cubic yard. There are 27 cubic feet in 1 cubic yard:
Volume (yd³) = volume (ft³) ÷ 27
A 20 ft³ slab is 20 ÷ 27 = 0.74 yd³, approximately. Use cubic feet when buying bagged mix if the bag yield is stated in cubic feet; use cubic yards when a supplier quotes a truck delivery in yards. Do not mix the units in one calculation.
Use the bag’s published yield to find the number of bags
The basic formula
Once you know the project volume and have selected a product, use:
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The yield is the volume of usable concrete produced by one bag when mixed according to the product instructions. It is not the bag’s weight and is not the volume of cement alone.
Example yields for one concrete-mix product line
The following approximate yields are listed on a QUIKRETE product data sheet. They apply to that product and package size; another manufacturer or formulation may yield differently.
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| Bag size | Approximate yield per bag | Use in the formula |
|---|---|---|
| 40 lb | 0.30 ft³ | Project ft³ ÷ 0.30 |
| 60 lb | 0.45 ft³ | Project ft³ ÷ 0.45 |
| 80 lb | 0.60 ft³ | Project ft³ ÷ 0.60 |
Read the yield printed on the actual bag or its current data sheet before ordering. A similar-looking bag with a different aggregate blend can produce a different volume.
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Bag calculations almost always produce a decimal. Round up to the next whole bag for purchasing. A result of 33.01 bags still requires 34 bags; buying 33 leaves less concrete than the calculated form volume. Keep any allowance for site variation separate from the mathematical minimum rather than hiding it by rounding down.
Worked example: a 10 ft × 6 ft × 4 in slab
- Convert thickness: 4 in ÷ 12 = approximately 0.33 ft.
- Calculate volume: 10 × 6 × 0.33 = approximately 20 ft³.
- Check the product label. Using a stated yield of 0.60 ft³ per 80 lb bag: 20 ÷ 0.60 = 33.33 bags.
- Round up for the minimum purchase: 34 bags.
A QUIKRETE training example reports 33 bags for these same dimensions because it rounds the intermediate result differently. The volume formula has not changed; 33.33 bags cannot be purchased as a fraction, so 34 is the whole-bag purchasing figure. The product’s stated yield and the actual form dimensions remain the controlling information.
How to calculate a separate Portland-cement batch
If you are batching cement, sand and gravel separately, do not use the bagged-mix table. You need a specified mix proportion and the yield of that particular combination of materials.
QUIKRETE’s Portland Cement product information states that five to six 94 lb bags of Portland cement are typically used to make 1 yd³ of concrete with appropriate sand and gravel proportions. This is a manufacturer-specific typical estimate, not a universal recipe. It does not mean that five bags will meet every strength, durability or code requirement.
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Why cement percentage cannot produce a shopping list
The 10% to 15% cement-by-volume figure from the American Cement Association is a broad description of concrete composition. It does not account for the voids and grading of aggregates, water, entrained or entrapped air, admixtures, moisture in the sand or the required design performance. Concrete yield is based on the combined absolute volumes of cementitious materials, aggregates, water, admixtures and air.
For a nonstructural repair where the product instructions permit field batching, follow the specified proportion and measure every ingredient consistently. For a footing, load-bearing slab, retaining element or any work governed by drawings, code or an engineer, use the specified mix or ask a qualified concrete supplier. Do not substitute a generic cement-to-aggregate ratio because its bag count appears convenient.
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Bagged mix versus separately batched ingredients
These options answer different project needs. Compare them on the factors that affect both quantity and performance:
| Question | Bagged concrete mix | Separate Portland cement batch |
|---|---|---|
| What is in the package? | Preblended cement and aggregates; water is added on site. | Portland cement supplied separately; sand, gravel, water and any admixtures are measured separately. |
| How is quantity calculated? | Project volume divided by the product’s published yield per bag. | Project volume converted to the selected batch’s ingredient quantities and expected yield. |
| Useful published figures | Example yields: 0.30, 0.45 and 0.60 ft³ for 40, 60 and 80 lb bags in one product line. | One manufacturer states five to six 94 lb cement bags per cubic yard with appropriate sand and gravel proportions. |
| When is it appropriate? | Small pours and projects where the product instructions meet the required performance. | Projects with an established mix design, measured aggregate supply and suitable batching equipment. |
| Main quantity risk | Using a yield from a different product or bag size. | Treating a typical cement proportion as an engineered mix or overlooking aggregate and moisture quantities. |
Product choice should follow the project specification, not simply whichever option appears to require fewer bags.
Account for the actual site without inventing a universal waste percentage
Manufacturer calculators describe their estimates as approximate and exclude uneven substrate and waste. No single waste allowance applies to every form, so inspect the conditions instead of automatically adding a fixed percentage.
Check the form and base
- Measure the inside dimensions of the completed form, not the rough excavation.
- Verify the depth at multiple points. High spots in the base can reduce volume in one area while soft or over-excavated spots increase it elsewhere.
- Confirm that thickened edges, steps, rebates, block-outs and post holes are included in the calculation.
- Recheck the final form immediately before ordering or mixing.
Separate mathematical volume from site variation
Your calculated quantity is the volume represented by the dimensions you measured. Uneven ground, spillage, material left in a mixer or changes made during placement can alter the amount required. Record the calculated minimum, then decide—based on the site and the consequences of running short—whether a separately documented additional purchase is prudent. Do not present that decision as a universal percentage established by the manufacturer.
A repeatable calculation worksheet
- Identify the material. Write down the exact product name, bag weight and whether it is concrete mix or Portland cement.
- Measure the form. Record length, width and every thickness or depth zone.
- Convert units. Change inches to feet by dividing by 12; keep the same unit in every dimension.
- Calculate each section. Multiply length × width × thickness for rectangles, or use the corresponding area formula for other shapes.
- Add sections. Sum the cubic-foot results to get total project volume.
- Convert if necessary. Divide cubic feet by 27 when a supplier quotes cubic yards.
- Read the current yield. Use the yield printed on the selected bag or its current data sheet, not a remembered number from another product.
- Divide and round up. Project volume ÷ yield gives the minimum whole-bag estimate.
- Review site conditions. Check the base, form depth and any features that could change the placed volume before purchasing.
Common calculation mistakes and their fixes
Calling concrete mix “cement”
Problem: You compare a bag of premixed concrete with a bag of Portland cement as though both produce the same material. Fix: Identify whether aggregates are already in the package. Use concrete-mix yield for a preblended product; use a specified ingredient schedule for separate batching.
Using inches without converting
Problem: You multiply 10 × 6 × 4 and treat the result as cubic feet. Fix: Convert 4 in to 0.333 ft first, then multiply.
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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteUsing bag weight as volume
Problem: You assume an 80 lb bag produces twice as much as a 40 lb bag without checking. Fix: Use the published yield. In the example product line, the approximate yields are 0.60 ft³ and 0.30 ft³, but that relationship is not a substitute for reading the label.
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Rounding down
Problem: You buy the integer below the decimal result. Fix: Round up to the next whole bag, then evaluate site-specific variation separately.
Applying one manufacturer’s numbers to another product
Problem: You reuse the 0.60 ft³ yield for a different formulation or package. Fix: Replace the example yield with the current yield for the exact product you will use.
Ignoring design requirements
Problem: You select a mix because it needs fewer bags, even though the project documents specify a different performance. Fix: Follow the drawing, code requirement or engineer’s specification first; quantity calculations come after the material is selected.
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Safety and when to obtain professional help
Cement-based products can irritate skin and eyes. QUIKRETE advises eye protection and waterproof gloves when working with cement-based materials. Follow the complete product instructions for mixing, handling, curing and disposal.
Obtain direction from a qualified concrete supplier, engineer or building professional when the work is structural, inspected, code-controlled, unusually large, or based on a specified strength or durability requirement. A supplier can confirm whether the selected product and calculated volume match the order, while an engineer or project specification determines whether the mix itself is acceptable.
Bottom line
Measure the finished form, convert all dimensions to one unit, calculate volume, divide by the selected product’s published yield and round up. For the 10 ft × 6 ft × 4 in example, that means about 20 ft³ and 34 bags when each 80 lb bag yields 0.60 ft³. If you are using Portland cement separately, treat manufacturer-specific figures as typical guidance only and follow the project’s required mix design.
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