For most home pours, you are estimating concrete mix, not bags of pure cement. Concrete mix combines Portland cement with sand, gravel or stone, and other ingredients. Start by finding the project’s volume, then divide that volume by the product’s stated yield per bag—or convert the volume to cubic yards for a ready-mix quote. Finally, price the complete order, including delivery and other fees.
The basic formulas are:
- Rectangular volume: length (ft) × width (ft) × thickness (ft) = cubic feet.
- Bags: project cubic feet ÷ stated cubic feet per bag, rounded up to a whole bag.
- Ready-mix quantity: cubic feet ÷ 27 = cubic yards.
Those calculations produce a material estimate, not a structural design or a guaranteed installed price. Uneven excavation, waste, reinforcement, finishing, access, local prices, and code requirements can change the final amount.
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Cement, concrete and concrete mix are not the same thing
Cement is the binding powder, usually Portland cement. Concrete is the finished composite made from cement, water and aggregates such as sand, gravel or crushed stone. A retail bag labeled concrete mix is a preblended product containing those ingredients. The QUIKRETE Concrete Mix 1101 specification identifies aggregates and Portland cement in the blend.
That distinction matters because a bag calculation uses the volume a particular concrete-mix product produces. It does not tell you how many pounds of pure cement to buy. The 80 lb QUIKRETE Concrete Mix listed by The Home Depot is model 110180, a finished concrete mix intended for uses including walls, sidewalks, curbs, steps, ramps and setting posts. Its listing specifies use for pours at least 2 inches thick. Confirm the current package, coverage and intended use before buying.
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What information to collect before calculating
Write down the dimensions of every section you will pour. Measure the actual planned thickness rather than relying on a visual estimate. Also record:
- Whether the shape is a rectangle, strip, circle, post hole or a combination.
- The product and bag size you intend to use, because yield is product-specific.
- Whether the project will be mixed from bags or supplied as ready-mix concrete.
- Your delivery location and access conditions if a truck may be needed.
- Any required base excavation, forms, reinforcement, demolition, pumping or finishing.
For a slab, check thickness at several points. A low spot or a sloped subgrade can increase volume even when the top surface appears rectangular.
Calculate volume in cubic feet
Convert thickness to feet
Use one unit system throughout the calculation. If length and width are in feet but thickness is in inches, divide the thickness by 12. For example, 4 inches is 4 ÷ 12 = 0.3333 feet.
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Multiply length in feet by width in feet by thickness in feet. QUIKRETE states the instruction plainly: First you need to determine the volume of the slab you wish to pour.
The statement appears in its concrete and cement mixes training document.
Example: A slab 10 feet long, 6 feet wide and 4 inches thick has a thickness of 4 ÷ 12 = 0.3333 feet. The calculation is 10 × 6 × 0.3333, or approximately 20 cubic feet. This is the volume before considering uneven subgrade or material loss.
Break irregular projects into sections
Divide an L-shaped patio, a walkway with a widening, or a set of steps into simple rectangles. Calculate each section separately and add the cubic-foot results. For a round pier or post hole, use the cylinder formula π × radius² × height, with all dimensions in feet, then add the results for multiple holes. Do not assume a calculator designed for common project shapes covers every geometry; enter only dimensions it actually supports.
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Convert volume into bags
Once you have cubic feet, divide by the yield printed for your exact product and bag size. Yield is the volume of fresh concrete produced by one bag; bag weight alone cannot provide that answer.
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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteBags required = total cubic feet ÷ cubic feet per bag. Round the result up to the next whole bag because a partial bag cannot complete the pour. The QUIKRETE Concrete Calculator also returns whole-bag estimates and warns that approximate yields do not account for an uneven subgrade or waste.
Published yields and the worked-example discrepancy
QUIKRETE’s training material gives these product-specific figures:
| Bag | Stated yield | 20-cubic-foot calculation at that yield | Training example count |
|---|---|---|---|
| 80 lb concrete mix | 0.60 cubic feet | 20 ÷ 0.60 = 33.33; strict whole-bag ceiling = 34 | 33 bags listed by QUIKRETE |
| 60 lb concrete mix | Approximately 0.45 cubic feet | 20 ÷ 0.45 = 44.44; strict whole-bag ceiling = 45 | 44 bags listed by QUIKRETE |
The source’s 10-by-6-foot, 4-inch example reports 33 80 lb bags and 44 60 lb bags, while the displayed yields produce fractional results that mathematically require 34 and 45 bags when rounded upward. Treat those counts as the manufacturer’s worked illustration, not a promise that every package produces exactly the nominal yield. Check the current bag label or technical sheet and the current calculator; for a purchase plan based strictly on the stated yields, use the ceiling of the division and confirm with the supplier.
Apply the method to your own slab
- Measure length and width in feet.
- Measure thickness in inches and divide by 12.
- Multiply the three dimensions to get cubic feet.
- Read the yield for your chosen bag from its current package or technical data.
- Divide volume by yield and round up.
- Check whether the calculator or supplier recommends an allowance for your particular ground conditions; do not assume a universal waste percentage.
For example, if your slab volume is 8.4 cubic feet and your package yield is 0.60 cubic feet, 8.4 ÷ 0.60 = 14 bags exactly. If the division produces 14.1, plan on 15 bags rather than 14.
Calculate cubic yards for ready-mix delivery
Ready-mix suppliers quote by cubic yard. Convert your cubic-foot total with the standard arithmetic conversion cubic yards = cubic feet ÷ 27. A 20-cubic-foot slab is 20 ÷ 27 = approximately 0.74 cubic yard.
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Give the supplier your dimensions, required thickness, project type, delivery address and access details. Ask whether the quote includes the concrete, delivery, minimum-load or short-load charges, waiting time, pumping and taxes. The supplier may recommend ordering quantities or a mix strength that cannot be selected from a volume calculation alone.
Estimate the cost of bagged concrete mix
The transparent DIY materials formula is:
Bagged-material cost = rounded-up bag count × current local price per bag.
Use the price for the exact bag size and product, not a price from a different yield. Keep tools, mixer rental, water, forms, base material, reinforcement, delivery of bags and disposal separate if you are building a complete budget. A bag count is a materials estimate; it is not an installed-project quote.
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Check the package before purchasing
- Confirm the bag weight and cubic-foot yield.
- Confirm the product is suitable for the planned thickness and use.
- Check the current instructions, mixing-water range, strength information and curing directions.
- Verify retailer stock and local price; a product listing is not evidence that every store has it.
- Buy from the same product specification when possible so yields and performance are consistent.
Estimate the cost of delivered ready-mix
For truck delivery, request a current local quote for the cubic yards you calculated. Compare the supplier’s complete order charge, not only the advertised per-yard number. Small pours can be disproportionately expensive because fixed delivery costs and minimum-load fees are spread over fewer yards.
Angi’s 2026 concrete-delivery guide reports a typical national delivered range of $110–$150 per cubic yard. It also reports $40–$60 per cubic yard for short-load fees on less-than-full-truck orders. Those are national guide figures, not a supplier offer; ask how your local company calculates the fee and whether it is a flat charge or multiplied by the short quantity.
The same Angi guide reports an average delivery-project estimate of $5,419, with a typical range of $2,111–$8,727 across 436 verified projects, including materials and labor. Those figures describe delivery projects in that guide, not a universal price for your slab, walkway or post holes.
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Compare bagged mix with ready-mix delivery
| Consideration | Bagged concrete mix | Ready-mix delivery |
|---|---|---|
| Quantity method | Divide cubic feet by the package yield and round up to whole bags. | Convert cubic feet to cubic yards and obtain a supplier quote. |
| Price basis | Local price per bag multiplied by the rounded-up count. | Quoted yards plus delivery, short-load, pump, waiting and other listed fees. |
| Work involved | You handle, measure, mix and place each bag; labor and mixer capacity matter. | The truck supplies mixed concrete, but access, placement and finishing still require planning and labor. |
| Best fit | Small pours, repairs and projects where bag handling is practical. | Volumes that would make hand mixing slow, or pours requiring a continuous supply. |
| Decision limit | No universal bag-count threshold is established; compare your local total. | No universal break-even volume is established; minimums and fees vary by supplier. |
Compare the two options using the same required volume and the same scope of work. Include material price, delivery or order fees, handling, available labor, placement time and suitability for the intended use. A low per-yard number can lose its advantage when a short-load fee applies, while a bag estimate can understate the job if mixing capacity or labor is limited.
Budget the costs that volume formulas do not include
Concrete quantity is only one part of a home-project budget. Angi’s 2026 slab-cost guide identifies thickness, site preparation and grading, reinforcement, finish and local concrete prices as major cost factors. It reports a broad installed slab benchmark of $4–$8 per square foot; this is an installed range, not a DIY materials price. Complicated or thicker foundations can cost more.
List these items separately so your estimate stays transparent:
- Excavation, grading and removal of unsuitable soil.
- Compacted base or gravel beneath the pour.
- Forms, stakes and edge restraints.
- Wire mesh, rebar or other reinforcement specified for the project.
- Vapor barrier or other site materials where required.
- Concrete delivery, pump rental, minimum-load and short-load fees.
- Mixing equipment, wheelbarrows, tools and personal protective equipment.
- Placement, screeding, edging, jointing, finishing and curing labor.
- Demolition, haul-away, permits and inspection fees.
Do not combine the $4–$8 installed benchmark with a bag-only calculation. They measure different scopes.
Use a calculator without losing control of the estimate
- Open the QUIKRETE calculator and select the project type it supports, such as a slab, sidewalk, step or post setting.
- Enter dimensions in the units requested. Convert inches to feet when doing an independent check.
- Record the displayed bag size, yield assumptions and whole-bag result.
- Repeat the volume calculation manually so a misplaced decimal or wrong thickness is visible.
- Compare the result with the current package or technical sheet for the product you will actually buy.
- Ask the supplier or contractor about subgrade conditions and any quantity allowance rather than inserting an unsupported percentage.
The calculator is an estimating aid. Its approximate yields do not include uneven subgrade or waste, and its supported project types do not establish a design for every shape or structural application.
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Using inches without converting
Multiplying 10 feet × 6 feet × 4 inches as though all three values were feet overstates the volume by a factor of 12. Divide every inch measurement by 12 before multiplying.
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Using bag weight instead of yield
An 80 lb bag is not automatically a fixed fraction of a cubic yard. Use the printed cubic-foot yield for that product and size.
Rounding down
Rounding 14.1 bags to 14 leaves less than the calculated volume. Round the mathematical result up to the next whole bag, then confirm the package’s current yield and the site’s conditions.
Forgetting the base and actual excavation
A nominal 4-inch slab over a subgrade that dips or has been over-excavated can require more concrete. Measure the formed space that will actually be filled, not only the drawing dimensions.
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Confusing cubic feet and cubic yards
One cubic yard contains 27 cubic feet. A ready-mix quote expressed in yards cannot be compared directly with a bag yield expressed in cubic feet until you convert the units.
Comparing an incomplete truck price with a bag total
Add delivery, short-load, pumping and other quoted fees to ready-mix. For bags, include the complete rounded-up count and the labor or equipment needed to mix them.
When a volume-and-cost estimate is not enough
A walkway, small pad or post-setting job can often begin with a quantity estimate, but a load-bearing foundation, structural slab or project controlled by local building code needs more than cubic-foot arithmetic. Soil, climate, intended loads, reinforcement, required compressive strength, joint layout, permits and inspection requirements may determine the design and mix. Angi notes that use, local soil, climate and permitting or load-bearing requirements affect slab costs. Obtain the applicable plans, code requirements or professional advice before ordering concrete for structural work.
For any project, plan safe handling. Concrete mix is heavy, dust from dry cementitious material can irritate the eyes and lungs, and wet concrete can burn skin. Follow the product’s current safety instructions, wear appropriate eye and skin protection, use safe lifting practices and arrange enough people and equipment to place the mix before it begins setting.
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A practical worksheet
| Item | Your value |
|---|---|
| Section length (ft) | Enter measurement |
| Section width or equivalent dimension (ft) | Enter measurement |
| Thickness (in ÷ 12 = ft) | Enter converted thickness |
| Section volume (cu ft) | Length × width × thickness |
| Total volume (cu ft) | Add all sections |
| Bag yield (cu ft per bag) | Copy from the current package or technical sheet |
| Rounded-up bag count | Total volume ÷ yield, rounded up |
| Bagged material cost | Bag count × current local price |
| Total volume (cu yd) | Total cubic feet ÷ 27 |
| Ready-mix order cost | Supplier’s quote plus every specified fee |
Keep the worksheet with the supplier quote and the product label. If dimensions, product, delivery location or site conditions change, update the calculation rather than carrying the old total forward.
Final decision rule
Find the real volume first, use the exact product yield, round bags up, or convert to cubic yards and request a complete local ready-mix quote. Then add preparation, reinforcement, equipment, labor and finishing as separate lines. National figures such as Angi’s 2026 ranges can frame a budget, but your current local bag price or supplier quote determines what the project will actually cost.
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