For a rectangular slab, strip footing, or driveway, calculate finished-concrete volume as length × width × thickness, with all dimensions in the same unit. Convert cubic feet to cubic yards by dividing by 27, add an allowance for uneven excavation and job variation, then divide by the yield printed on your selected premixed-concrete bag and round up. If you order ready-mix, order the adjusted result in cubic yards. The calculation estimates material quantity; it does not determine the structural thickness, reinforcement, strength, drainage, or footing dimensions required for your project.
Start with the right material: cement is not the same as concrete
Homeowners commonly use “cement” to mean the material poured into a slab or driveway. Technically, cement is the binder in concrete. Concrete also contains aggregate (sand and stone) and water. The American Cement Association says cement typically makes up only 10 to 15 percent of concrete by volume. A bag labeled as premixed concrete is therefore sold by the volume of finished concrete it can produce, not by the volume of cement powder alone.
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For quantity planning, calculate the finished concrete required. Do not try to infer a universal number of cement-only bags from the dimensions. Use the yield printed on the exact premixed product you intend to buy, because yields vary by formulation and package size.
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For a uniform rectangular pour, use:
Volume (ft³) = length (ft) × width (ft) × thickness (ft)
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Thickness is usually given in inches, so convert it before multiplying:
- 4 inches =
4 ÷ 12 = 0.3333 ft - 6 inches =
6 ÷ 12 = 0.5 ft - 8 inches =
8 ÷ 12 = 0.6667 ft
Keep length, width, and thickness in feet for a result in cubic feet. One cubic yard contains 27 cubic feet:
Volume (yd³) = Volume (ft³) ÷ 27
Ready-mix suppliers quote concrete by cubic yard. Premixed bags state a yield in cubic feet per bag.
A reliable calculation process
- Measure the pour. Record the actual length and width. For a footing, measure each section whose width or depth changes.
- Convert every dimension to one unit. Convert inches to feet before multiplying; do not mix inches and feet in the same equation.
- Calculate the theoretical volume. Multiply length × width × thickness for each rectangular section.
- Convert to cubic yards when ordering ready-mix. Divide the cubic-foot total by 27.
- Add an allowance. Increase the theoretical volume to account for job variation, uneven subgrade, over-excavation, and spillage. The Federal Highway Administration notes that “Slightly more material is needed to compensate for job variations.”
- Apply the product yield. For bags, divide the allowance-adjusted cubic feet by the package yield in cubic feet per bag.
- Round up to a whole bag or practical delivery quantity. You cannot order a fraction of a bag, and a short pour can create a cold joint or an unfinished edge.
The allowance-adjusted formulas are:
V_order = V × (1 + allowance)
Bags = ceiling(V_order ÷ bag yield in ft³)
Choose the allowance based on how accurately the excavation is formed and how much the subgrade varies. A 10 percent allowance is shown in the example below; it is not a universal requirement for every site.
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Bag yields and what they mean
The following approximate yields are reported in the cited product reference. They are not universal constants. Read the yield on the package you will actually use, and verify the required water amount and placement instructions.
| Bag size | Approximate yield per bag | Approximate bags per cubic yard at that yield |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
| 90 lb | 0.675 ft³ | 40 |
The “bags per cubic yard” column is simply 27 divided by the stated yield, rounded to a whole bag for planning. It does not replace the package label. Different mixes, aggregate sizes, or revised packaging can change the yield.
Worked example: 10 ft × 10 ft slab at 4 inches
1. Find the theoretical volume
Convert the thickness: 4 ÷ 12 = 0.3333 ft. Then:
10 × 10 × 0.3333 = 33.33 ft³
In cubic yards, that is 33.33 ÷ 27 = 1.23 yd³.
2. Convert to bags
Using the cited 80 lb yield of 0.60 ft³ per bag:
33.33 ÷ 0.60 = 55.6 bags
Round up to 56 bags before adding an allowance. If the slab excavation is uneven or access makes a second trip difficult, apply your chosen allowance first, then divide by 0.60 and round up again.
Worked example: 12 ft × 20 ft slab at 6 inches with 10 percent extra
1. Calculate the base quantity
Six inches is 6 ÷ 12 = 0.5 ft. The base volume is:
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12 × 20 × 0.5 = 120 ft³
120 ÷ 27 = 4.44 yd³
2. Add the stated allowance
With 10 percent extra:
120 × 1.10 = 132 ft³
132 ÷ 27 = 4.89 yd³
3. Convert to 60 lb bags
At the cited 60 lb yield of 0.45 ft³:
132 ÷ 0.45 = 293.3 bags
Round up to 294 bags. This illustrates why a pour of several cubic yards is usually evaluated for ready-mix delivery rather than mixed bag by bag.
Driveway quantity: calculate volume, then make structural decisions separately
For a 30 ft × 10 ft driveway at 4 inches:
30 × 10 × (4 ÷ 12) = 100 ft³
100 ÷ 27 = 3.70 yd³
That is the theoretical volume before an allowance. Apply the same allowance and product-yield steps used for a slab. The arithmetic does not tell you whether a driveway should be 4, 5, or 6 inches thick, what reinforcement is needed, or what strength class is appropriate. Those decisions depend on vehicle loading, soil, climate, drainage, joints, local practice, and applicable code. Obtain the design or permit guidance required in your jurisdiction before ordering.
Footings with changing dimensions
A straight, uniform strip footing can use the same length × width × depth formula. Many footings, however, have steps, thickened portions, corners, pads, or grade changes. Break the geometry into separate rectangular sections.
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Section-by-section method
- Draw or list each section with a constant width and depth.
- Convert each depth to feet.
- Calculate each section in cubic feet.
- Add all section volumes.
- Convert the total to cubic yards or apply the bag yield.
- Add an allowance after summing the sections, unless you have a specific reason to use different allowances for different pours.
For example, if a footing has a 40 ft × 1 ft × 0.667 ft stem section and a 10 ft × 2 ft × 0.833 ft thickened section, calculate both products separately and add them. Do not average the depths; averaging can hide the extra concrete in the thickened area.
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Footing quantity is not footing design. Width, depth below grade, reinforcement, bearing conditions, frost protection, drainage, and connection to walls or columns may be structural or code-controlled. A building official, engineer, or applicable construction standard may be required to establish them.
Choosing bags versus ready-mix
| Consideration | Premixed bags | Ready-mix delivery |
|---|---|---|
| Quantity basis | Package yield in ft³ per bag | Supplier quote in yd³ |
| Labor | Every bag must be moved, measured with water, and mixed | Truck supplies a continuous load, but placement and finishing must be ready |
| Site access | Useful where a truck cannot reach the pour | Requires truck access, a discharge point, and a plan for traffic and washout |
| Cost comparison | Compare total bags, water, mixing equipment, labor, and waste | Compare delivered cubic-yard price, minimum load, delivery charge, and any waiting or short-load fees |
| Mix selection | Check the label for intended strength and exposure | Specify strength, aggregate size, slump, and other requirements with the supplier |
For either method, confirm the actual quantity, product suitability, and site preparation before the pour. A lower material price can be offset by mixing labor, equipment, delivery constraints, or an unsuitable mix.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Strength, exposure, and the 3,000 psi label
Quantity and performance are separate questions. A 3,000 psi designation generally refers to the concrete’s specified 28-day compressive strength, not to the amount of material needed. The American Cement Association notes that cement specifications vary by jurisdiction. The required strength, air entrainment, water-cement ratio, aggregate, and exposure class may therefore differ between locations and applications.
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Use a product whose label or supplier ticket matches the project requirements. Do not increase or decrease the amount of concrete to compensate for a strength specification; change the approved mix or design instead.
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Common mistakes and how to correct them
Using inches directly in the formula
Multiplying feet by inches produces the wrong units. Convert every thickness and depth to feet first, or convert all dimensions to inches and divide the resulting cubic inches by 1,728. The feet-and-cubic-yards method is usually less error-prone.
Confusing cubic feet with cubic yards
A result of 120 ft³ is not 120 yd³. Divide by 27: 120 ft³ equals 4.44 yd³ before allowance.
Relying on a nominal slab size
Excavation can be wider or deeper than the drawing, and subgrade can be irregular. Measure the actual formed area and include an allowance rather than ordering only the theoretical rectangle.
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Bag weights do not guarantee a universal yield. A 60 lb product in one formulation may not yield the same volume as another. Read the package and calculate with its stated ft³-per-bag value.
Ordering exactly the calculated fraction
Concrete volume is rounded up to whole bags, and ready-mix orders should account for the supplier’s practical delivery increments. Running short after placement starts is more disruptive than having a small remainder, provided the unused material can be handled legally and safely.
Calling a footing or driveway calculation a design
The volume equation cannot select reinforcement, joints, subgrade preparation, drainage, frost depth, or strength. Treat those as separate design and compliance decisions.
Before you place an order
- Confirm the measured length, width, and thickness or depth for every section.
- Verify that all dimensions were converted to the same unit.
- Record the theoretical total in cubic feet and cubic yards.
- Choose and document the allowance used for site variation.
- Check the exact bag yield or obtain a supplier ticket stating the ready-mix volume.
- Confirm mix strength, aggregate size, slump, and exposure requirements with the label, supplier, or project designer.
- Check minimum loads, delivery charges, truck access, discharge distance, crew, forms, reinforcement, tools, and finishing capacity.
- Arrange a plan for washout and leftover material.
These checks prevent a quantity calculation from becoming a scheduling or quality failure. The volume itself is simple; accurate measurement, a verified yield, and a suitable mix determine whether the order works in practice.
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