How Much Concrete Do I Need?
For a rectangular slab, multiply length × width × thickness after converting thickness to feet, then divide cubic feet by 27 to get cubic yards. Use the Concrete Calculator when you want the conversion handled from your own dimensions.
How to calculate concrete yards
To calculate concrete yards, start with dimensions in compatible units. If length and width are in feet and thickness is in inches, divide thickness by 12 before multiplying, calculate cubic feet, then divide by 27.
cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
cubic yards = cubic feet ÷ 27
The cubic-yard conversion is exact. The amount you finally order can still depend on project geometry, grade variation, forms, spillage, supplier minimums and the extra-material allowance you intentionally choose.
Example: 10 ft × 10 ft slab at 4 inches thick
A 4-inch thickness is 4 ÷ 12 = 0.3333 ft. The slab volume is therefore approximately:
10 × 10 × 0.3333 = 33.33 ft³
Convert cubic feet to cubic yards:
33.33 ÷ 27 ≈ 1.23 yd³
That 1.23 yd³ is the geometric volume before any project-specific extra allowance. The calculator keeps the base quantity and the extra percentage separate so you can see exactly what changed the final planning quantity.
For driveways, footings and irregular projects, calculate sections separately
A project does not need to be one perfect rectangle. Divide it into simple sections that can be measured reliably, calculate the volume of each section and add the results. A driveway with a thicker apron, a footing plus slab, or several pads is easier to audit when each thickness is calculated separately.
Avoid hiding different thicknesses inside one rough average when the sections can be measured independently. The arithmetic becomes easier to check and the resulting concrete quantity is easier to compare with drawings or a supplier quote.
Ready-mix yards and bagged concrete are different calculations
Ready-mix quantity is commonly planned in cubic yards. Bagged products are purchased by package, and the mixed yield per bag can vary by product and package size. Do not assume one universal number of bags per cubic yard.
If you are using bagged concrete, enter the exact mixed yield printed on the product or manufacturer information in the Concrete Calculator. The tool can then convert the project volume into a whole-bag quantity.
Quantity first, project cost second
The amount of concrete is a geometry question; project cost depends on local material pricing, labor and fixed extras. Keep those stages separate. After calculating cubic yards, continue to the Concrete Cost Calculator to apply the rates and extras that actually match your project.
For a driveway-specific budget, use the Concrete Driveway Cost Calculator. Neither cost page inserts a hidden national installed-price assumption.
Concrete quantity checklist
- Measure the dimensions of the concrete section.
- Convert thickness to feet before multiplying.
- Calculate cubic feet, then divide by 27 for cubic yards.
- Calculate sections with different thicknesses separately.
- Keep any extra-material percentage visible rather than hiding it in the dimensions.
- For bagged mix, use the exact product yield instead of a generic bags-per-yard assumption.
Why concrete bag count depends on bag yield
Use the same illustrative 10 × 10 ft slab at 4 inches from the example above. Its volume is 100 × (4 ÷ 12) = about 33.33 cubic feet. If the specific bagged mix yields 0.60 cubic feet per bag, you need ceiling(33.33 ÷ 0.60) = 56 bags before any extra concrete allowance. A different product yielding 0.45 cubic feet per bag would require ceiling(33.33 ÷ 0.45) = 75 bags. These yields are arithmetic examples, not product recommendations.
For significant pours, supplier delivery, batching and appropriate reinforcement may be more important than a simple bag count. Confirm the product's yield and mixing instructions, and do not confuse the geometric slab volume with a structural or code-compliant slab specification. The Concrete Calculator allows the actual yield per bag to be entered.
Buying decision: bagged concrete versus ready-mix on the same measured pour
Start from one explicitly hypothetical 10 × 10 ft slab at 4 inches of depth. Its geometric volume is 100 × (4 ÷ 12) = 33.33 cubic ft. For this comparison only, choose a 5% additional quantity allowance. The planned volume becomes 35 cubic ft, or about 1.296 cubic yd. That allowance is not a structural specification or a universal waste recommendation; use the actual project plan and supplier requirements.
| Purchase method | Quantity calculation | Illustrative cost |
|---|---|---|
| Bagged mix, hypothetical yield 0.60 cubic ft per bag | ceil(35 ÷ 0.60) = 59 bags | 59 × $6.40 = $377.60 |
| Ready mix with hypothetical 0.1 cubic-yard order increments | Round 1.296 cubic yd up to 1.3 cubic yd | (1.3 × $180) + $160 delivery/short-load = $394.00 |
On the entered purchase lines, the two examples differ by $16.40. This is not a real supplier comparison: unloading, mixing, access, labor, equipment, delivery windows, short-load thresholds and the exact mix requirements may reverse the result. A bag quote is incomplete if you ignore the cost and practical feasibility of mixing 59 bags; a truck quote is incomplete if it excludes discharge or access charges.
What to verify before placing an order
Confirm the volume and any reinforcement, mix, finishing or structural specification with the relevant professional. Then get the exact bag yield from the manufacturer's label and the supplier's actual rounding and fees for ready mix. Use the Concrete Calculator for volume and bags, and the Concrete Cost Calculator for a quote built from the supplier's real pricing. The calculation cannot certify that a slab thickness or concrete mix is suitable for a specific use.