Concrete Yardage Calculator: Slab Formulas, Tables, and Order Checks
A concrete yardage calculator converts a project’s dimensions into the volume of concrete it needs. For a rectangular slab measured in feet, the formula is length × width × thickness in feet ÷ 27. If thickness is in inches, use length × width × thickness ÷ 324 instead.
Below, you will find worked slab and footing examples, a quick volume table, bag conversions, and checks before ordering. The calculations estimate material volume. They do not determine structural thickness, reinforcement, foundation design, or the right mix for a project.
Concrete yardage calculator: the core formulas
One cubic yard contains 27 cubic feet. Meanwhile, one foot contains 12 inches. Combining those conversions gives the divisor 324 when length and width use feet and thickness uses inches.
- Rectangular volume in cubic yards = length (ft) × width (ft) × thickness (in) ÷ 324.
- Rectangular volume in cubic yards = length (ft) × width (ft) × thickness (ft) ÷ 27.
- Metric volume in cubic meters = length (m) × width (m) × thickness (m).
Keep all dimensions tied to the same portion of the work. For example, do not use the full building width for one line and the width of only one slab bay for another. Sketching each section before calculation helps avoid that error.
For an independent product check, QUIKRETE’s concrete calculator includes slab and other product calculations. Its page also notes that yields are approximate and do not cover every uneven thickness or waste condition.
Worked example: a 20-by-12-foot slab
Assume a fictional slab measures 20 feet long and 12 feet wide, with a specified thickness of 4 inches. First, calculate the area: 20 × 12 = 240 square feet.
Next, calculate volume: 240 × 4 ÷ 324 = 2.963 cubic yards. This is the geometric volume for those dimensions before any project-specific allowance.
If the estimator chooses a 7% planning allowance for this example, the adjusted quantity is 2.963 × 1.07 = 3.170 cubic yards. That percentage is an assumption, not a universal rule. Confirm the appropriate allowance and order increment with the supplier and project team.
Keep the measured volume, allowance, and final order separate. A supplier-rounded order of 3.25 cubic yards should not replace the original calculation in your estimate.
Quick slab volume table
The table shows theoretical cubic yards for selected areas and thicknesses. It excludes waste, thickened edges, slopes, and other separate features. Also, the listed thicknesses illustrate the math rather than recommend a design.
| Area | 4 inches thick | 6 inches thick | 8 inches thick |
|---|---|---|---|
| 100 sq ft | 1.23 cu yd | 1.85 cu yd | 2.47 cu yd |
| 200 sq ft | 2.47 cu yd | 3.70 cu yd | 4.94 cu yd |
| 300 sq ft | 3.70 cu yd | 5.56 cu yd | 7.41 cu yd |
| 500 sq ft | 6.17 cu yd | 9.26 cu yd | 12.35 cu yd |
| 1,000 sq ft | 12.35 cu yd | 18.52 cu yd | 24.69 cu yd |
Use the table as a reasonableness check. For instance, a 200-square-foot slab at 4 inches should not produce 20 cubic yards. If your result differs sharply, review the unit conversion before changing the allowance.
How to calculate footings
For a rectangular footing, multiply length, width, and depth using consistent units. If the length is in feet and the other dimensions are in inches, divide by 144 to get cubic feet. Then divide by 27 for cubic yards.
Consider a fictional footing 40 feet long, 18 inches wide, and 12 inches deep. Convert the width and depth first: 18 inches = 1.5 feet, and 12 inches = 1 foot.
The volume is 40 × 1.5 × 1 = 60 cubic feet. Next, 60 ÷ 27 = 2.222 cubic yards before allowance. Use the actual specified geometry, including steps or changes in section.
At corners and intersections, avoid counting the same space twice. Divide the layout into non-overlapping shapes or deduct overlap explicitly. Anomoz’s construction takeoff guide explains the value of a traceable quantity breakdown.
How to calculate round piers and columns
A cylinder’s volume equals π × radius² × height. Use the radius, which is half the diameter. If all dimensions use feet, divide the cubic-foot result by 27 for cubic yards.
For example, a fictional round pier has a 12-inch diameter and a 4-foot height. The radius is 0.5 feet, so its volume is π × 0.5² × 4 = about 3.142 cubic feet.
For six identical piers, multiply by six: about 18.850 cubic feet. Then divide by 27 to get about 0.698 cubic yards. Add any separate base or enlarged section using its own geometry.
For a post hole, the hole’s full cylinder is not always the concrete volume. A post or other object may displace part of the space. Use the actual geometry and the installation requirements for the selected product.
Thickened edges and stepped sections
When a slab has deeper edges, calculate the ordinary slab volume first. Next, add only the extra depth below that slab section. Otherwise, the edge calculation can count some concrete twice.
For a fictional example, suppose a 30-foot edge strip is 1 foot wide. The slab is 4 inches thick, while the total edge depth is 12 inches. The extra depth is 8 inches, not 12 inches.
The added volume is 30 × 1 × (8 ÷ 12) = 20 cubic feet, or 0.741 cubic yards. This example explains quantity accounting only; the actual edge dimensions must come from the project design.
Similarly, calculate ramps, steps, and pads as separate sections. If thickness varies linearly, an average thickness may represent that section. However, complex profiles need a more detailed breakdown.
Convert concrete volume into bags
Use the stated yield of the exact bagged product. Bag weight alone does not tell you the finished volume, especially when comparing different mixes or high-yield products.
The formula is bags = required cubic feet ÷ cubic feet yielded per bag. Round up to whole bags only after calculating the required volume and the chosen allowance.
For a fictional product with a listed yield of 0.60 cubic feet per bag, a 12-cubic-foot requirement needs 12 ÷ 0.60 = 20 bags before allowance. If a different product yields 0.45 cubic feet, the same volume needs 26.667 bags, so at least 27 whole bags before any additional allowance.
Sakrete’s product calculators provide another manufacturer-based check. Select the relevant project and product rather than applying one bag yield to every material sold as concrete.
Why measured dimensions matter more than extra decimals
A calculator can return six decimal places from uncertain measurements. That does not make the estimate precise. Small thickness differences can have a large effect across a wide area.
For example, an extra half inch across 600 square feet adds 600 × 0.5 ÷ 324 = 0.926 cubic yards. This is a volume sensitivity check, not a recommendation to change the design.
Therefore, review the prepared dimensions and relevant levels before confirming the order. Uneven excavation, overbreak, and unplanned sections can all affect the actual volume. Discuss uncertain conditions with the responsible team.
A simple estimate worksheet
Use one row per distinct section. Record the drawing or field reference so another person can reproduce the result.
- Section name and reference.
- Length and width, with units.
- Specified thickness or depth, with units.
- Calculated volume before allowance.
- Any overlap or displacement adjustment.
- Chosen allowance and its reason.
- Supplier order unit and rounded quantity.
- Revision date and reviewer.
Keep price outside the volume formula. Multiply the agreed order quantity by the quoted unit rate, then add delivery, minimum-load charges, or other quoted items separately. That makes the cost easier to explain.
Anomoz’s construction estimate template can help structure the cost side. Meanwhile, concrete contractor software can connect quantities with crew and pour planning when the business needs a shared workflow.
Metric example and conversion checks
Suppose a fictional pad measures 6 meters by 3 meters with a specified thickness of 100 millimeters. Convert the thickness to meters first: 100 ÷ 1,000 = 0.1 meter. Then calculate 6 × 3 × 0.1 = 1.8 cubic meters.
If the supplier quotes cubic meters, keep the order in that unit. If you need cubic yards for comparison, multiply cubic meters by about 1.308. In this example, 1.8 cubic meters equals about 2.354 cubic yards before allowance.
Do not multiply a cubic-meter result by a length-only conversion. Volume conversions involve three dimensions. Likewise, record whether a drawing uses millimeters or centimeters before entering the numbers in a worksheet.
Separate material quantity from the project quote
Suppose the reviewed order quantity is 3.25 cubic yards and a fictional supplier quotes $180 per cubic yard. The concrete line would equal $585 before other charges. If the quote includes a separate $90 delivery charge, the subtotal becomes $675 before tax or any other agreed items.
This example uses made-up rates solely to show the calculation. Actual pricing depends on location, product, quantity, scheduling, and supplier terms. Therefore, compare written quotes on the same mix and delivery basis.
Bagged concrete has a different cost structure. Include the exact product quantity, transport, handling, and labor needed to mix and place it. A lower material total does not necessarily make bagged material the best method for a larger placement.
For repeat jobs, retain the original estimate beside the final order and actual tickets. Note whether the variance came from changed dimensions, a separate feature, ordering increments, or site conditions. That record is more useful than applying a larger allowance to every future job.
Checks before placing the order
Confirm the latest dimensions and revision first. Next, verify the mix specification, delivery location, access, timing, order increment, and any supplier minimums. A correct yardage calculation does not choose those details.
Also confirm whether the quantity covers one placement or several. Combining separate stages into a single total can hide different delivery needs. Keep the order breakdown aligned with the actual work plan.
Finally, record the supplier confirmation and actual delivery tickets. Later, compare estimated and delivered quantities with the completed geometry. Use the difference to improve future estimates rather than treating every variance as waste.
Common concrete calculator mistakes
The largest unit error is entering 4 inches as 4 feet. Another is dividing by 27 after using inches without first converting them. Both can produce a result that looks plausible on a screen but is far too large.
People also confuse square yards with cubic yards. Square yards measure area; cubic yards measure volume. A slab needs both area and thickness before you can calculate volume.
Finally, avoid rounding every small section upward before adding the total. Keep useful precision through the calculation, apply the allowance clearly, and round the final order to the supplier’s increment.
Frequently Asked Questions
How many cubic yards does a 10-by-10 slab need at 4 inches?
The theoretical volume is 10 × 10 × 4 ÷ 324 = 1.235 cubic yards. This excludes any project-specific allowance and separate deeper sections.
Can a concrete calculator determine slab thickness?
No. It uses an input thickness to calculate volume. The appropriate thickness and structural details come from the project’s design and requirements.
Should I always add the same waste percentage?
No. Conditions, geometry, product, and supplier practice affect the allowance. Keep it explicit and confirm it for the particular job.
Calculate the volume, then confirm the order
A reliable concrete yardage calculator starts with consistent units and a clear section breakdown. Calculate the geometric volume, show adjustments separately, and verify the selected product and delivery details. That gives you a quantity another person can check and a better basis for ordering.
