Concrete Calculator
Estimate rectangular slab volume, ready-mix allowance, and whole bags from package-specific yield.
Content updated August 5, 2026
Rectangular-volume estimate only. Verify dimensions, mix, bag yield, structural design, site conditions, supplier batch size, and safe handling before buying or placing concrete.
Estimate the concrete volume for a rectangular slab from length, width, and thickness. Length and width can use one convenient unit while thickness uses another, such as feet for plan dimensions and inches for slab depth.
The tool separates the net plan volume from an editable contingency allowance, then reports cubic yards, cubic metres, cubic feet, and a whole-bag estimate. Bag yield stays user-entered because mix, bag size, and manufacturer determine it.
The Concrete formula
Net volume = length × width × thickness · order volume = net volume × (1 + allowance% ÷ 100) · bags = ceiling(order ft³ ÷ yield ft³/bag)All dimensions are converted before multiplication · 1 yd³ = 27 ft³ exactly · entered bag yield must come from the applicable product/package.
Worked example
A 12 ft × 10 ft slab at 4 in thick has a net volume of 40 ft³, or about 1.481 yd³. With 10% allowance the order volume is 44 ft³, about 1.630 yd³; at 0.6 ft³ per bag, 73.333 bags rounds up to 74.
Assumptions, rounding, and limitations
Assumptions
- The placement is a rectangular prism with uniform finished length, width, and thickness.
- The entered contingency percentage is applied directly to net plan volume.
- Bag yield is mixed-concrete volume per bag copied for the exact product, not inferred from bag weight.
- International-foot and metric conversions use NIST exact relationships.
Rounding: Volume math retains floating-point precision. Displayed volumes show up to three decimal places; the purchase-oriented bag result always rounds the exact quotient upward to a whole bag.
Limitations
- Does not model footings, columns, tubes, stairs, slopes, voids, reinforcement displacement, uneven subgrade, form deflection, excavation tolerance, spillage, pumping loss, or supplier batch increments.
- Does not select a concrete mix, strength, aggregate, slump, exposure class, curing method, structural design, or safe placement procedure.
- The result is a planning estimate and should be checked against drawings, field measurements, packaging, the concrete producer, and applicable project requirements.
Sources
- CIP 31 — Ordering Ready Mixed Concrete — National Ready Mixed Concrete Association
- NIST Guide to the SI, Appendix B: Conversion Factors — National Institute of Standards and Technology
Plan volume, then a visible contingency
A rectangular slab volume is length × width × thickness after all three dimensions are converted to compatible units. NIST's exact international-foot relationships make the cubic-foot, cubic-yard, and cubic-metre conversions reproducible.
NRMCA states that ready-mix concrete is sold by volume and that an order should generally be 4%–10% above an estimate from plan dimensions to account for contingencies. The default is 10%, but the input is exposed so you can use a project-specific decision.
Bag count depends on the package yield
Dry-mix bags are not interchangeable volume units. The mixed yield depends on the exact product and bag size, so copy the cubic-foot yield from the packaging or supplier information instead of relying on a universal weight-to-volume assumption.
The exact volume quotient is shown for transparency, while the purchase count rounds up to the next whole bag. Supplier minimums, batch size, partial-bag handling, and leftover policy remain outside the calculation.
Frequently asked questions
▶How do I calculate concrete for a slab?
Convert length, width, and thickness to compatible units, multiply them for net volume, then apply the contingency allowance you choose.
▶Why does the default include 10% extra?
NRMCA recommends ordering 4%–10% more than plan-dimension estimates for contingencies. Ten percent is the editable default, not a rule for every job.
▶How many cubic feet are in a cubic yard?
There are exactly 27 cubic feet in one cubic yard because one yard equals three feet and volume cubes that relationship.
▶How is the number of concrete bags calculated?
The order volume in cubic feet is divided by your entered per-bag yield, then rounded up to a whole bag.
▶Will the estimate match the final amount used?
Not necessarily. Actual excavation, form dimensions, uneven subgrade, spillage, over-excavation, placement, supplier batching, and field changes can alter the amount.
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Open calculator →Disclaimer: Concrete Calculator results are estimates for general information and education only, and are not financial, tax, legal or medical advice. Verify important decisions with a qualified professional.