The quick formulaLength × width × thickness gives cubic feet. Divide by 27 for cubic yards, then add a project-specific allowance.

How the concrete estimate works

A slab is a rectangular volume. Multiply length in feet by width in feet, then multiply by thickness converted from inches to feet. The calculator adds your selected allowance before converting the result to cubic yards.

Bag counts use typical approximate yields of 0.30 cubic feet for a 40 lb bag, 0.45 for a 60 lb bag, and 0.60 for an 80 lb bag. Product yield can vary, so the bag label is the final reference.

When to order ready-mix

Bagged mix can make sense for a small repair or a few post holes. Larger pads quickly require dozens or hundreds of bags, plus mixing time and water. Ask local suppliers about minimum orders, short-load charges, access, and the amount they round to for delivery.

Confirm finished thickness at several points.
Include thickened edges or footings as separate volumes.
Keep forms, base, reinforcement, and finishing labor outside this material-only estimate.

Choose an allowance intentionally

A flat, compacted base and accurate forms need less cushion than an irregular excavation. Ten percent is a common planning value, but the correct number depends on subgrade, form accuracy, spill, and the supplier’s order increments.

Worked example: 12 × 10 ft slab at 4 in

Convert thickness: 4 in ÷ 12 = 0.333 ft.
Net volume: 12 × 10 × 0.333 = 40.0 ft³.
Convert: 40.0 ÷ 27 = 1.48 yd³ before allowance.
At an example 10% allowance: 1.48 × 1.10 = 1.63 yd³.

Result and limit. 1.48 yd³ is the geometric audit number and 1.63 yd³ is the example planning quantity. Bag count must use the yield printed on the selected product; the calculator defaults are editable planning values, not universal label claims.

Decision rule

Keep net volume, allowance, supplier increment, and bag yield separate. Use ready-mix or bags only after confirming placement size, access, mixing capacity, product label, and the supplier's delivery terms.

Primary and official references

NIST: SI Units — Volume
Volume measurement and unit-conversion context; it does not establish a bag yield or order quantity.
OSHA: Concrete and masonry construction
Construction-safety context for covered work; it does not size or price a slab.

BuildMath Lab provides planning estimates, not engineering, code approval, or a supplier quote. Confirm the final order with the product manufacturer, supplier, plans, and qualified professional.