Concrete Calculator
Size a rectangular concrete pour by finished length, width, and thickness. See exact volume, order volume, and an approximate 80-pound bag equivalent.
What does the concrete calculator calculate?
For a rectangular pour, multiply length by width by thickness converted from inches to feet, then divide cubic feet by 27. Add 5%–10% for subgrade variation, spillage, and forms. Order ready-mix by cubic yard or use the bag calculator with the manufacturer’s stated yield.
Concrete volume for a rectangular pour
The dimensions produce placed cubic feet after thickness is converted to feet. Dividing by 27 gives cubic yards, and the selected allowance is added to reduce the chance of a short pour.
How to measure for an accurate estimate
- Measure inside the forms rather than using the excavation’s outside dimensions.
- Check thickness at several points because an uneven subgrade increases real concrete volume.
- Calculate thickened edges, grade beams, steps, and footings as separate shapes.
- Use finished dimensions from approved plans for structural work instead of estimating by eye.
Small patio pour
A patio is 12 ft long, 10 ft wide, and 4 inches thick with 10% extra.
12 × 10 × (4 ÷ 12) ÷ 27 = 1.48 cu yd exact; order estimate = 1.63 cu yd.Before you purchase material
- Ask ready-mix suppliers about minimum loads, short-load fees, delivery windows, and washout rules.
- Confirm strength, slump, aggregate, air entrainment, and admixtures with the project specification.
- Use bags only when mixing capacity and placement time allow a continuous, properly finished pour.
- Arrange labor, tools, access, weather protection, and curing materials before the truck arrives.
Common questions
How much extra concrete should I order?
Five to ten percent is a common planning range for simple formed work, but irregular excavation or uncertain subgrade can require more. Measure carefully and ask the supplier or contractor; excess concrete also needs a lawful disposal plan.
How many 80-pound bags equal one cubic yard?
At a typical listed yield of 0.60 cubic foot per 80-pound bag, one cubic yard requires 45 bags because 27 ÷ 0.60 = 45. Product yields vary, so the package label controls your final count.
Is four inches thick enough for a concrete slab?
Four inches is common for some residential pedestrian slabs, but it is not universally sufficient. Loads, soil, climate, reinforcement, joints, and local code all matter. Have the designer or local building authority verify structural requirements before construction.
Can I calculate concrete for multiple sections together?
Yes. Calculate the exact cubic yards of each distinct slab, footing, step, or thickened edge, then add those exact volumes. Apply the placement allowance once to the combined total to avoid compounding waste percentages.
Sources and reference standards
- QUIKRETE Concrete Mix Product DataManufacturer product data used to cross-check typical bag yields; verify the yield printed on the bag you buy.
- International Code Council — 2024 International Residential CodePrimary model-code source for residential foundations, reinforcement, masonry, and stair safety requirements.