Concrete is ordered by volume, so a small measuring error can become a large material shortage. A sound estimate starts with the finished length, width, and average thickness of each pour.
Use the Concrete Calculator to estimate cubic yards, 80-pound bags, and basic ready-mix cost.
Calculate the project volume
For a rectangular slab, multiply length by width by thickness. Keep units consistent. If length and width are in feet, convert thickness from inches to feet by dividing by 12. The result is cubic feet. Divide cubic feet by 27 to convert to cubic yards.
Calculate footings, thickened edges, piers, steps, and separate sections independently. Add the volumes together only after checking each shape.
Use average depth carefully
Excavated ground is rarely perfectly flat. Measure depth in several places and use a realistic average. A base that is one inch deeper across a large slab can add substantial volume. Proper grading and compacted base material help control both thickness and performance.
Add an appropriate allowance
An allowance helps cover uneven excavation, spillage, form movement, and small measurement errors. The right amount depends on the project, site, delivery method, and supplier requirements. Do not use extra concrete to hide poor base preparation or create an unplanned thickness.
Bagged mix or ready-mix?
Bagged concrete can be practical for small repairs, isolated posts, or locations that a truck cannot reach. Large pours require many bags, consistent mixing, enough helpers, and a plan to place and finish material before it begins setting.
Ready-mix is commonly ordered in cubic yards and may involve minimum quantities, short-load fees, delivery charges, waiting time, or access restrictions. Ask the supplier about truck dimensions, chute reach, washout rules, and cancellation terms.
Plan beyond the concrete itself
- Forms, stakes, release agent, and bracing
- Compacted base material
- Reinforcement specified for the project
- Control or isolation joints
- Finishing tools and curing materials
- Personal protective equipment and a safe washout area
Safety and structural requirements
Wet concrete is caustic and can cause serious burns. Wear appropriate protection and wash exposed skin promptly. Slabs supporting buildings, vehicles, retaining structures, or other critical loads may require engineered thickness, reinforcement, drainage, frost protection, and permits. Confirm local requirements before work begins.
The calculator is intended for preliminary budgeting. Verify dimensions, specifications, and order quantity with your contractor, engineer, or concrete supplier.
Worked example: a 12-by-20-foot patio
Suppose a patio will be 12 feet wide, 20 feet long, and 4 inches thick. The surface area is 12 × 20 = 240 square feet. Convert the thickness to feet: 4 ÷ 12 = 0.333 foot. Multiply 240 × 0.333 to get about 80 cubic feet, then divide by 27 to get 2.96 cubic yards.
If the site and forms justify a 10% ordering allowance, 2.96 × 1.10 = 3.26 cubic yards. That is a planning figure, not an automatic order quantity. Ask the ready-mix supplier how it sells partial yards, whether a minimum or short-load charge applies, and whether the supplier recommends a different allowance for the site. If using bags, divide the required cubic feet by the yield printed on the exact bag; do not assume every bag size or mix has the same yield.
Verify these details before ordering
- Measure the inside dimensions of the finished forms, not the disturbed excavation.
- Check depth at several points and calculate footings, piers, steps, and thickened edges separately.
- Confirm the specified concrete strength, air entrainment, slump, aggregate, and curing plan with the project designer or supplier.
- Plan truck access, chute reach, pumping or wheelbarrow distance, washout, crew size, and finishing time.
- Check the weather and have protection ready for rain, heat, cold, or rapid surface drying.
Costs this estimate may not include
A concrete-volume estimate normally does not include demolition, excavation, spoil disposal, gravel base, compaction, forms, reinforcement, vapor barrier, pumping, short-load or waiting fees, permits, finishing labor, curing materials, testing, or repairs to truck access. List those items separately so a low material estimate does not become an unrealistic total budget.
When to get professional help
Consult a qualified contractor or engineer when the concrete supports a building, vehicle, retaining load, elevated structure, or other safety-critical use. Local frost depth, drainage, soil bearing, reinforcement, joints, and code requirements cannot be determined from volume alone. Wet concrete can also cause serious chemical burns; use appropriate protection and follow current safety guidance.
Method and references
See How Our Calculators Work for the formulas, rounding rules, assumptions, and limitations used across DIY Cost Tools. For jobsite safety, review the OSHA concrete and concrete-products guidance. Recheck the product data, supplier instructions, and local requirements for your specific project.
Calculation example and guidance reviewed September 21, 2026.
Continue planning this project
A concrete order is easier to verify when the slab area and supporting base are calculated separately.