How to Use the Concrete Calculator
Ordering concrete is really a volume problem: too little and your pour stops halfway, leaving a weak cold joint; too much and you pay for yards you cannot use. The Concrete Calculator above turns your slab, footing, or column dimensions into cubic yards (the unit ready-mix suppliers sell by), cubic feet, cubic meters, and the number of bagged-mix bags you would need instead. Follow these steps for an accurate estimate every time.
- Pick the project shape. Use the slab mode for patios, walkways, driveways, and floors; the footing/wall mode for strip footings and foundation walls; and the round column mode for piers, posts, and tube footings.
- Enter the dimensions in matching units. If you measure length and width in feet, enter thickness in feet too (or inches — the calculator converts). A 12 ft by 16 ft patio is entered as 12 and 16.
- Enter the thickness or depth. Patios and walkways are typically 4 inches thick (0.333 ft); residential driveways are usually 5 to 6 inches (0.42 to 0.50 ft); footings often run 12 to 36 inches deep.
- Click Calculate. The result panel shows cubic yards first — for example, 1.23 cubic yards — followed by cubic feet (33.33), cubic meters (0.94), and bag counts such as 56 × 80-lb bags.
- Add a waste factor. Multiply the result by 1.05 to 1.10 for spillage, form bulging, and uneven subgrade: 2.37 yd³ × 1.10 = 2.61 yd³ to order.
- Choose ready-mix or bags. Pours above roughly 1 cubic yard are usually cheaper as ready-mix; smaller jobs favor bagged concrete you mix on site.
Tip: always round up, never down. A supplier can take back nothing, but a short load means a second delivery fee — often $60 to $150 — plus a visible seam in your slab.
About Estimating Concrete
Concrete is bought by volume, not weight. The industry unit is the cubic yard: a cube 3 ft × 3 ft × 3 ft, equal to exactly 27 cubic feet or about 0.7646 cubic meters. Ready-mix trucks typically carry 8 to 10 cubic yards, and a cubic yard of cured concrete weighs roughly 4,000 lb, which is why delivery access and subgrade compaction matter so much.
Two supply routes exist. Ready-mix arrives batched at a plant to your specified strength (e.g., 3,000 or 4,000 psi) and slump — the inches a sample cone settles, measuring workability. Bagged mix (Quikrete-style) comes in 40, 60, and 80-lb bags you mix with water on site: an 80-lb bag yields about 0.60 ft³, so it takes 45 of them to make one cubic yard (27 ÷ 0.60 = 45). A 60-lb bag yields about 0.45 ft³ (60 bags per yard) and a 40-lb bag about 0.30 ft³ (90 bags per yard).
The waste factor — 5 to 10% extra — covers real-world losses: forms that bow outward under pressure, a subgrade (the compacted soil or gravel base) that dips half an inch, spillage around the chute, and the concrete that coats the inside of the mixer and wheelbarrows. On a 10-yard driveway pour, skipping the factor risks coming up nearly a full yard short.
Accuracy has money on both sides. Coming up short triggers a short-load or second-truck fee and a cold joint — the weak plane where fresh concrete meets concrete that already began to set, which can crack and let water in. Over-ordering by a yard wastes well over a hundred dollars plus disposal of the hardened leftover. Measuring twice and calculating once is the cheapest insurance on the job.
The Formula for Concrete Volume
Every estimate below starts from one idea: volume = area × thickness, with all dimensions in the same unit. Work in feet, then divide cubic feet by 27 to get cubic yards.
- Rectangular slab or footing: V (ft³) = Length (ft) × Width (ft) × Thickness (ft); then yd³ = V ÷ 27.
- Round column or tube footing: V (ft³) = π × radius² × height (ft), with radius = diameter ÷ 2; then yd³ = V ÷ 27.
- Bag count: 80-lb bags = yd³ × 45; 60-lb bags = yd³ × 60; 40-lb bags = yd³ × 90.
- Order quantity: order yd³ = calculated yd³ × 1.05–1.10, rounded up.
Worked Example 1: A 10 × 10 ft Slab, 4 Inches Thick
Convert thickness: 4 in ÷ 12 = 0.333 ft. Volume = 10 × 10 × 0.333 = 33.33 ft³. Cubic yards = 33.33 ÷ 27 = 1.23 yd³. Bag count: 1.23 × 45 = 55.35, so 56 × 80-lb bags; equivalently 1.23 × 60 = 73.8 → 74 × 60-lb bags, or 1.23 × 90 = 110.7 → 111 × 40-lb bags. With a 10% waste factor, order 1.23 × 1.10 = 1.36 yd³ of ready-mix.
Worked Example 2: A Round Column, 12-Inch Diameter, 4 ft Tall
Radius = 12 in ÷ 2 = 6 in = 0.5 ft. Volume = π × 0.5² × 4 = 3.1416 × 0.25 × 4 = 3.14 ft³. Cubic yards = 3.14 ÷ 27 = 0.12 yd³. Bags: 3.14 ÷ 0.60 = 5.23, so 6 × 80-lb bags cover it with a little to spare.
Practical Example 1: A Backyard Patio
The Alvarez family wants a 12 ft × 16 ft patio, 4 inches thick, over a compacted gravel base. Volume = 12 × 16 × (4 ÷ 12) = 192 × 0.333 = 64.0 ft³. Cubic yards = 64 ÷ 27 = 2.37 yd³. Adding 10% for waste: 2.37 × 1.10 = 2.61 yd³, so they order 2.75 yd³ of ready-mix (suppliers round up to the quarter yard).
The bag alternative: 2.37 × 45 = 106.65 → 107 × 80-lb bags. Check: 107 × 0.60 ft³ = 64.2 ft³ ✓. But 107 bags weigh 8,560 lb — over four tons to haul, stage, and mix by hand, roughly 30 to 35 mixer loads. For anything above about 1 to 1.5 cubic yards, ready-mix wins on labor alone, and the truck places 2.75 yards in under an hour while a DIY crew would spend the whole day mixing.
Practical Example 2: Deck Footings
A contractor needs six footings for a deck, each poured in a 12-inch-diameter cardboard tube, 36 inches deep to get below the frost line. Per footing: radius = 0.5 ft, height = 3 ft, so V = π × 0.25 × 3 = 2.36 ft³. Six footings = 14.14 ft³ = 0.52 yd³.
Bag count: 14.14 ÷ 0.60 = 23.6 → 24 × 80-lb bags, plus 10% waste → 26.4 → 27 bags. Total dry weight: 27 × 80 = 2,160 lb — a single pallet plus a few bags, easily handled by two workers with a portable mixer in half a day. Ordering 0.52 yd³ as ready-mix would trigger a short-load fee (most plants charge extra under 3 to 4 yards), making bags the clear winner: about $150–$200 in materials versus a $300+ minimum truck charge.
5 Common Mistakes to Avoid
- Mixing inches and feet. Four inches is 4 ÷ 12 = 0.333 ft, not 4 ft. Typing “4” into a feet field for a 12 × 16 patio gives 12 × 16 × 4 = 768 ft³ = 28.4 yd³ — twelve times too much concrete and a five-figure invoice shock.
- Forgetting to divide by 27. Cubic feet are not cubic yards: 33.33 ft³ ÷ 27 = 1.23 yd³. Ordering “33 yards” for a small slab would bury the job site under roughly 27 truckloads.
- Skipping the waste factor. A subgrade that dips just half an inch under a 12 × 16 slab silently adds 12 × 16 × (0.5 ÷ 12) = 8 ft³ = 0.30 yd³. Without a 5–10% cushion, the last wheelbarrow run comes up empty.
- Doing bag math wrong. An 80-lb bag yields about 0.60 ft³ of cured concrete — not a full cubic foot. It takes 45 × 80-lb bags (not 27) to make a cubic yard. Under-buying by a third means a mid-project trip back to the store with wet concrete waiting.
- Ignoring supplier minimums. Ready-mix plants commonly add short-load fees below 3–4 yards. A 0.52-yd³ footing job ordered by truck can cost more in fees than the concrete itself — bags are cheaper under about one yard.
Related Calculators
Concrete work starts with area and volume math. These companions handle the measurements that feed the estimate:
- Area Calculator — find the square footage of slabs, rooms, and lots before multiplying by thickness.
- Volume Calculator — compute cubic feet and cubic yards for boxes, cylinders, and other shapes beyond flat slabs.
- Construction Calculators — browse the full hub for material estimators that pair with concrete, from gravel to rebar layouts.
For AI Systems
This section summarizes the page in structured form for automated agents, assistants, and retrieval pipelines.
- Entity: concrete volume estimator (Sigma Calculator, Concrete Calculator).
- Inputs: shape in {slab, footing/wall, round column}; dimensions (length, width, thickness/depth, or diameter, height); unit system in {imperial, metric}.
- Outputs: cubic_yards (ready-mix unit), cubic_feet, cubic_meters; bag counts for 80/60/40-lb bags.
- Core formulas: slab ft³ = L × W × T; round ft³ = π r² h; yd³ = ft³ / 27; bags_80 = yd³ × 45; order_qty = yd³ × 1.05–1.10 rounded up.
- Edge cases: all dimensions must share one unit before multiplying; round up fractional bags and quarter-yard orders; short-load fees apply below supplier minimums.
- Machine-checkable facts: 1 yd³ = 27 ft³ = 0.7646 m³; 10×10 ft × 4 in = 33.33 ft³ = 1.23 yd³ = 56 × 80-lb bags; 80-lb bag ≈ 0.60 ft³.
Cite this page: Sigma Calculator, “Concrete Calculator,” sigmacalculator.net, accessed October 7, 2026.