Concrete Calculator

How Concrete Calculator Works

Cubic Yards = (Length × Width × Thickness in ft) ÷ 27

27 cu ft

Cubic Yards Conversion — One cubic yard equals 27 cubic feet (3ft × 3ft × 3ft).

How to Use

1. Enter the length and width of the slab area in feet.
2. Select the concrete thickness for your specific application (e.g., 4 inches for driveways).
3. Select a waste allowance based on site complexity.
4. The result shows cubic yards for ordering ready-mix and equivalent 60-lb and 80-lb bag counts.
5. Compare bag counts against ready-mix for your project size: under 1 yard = bags are practical, over 2 yards = ready-mix saves labor and ensures consistent quality.

Quick Reference

Common Uses

• •Ordering the right amount of ready-mix concrete for a patio, driveway, or foundation to avoid costly shortages or waste
• •Estimating bagged concrete quantity for small DIY projects like fence posts, walkways, or shed pads
• •Comparing costs between ready-mix delivery and bagged concrete for projects of different sizes
• •Planning slab thickness based on expected loads — from light pathways to heavy vehicle parking areas

Understanding the Result

Worked Examples

Frequently Asked Questions

How thick should a concrete driveway be?

Residential driveways handling passenger cars should be at least 4 inches thick. If you park heavy vehicles like trucks or RVs, use 5-6 inches. Commercial lots often use 6-8 inches. Thicker slabs also require more reinforcement and a stronger subbase.

When should I use ready-mix concrete vs. bags?

Ready-mix is cost-effective for pours over 1 cubic yard. Below that, bagged concrete is practical. Pouring more than 2-3 yards from bags is extremely labor-intensive. Ready-mix also ensures consistent quality, while bag mix quality can vary if not properly measured and mixed.

How long does concrete take to cure?

Concrete reaches about 70% of its full strength within 7 days and achieves full design strength after 28 days of proper curing. Keep the concrete moist for at least the first 7 days by covering with wet burlap, applying curing compound, or using a soaker hose. Avoid heavy loads on the slab for at least 7 days.

Why do concrete slabs crack?

Concrete shrinks as it cures and expands and contracts with temperature changes. Control joints (cut grooves) direct cracking to predetermined locations where it is less visible and structurally harmless. Without control joints, cracks will still occur but in random, potentially unsightly patterns. Proper reinforcement, subbase preparation, and curing all help minimize cracking.

What is the best concrete mix for a DIY slab?

For DIY slabs, use a pre-mixed concrete blend designed for projects like walkways, patios, and driveways. 4,000 PSI mix is standard for residential slabs — it provides good strength for most applications. Ready-mix concrete labels typically specify the PSI rating. For bagged concrete, look for "high-strength" or "4000 PSI" mixes. Add fiber reinforcement or wire mesh for crack resistance. For structural applications like footings or foundations, use 4,500+ PSI mix.

Can I pour concrete in cold or hot weather?

Ideal pouring temperature is 50–80 degrees Fahrenheit. In cold weather (below 40 degrees F), concrete sets very slowly and can freeze before gaining strength — frozen concrete must be removed and re-poured. Use cold-weather admixtures or heated water, and cover with insulated blankets. In hot weather (above 90 degrees F), concrete sets too quickly, which can cause cracking from rapid drying. Use cool water, pour early morning or evening, apply evaporation retarder, and start curing immediately. Never pour on frozen or muddy ground.

Pro Tips

• →Never pour concrete on dry, uncompacted, or frozen ground. Excavate to the required depth, compact the soil with a plate compactor (rent for ~$50/day), add 4–6 inches of compacted gravel base for drainage, and dampen the gravel before pouring to prevent the dry base from sucking water out of the concrete too quickly, which weakens the slab.
• →Order slightly more concrete than calculated — coming up short mid-pour creates a "cold joint" (a weak seam between old and new concrete) that will crack and looks terrible. Most suppliers allow you to return unused concrete (check their policy), so the cost of over-ordering 0.25–0.5 yards is far less than the cost of a short load or structural failure.
• →Cut control joints within 6–18 hours of finishing — while the concrete is still green but has hardened enough to hold an edge. Joints should be 1/4 the slab thickness deep and spaced at 2–3 times the slab thickness in feet (e.g., for 4-inch slab, cut joints every 8–12 ft). Control joints don't prevent cracking; they direct cracks to a straight, hidden line where they are less visible.
• →Curing is not the same as drying — concrete cures through a chemical reaction with water (hydration), not by drying out. Keep the slab continuously moist for at least 7 days after pouring. Use a soaker hose, wet burlap, plastic sheeting, or spray-on curing compound. Concrete that dries out too quickly can lose up to 40% of its design strength.
• →Use reinforcement: welded wire mesh (WWF) or rebar is cheap insurance. For a 4-inch residential slab, 6×6 W1.4/W1.4 wire mesh placed at mid-depth (use chairs or pull it up during the pour — never lay it on the ground and pour over it) adds about $0.25/sq ft and dramatically reduces random cracking. For structural slabs, use #4 rebar spaced 12–18 inches.
• →When mixing from bags, rent a concrete mixer (about $40–60/day) instead of mixing in a wheelbarrow. A mixer produces more consistent concrete, saves hours of labor, and reduces the risk of weak spots from improperly mixed bags. One person operating a mixer can produce about 1 cubic yard per hour — enough for a small patio. For anything over 1.5 yards, ready-mix is more economical.

Limitations to Know

• •This calculator provides an estimate for rectangular slabs with uniform thickness. It does not account for irregular slab shapes (L-shaped patios, curved walkways), variable-depth sections like thickened edges, or below-grade foundation walls and footings.
• •Quantities for reinforcement (rebar, wire mesh), formwork lumber, gravel base material, and finishing tools are not included — these must be estimated separately based on your specific project and local building codes.
• •The bag conversion uses approximate yields at standard water ratios: 80-lb bag yields ~0.6 cu ft, 60-lb bag yields ~0.45 cu ft. Actual yields vary slightly by mix design and water content.
• •For structural slabs, foundations, or any load-bearing application, consult a licensed structural engineer. For projects over 1 cubic yard, always compare ready-mix pricing — it is typically cheaper, faster, and higher quality than bagged concrete.

Sources

• Portland Cement Association
• International Building Code