The standard ordering unit for ready-mix concrete. One cubic yard equals 27 cubic feet and weighs approximately 4,050 lbs.
PSI Rating
The compressive strength of cured concrete in pounds per square inch. Residential slabs typically use 3,000-4,000 PSI concrete.
Ready-Mix
Concrete delivered by a truck already mixed and ready to pour. Minimum orders are usually 1 cubic yard, with short-load fees for small quantities.
Rebar
Steel reinforcing bars placed within concrete to add tensile strength. Number 4 (#4) rebar at 12-18 inch spacing is common for residential work.
Cure Time
Concrete reaches about 70% of its rated strength in 7 days and full strength in 28 days. Keep it moist during the first week for best results.
Real-World Examples
Example 1
Driveway Slab
Dimensions: 40ft × 12ft × 4 inches thick
Result: 40 × 12 × 0.333 / 27 = 5.93 cubic yards. Order 6.5 yards to allow for waste and grade variations. At $150/yard delivered, cost is approximately $975.
Example 2
Deck Footings
8 footings, each 12 inches diameter × 42 inches deep
Result: Each footing = π × 0.5² × 3.5 = 2.75 cu ft. Total = 22 cu ft = 0.81 cubic yards. About 37 bags of 80-lb concrete (0.6 cu ft yield each).
Concrete Strength Guide
PSI Rating
Typical Use
Bag Type
Cost per Yard
2,500 PSI
Sidewalks, patios
Standard mix
$120 - $140
3,000 PSI
Driveways, slabs
Standard mix
$130 - $150
4,000 PSI
Garage floors, footings
High-strength
$140 - $165
5,000 PSI
Commercial, structural
High-strength
$155 - $180
Working with Concrete Successfully
Bags vs. Ready-Mix: When to Switch
For projects under 1 cubic yard (about 45 bags of 80-lb mix), hand-mixing bags with a mixer or wheelbarrow is practical. Between 1 and 2 yards, consider a trailer-mounted mixer from a rental center. Above 2 yards, ready-mix delivery is almost always faster, produces more consistent results, and often costs less per yard than bagged concrete.
Proper Curing Prevents Cracking
Concrete that dries too fast develops surface cracks. After finishing, cover the slab with plastic sheeting or apply a curing compound to retain moisture. Keep it damp for at least 7 days. Avoid pouring in extreme heat (above 90°F) or freezing conditions without additives, as both compromise final strength.
Related Calculators
How to Use This Calculator
1
Choose Shape
Select slab (rectangle), column (cylinder), footing, or steps. Each shape uses the appropriate volume formula.
2
Enter Dimensions
Input length, width (or diameter), and thickness/depth in feet and inches. The calculator converts to cubic feet automatically.
3
Select PSI Strength
Choose the concrete compressive strength -- 3,000 PSI for sidewalks and slabs, 4,000 PSI for driveways and structural elements.
4
Add Waste Percentage
Add 10% for formed pours and 15% for irregular shapes to account for spillage, pump waste, and surface variation.
5
Get Cubic Yards and Bags
Review the total cubic yards for ready-mix ordering or the equivalent number of 80-lb bags for small pours.
Key Formulas
Slab VolumeCY = L x W x D / 27
Bags EquivalentBags = CY x 45 (80-lb bags per CY)
Typical Slump4-5 inches for most residential pours
Cubic Yard — The standard unit for ordering ready-mix concrete. One cubic yard equals 27 cubic feet or approximately 45 bags of 80-lb concrete mix.
Slump — A measure of concrete workability. Higher slump = more water = easier to pour but weaker. Residential slabs use 4-5 inch slump; structural elements 3-4 inches.
w/c Ratio — Water-to-cement ratio by weight. Lower ratios (0.40) produce stronger, more durable concrete; higher ratios (0.55+) reduce strength and durability.
PSI Strength — Compressive strength measured at 28 days. 3,000 PSI suits sidewalks and patios; 4,000 PSI for driveways; 5,000+ PSI for structural applications.
Rebar Reinforcement — Steel reinforcing bars embedded in concrete to resist tensile cracking. Driveways typically use #4 rebar on 18-inch centers; slabs use #3 or wire mesh.
Curing — Keeping concrete moist and at proper temperature for 7-28 days after pouring to allow full hydration and achieve design strength.
Frequently Asked Questions
When should I use 3,000 vs 4,000 PSI concrete?
Use 3,000 PSI (general purpose) for sidewalks, patios, and garage floors with light vehicle traffic. Use 4,000 PSI for driveways with frequent vehicle traffic, pool decks, and areas subject to freeze-thaw cycles. Structural footings and slabs supporting loads typically require 4,000 PSI or higher per engineer specifications.
Should I use fiber reinforcement or rebar?
Synthetic fiber (polypropylene) controls plastic shrinkage cracking during the initial curing period but does not replace structural rebar. Rebar and wire mesh provide long-term tensile strength to resist cracking under load. For driveways and slabs, use both: fiber in the mix for early-age crack control plus rebar or mesh for structural reinforcement.
How long before I can drive on new concrete?
Wait at least 7 days before light foot traffic and 28 days before allowing vehicle traffic on a standard residential driveway. The 28-day mark is when concrete reaches its design compressive strength. Cold weather slows curing -- if temperatures drop below 50 degrees F, protect the pour with insulating blankets and extend the waiting period accordingly.
When should I order ready-mix vs. use bags?
Order ready-mix when you need more than 1 cubic yard (about 45 bags). Bagged concrete is economical for small repairs and footings under 1 CY, but becomes expensive and labor-intensive above that volume. Most ready-mix plants have a minimum order of 1 CY with a short-load fee -- factor this in when comparing costs for borderline volumes.