Planning a concrete project and need to know how much material to order? Whether you’re pouring a new patio, replacing a driveway, or setting fence posts, getting the calculation right saves money and prevents the headache of running short mid-pour.
The quick answer: multiply your length × width × thickness (all in feet) to get cubic feet, then divide by 27 to convert to cubic yards. At 4 inches thick, one cubic yard covers about 81 square feet. Our calculator below handles all the math and gives you bag counts too.
🧱 Your Concrete Estimate
📦 Bags Needed by Size
💰 Estimated Cost Range
📊 Quick Reference: Recommended Thickness
| Project Type | Minimum | Recommended |
|---|---|---|
| Sidewalks & Walkways | 4″ | 4″ |
| Patios | 4″ | 4″ |
| Residential Driveways | 4″ | 5-6″ |
| Garage Floors | 4″ | 5-6″ |
| House Foundations | 6″ | 6-8″ |
| Commercial/Heavy Traffic | 6″ | 8-12″ |
📝 The Formula
Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet
Cubic Feet ÷ 27 = Cubic Yards
Quick Tip: At 4″ thick, 1 cubic yard covers approximately 81 square feet. Divide your square footage by 81 for a quick estimate.
Understanding Concrete Measurements
Concrete is sold in two ways: by the cubic yard for ready-mix delivery, or by the bag for DIY projects. Understanding both measurements helps you choose the most cost-effective option for your project.
What is a Cubic Yard?
A cubic yard equals 27 cubic feet—imagine a cube measuring 3 feet on each side. This is the standard unit for ordering ready-mix concrete from a supplier. One cubic yard weighs approximately 4,000 pounds when cured.
At 4 inches thick (the most common residential thickness), one cubic yard covers about 81 square feet. This “81 rule” provides a quick mental calculation: divide your square footage by 81 to estimate cubic yards needed.
Bag Yields
Bagged concrete from hardware stores comes in several sizes, each yielding a different volume when mixed:
- 80 lb bag: 0.60 cubic feet (about 45 bags per cubic yard)
- 60 lb bag: 0.45 cubic feet (about 60 bags per cubic yard)
- 40 lb bag: 0.30 cubic feet (about 90 bags per cubic yard)
The Concrete Calculation Formula
The basic formula for calculating concrete volume is straightforward:
Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet
Cubic Feet ÷ 27 = Cubic Yards
The key is converting thickness from inches to feet by dividing by 12. A 4-inch slab is 0.333 feet thick, a 6-inch slab is 0.5 feet thick.
Example Calculation
For a 20-foot by 10-foot patio at 4 inches thick:
- 20 × 10 × 0.333 = 66.6 cubic feet
- 66.6 ÷ 27 = 2.47 cubic yards
- Add 10% waste: 2.72 cubic yards
- Round up to 2.75 or 3 cubic yards when ordering
Recommended Concrete Thickness by Project
Different projects require different slab thicknesses based on the loads they’ll support. Using the wrong thickness can lead to cracking, settling, and premature failure.
Sidewalks and Walkways (4 inches)
For foot traffic only, 4 inches is standard and sufficient. Most building codes require a minimum of 4 inches for residential sidewalks. No need to go thicker unless you expect occasional vehicle traffic.
Patios (4 inches)
Standard patios supporting outdoor furniture and foot traffic need only 4 inches of concrete. However, if you’re planning a hot tub or heavy outdoor kitchen, consider 5-6 inches in that area.
Residential Driveways (4-6 inches)
While 4 inches is the code minimum for residential driveways, 5-6 inches is recommended for better durability. Increasing from 4 to 5 inches costs about 20% more but increases load-bearing capacity by nearly 50%. For areas with freeze-thaw cycles or heavy vehicles, thicker is always better.
Garage Floors (4-6 inches)
Most residential garages use 4-inch slabs, but 5-6 inches provides better durability for parking vehicles and supporting heavy equipment. The floor apron (where vehicles enter) should be the same thickness or slightly thicker than the main floor.
Foundations and Footings (6-8 inches)
House foundations and load-bearing slabs require 6-8 inches minimum, often reinforced with rebar. Always check local building codes, as foundation requirements vary based on soil conditions, frost depth, and structure type.
Ready-Mix vs. Bagged Concrete: Which to Choose
The decision between ready-mix delivery and bagged concrete depends on project size, budget, and your willingness to do physical labor.
When to Use Bagged Concrete
- Projects under 1 cubic yard (about 45 bags of 80 lb mix)
- Small repairs and patches
- Fence post holes
- Small walkway sections
- When you have time and don’t mind mixing
Bagged concrete costs $4-8 per bag at hardware stores. For one cubic yard, you’ll spend roughly $225-360 on bags alone, plus significant time and labor mixing.
When to Use Ready-Mix
- Projects over 1 cubic yard
- Driveways, patios, garage floors
- Any slab requiring continuous pour
- When you want consistent, professional-grade concrete
Ready-mix concrete costs $120-170 per cubic yard delivered. For larger projects, it’s often more economical than bags, and significantly faster. A concrete truck can deliver and pour 10 yards in about an hour—the same volume would require mixing over 400 bags by hand.
Concrete Costs Explained
Understanding the full cost picture helps you budget accurately for your project.
Ready-Mix Pricing
- Standard concrete (3000 PSI): $130-150 per cubic yard
- High-strength (4000 PSI): $150-170 per cubic yard
- Short load fee (under 5 yards): $50-100 extra
- Saturday/after-hours delivery: $30-50 extra
- Pump truck (if needed): $150-500
Bagged Concrete Pricing
- 80 lb bag: $5-8 each
- 60 lb bag: $4-6 each
- Fast-setting mix: $1-2 more per bag
Total Project Costs
For a complete installation including materials, labor, forms, and finishing:
- Basic concrete slab: $6-10 per square foot
- Stamped or colored: $8-18 per square foot
- Exposed aggregate: $10-15 per square foot
Tips for Ordering Concrete
Getting your order right prevents costly problems on pour day.
Always Order Extra
Add 5-10% to your calculated volume for waste, spillage, and uneven subgrade. Running short during a pour is far more expensive than having a little left over. When ordering ready-mix, round up to the nearest quarter yard.
Know Your PSI Requirements
PSI (pounds per square inch) indicates concrete strength:
- 2500 PSI: Light residential use
- 3000 PSI: Standard driveways and patios
- 4000 PSI: Heavy-duty applications and foundations
- 5000+ PSI: Commercial and industrial
Schedule Strategically
Concrete trucks typically wait 7 minutes per yard at no charge, then charge by the hour. Have your forms ready, reinforcement in place, and crew prepared before the truck arrives. Morning pours are ideal in hot weather, as afternoon heat can cause concrete to set too quickly.
Concrete Reinforcement
Reinforcement won’t prevent cracks but helps hold concrete together if cracks occur.
Wire Mesh
Standard for 4-5 inch residential slabs. Wire mesh (also called welded wire fabric) provides reinforcement against shrinkage cracks and minor settlement. Position mesh in the middle of the slab, not at the bottom.
Rebar
Required for slabs 5 inches or thicker, and for any load-bearing applications. #4 (1/2 inch) rebar is most common for residential work. Space bars on a grid pattern, typically 12-18 inches apart.
Fiber Reinforcement
Synthetic or steel fibers mixed into concrete provide secondary reinforcement against shrinkage cracks. Fibers don’t replace structural reinforcement but can reduce surface cracking.
Subgrade Preparation
Even perfect concrete fails on a poor foundation. Proper subgrade preparation is essential for long-lasting results.
- Remove all organic material, topsoil, and debris
- Compact the soil thoroughly
- Add 4-6 inches of compacted gravel base for drainage
- Ensure uniform depth—low spots mean thin concrete
- Install forms at the correct height and slope (1/8 inch per foot minimum for drainage)
Curing Concrete Properly
How concrete cures determines its final strength and durability.
- First 24-48 hours: Keep covered and moist, no foot traffic
- Days 2-7: Mist regularly or cover with plastic; light foot traffic OK after day 2
- Day 7: 70% strength reached; vehicle traffic OK for driveways
- Day 28: Full design strength achieved
Avoid pouring concrete when temperatures are below 40°F or above 90°F. Extreme temperatures affect curing time and can reduce final strength.
Common Concrete Projects and Requirements
10×10 Shed Pad
100 square feet at 4″ thick = 1.24 cubic yards = 56 bags of 80 lb mix. Consider bagged concrete for this size, or a short-load delivery if you prefer not to mix.
Standard 2-Car Driveway (20×24 feet)
480 square feet at 5″ thick = 7.4 cubic yards. Ready-mix is the clear choice. Budget $900-1,300 for materials only; $3,000-5,000 with professional installation.
Patio (12×16 feet)
192 square feet at 4″ thick = 2.4 cubic yards = about 108 bags of 80 lb mix. On the borderline between bags and ready-mix—consider your labor capacity.
Sidewalk (4 feet wide, 50 feet long)
200 square feet at 4″ thick = 2.5 cubic yards = about 113 bags. Ready-mix recommended for consistent results on a long pour.
Quick Summary
- Formula: Length × Width × Thickness (in feet) ÷ 27 = Cubic Yards
- 81 rule: At 4″ thick, 1 cubic yard covers 81 square feet
- Bags per yard: ~45 bags (80 lb), ~60 bags (60 lb), ~90 bags (40 lb)
- Cost: $120-170 per cubic yard (ready-mix), $5-8 per bag
- Always add 5-10% extra for waste
With accurate calculations and proper preparation, your concrete project will last for decades. Use our calculator above to get your exact requirements!
❓ Frequently Asked Questions
Ready-mix concrete costs between $120 and $170 per cubic yard delivered, with the national average around $140-150 per yard in 2024-2025. Prices vary by region, concrete strength (PSI), and delivery distance. Short loads under 5 cubic yards typically incur additional fees of $50-100. Higher PSI mixes (4000+ for foundations) cost more than standard 3000 PSI mixes.
One cubic yard (27 cubic feet) requires approximately 45 bags of 80-pound concrete mix, 60 bags of 60-pound mix, or 90 bags of 40-pound mix. An 80-pound bag yields about 0.60 cubic feet, a 60-pound bag yields 0.45 cubic feet, and a 40-pound bag yields 0.30 cubic feet. For large projects, ready-mix delivery is more practical than mixing dozens of bags by hand.
A residential concrete driveway should be at least 4 inches thick, though 5-6 inches is recommended for heavy vehicles, frequent use, or areas with freeze-thaw cycles. Commercial driveways typically require 6-8 inches. Increasing thickness from 4 to 5 inches adds about 20% to your concrete cost but increases load-bearing capacity by nearly 50%.
For a 10×10 foot slab at 4 inches thick, you need approximately 1.24 cubic yards of concrete (about 56 bags of 80-pound mix). At 6 inches thick, you need 1.85 cubic yards (about 84 bags). The formula is: 10 × 10 × (thickness in feet) ÷ 27 = cubic yards. Always add 5-10% extra for waste and spillage.
For projects under 1 cubic yard (about 45 80-lb bags), bagged concrete is often more practical and cost-effective, avoiding delivery fees. For larger projects, ready-mix saves significant time and labor—mixing and pouring 45+ bags is extremely labor-intensive. Ready-mix is also mixed to precise specifications for consistent strength. Consider that a 10-yard truckload can be poured in about 90 minutes, while mixing that many bags could take all day.
Concrete reaches about 70% of its design strength in 7 days and full strength in 28 days. You can walk on fresh concrete after 24-48 hours and drive on a driveway after 7 days. For best results, keep concrete moist during the first week by misting with water or covering with plastic sheeting. Avoid pouring concrete in extreme heat or cold, as temperatures affect curing time and final strength.
Reinforcement helps prevent cracks and holds concrete together if cracks do occur. Use wire mesh for slabs 4-5 inches thick, and rebar for slabs 5 inches or thicker. For residential driveways and patios, wire mesh is usually sufficient. Foundations and load-bearing slabs typically require rebar. Adding fiber mesh to the concrete mix provides secondary reinforcement against shrinkage cracks.
At 4 inches thick, one cubic yard of concrete covers approximately 81 square feet. This provides a quick mental calculation: divide your total square footage by 81 to estimate cubic yards needed for a 4-inch slab. For example, a 200 square foot patio needs about 2.5 cubic yards (200 ÷ 81 = 2.47). Adjust for different thicknesses—at 6 inches thick, one cubic yard covers only 54 square feet.
