Concrete Calculator
Calculate how much concrete you need for your project
How to Use This Calculator
Our concrete calculator makes it easy to determine exactly how much material you need for any project. Start by selecting your project type from the three available options: slab for flat surfaces like patios, driveways, and floors; footing for wall foundations and structural supports; or column for fence posts, deck supports, and pier footings.
For slabs and footings, enter your length and width in feet, then specify the thickness in inches. Common thicknesses range from 4 inches for sidewalks and patios to 6 inches or more for driveways and structural applications. The calculator instantly displays results in cubic yards, cubic feet, and the number of bags needed in 40-pound, 60-pound, and 80-pound sizes.
For column projects, enter the diameter and height of each hole in inches, then specify how many columns you need. This is perfect for planning fence post installations or deck footings. Always remember to add 10 percent to your calculated amount to account for waste, spillage, and uneven ground conditions. Review the thickness guidelines below to ensure your project meets minimum structural requirements.
Understanding Concrete Calculations
Cubic Yards Explained
Concrete is sold and delivered by the cubic yard, which equals 27 cubic feet. One cubic yard covers 81 square feet at 4 inches thick, 54 square feet at 6 inches thick, or 27 square feet at 12 inches thick. Understanding this relationship helps you estimate costs and plan your project timeline. Ready-mix trucks typically hold 8 to 10 cubic yards, and most suppliers have minimum delivery requirements of 1 to 2 cubic yards.
Bags Versus Ready-Mix Delivery
Bagged concrete works best for small projects under one cubic yard, such as fence posts, small repairs, and stepping stones. Each 80-pound bag yields approximately 0.6 cubic feet of concrete when mixed. For larger projects, ready-mix delivery is more economical and practical. Mixing 50 or more bags by hand is exhausting, time-consuming, and makes it difficult to achieve a consistent pour before the concrete begins to set. Ready-mix ensures uniform quality throughout your project.
PSI Ratings and Strength
Concrete strength is measured in pounds per square inch (PSI), indicating how much compression force it can withstand. Standard bagged concrete provides 3,000 to 4,000 PSI, which is adequate for most residential projects. High-strength mixes reach 5,000 PSI or higher, recommended for driveways with heavy vehicle traffic or structural applications. The water-to-cement ratio significantly affects final strength, so avoid adding extra water even if it makes mixing easier.
Reinforcement Requirements
Wire mesh or rebar reinforcement prevents cracks from spreading and adds tensile strength to concrete, which is naturally strong in compression but weak in tension. For 4-inch residential slabs, wire mesh provides adequate reinforcement. Driveways and slabs supporting heavy loads benefit from number 3 or number 4 rebar placed in a grid pattern. Always position reinforcement in the middle to lower third of the slab thickness for maximum effectiveness.
Curing Time and Conditions
Concrete does not dry; it cures through a chemical reaction called hydration. This process requires moisture and time. Concrete reaches approximately 70 percent of its design strength in 7 days and full strength in 28 days. Keep the surface moist during the first week by covering with plastic sheeting or spraying with water several times daily. Proper curing can increase final strength by 50 percent compared to concrete that dries out too quickly.
Weather Considerations
Temperature dramatically affects concrete curing. Ideal conditions range from 50 to 90 degrees Fahrenheit. In cold weather below 50 degrees, the curing reaction slows significantly, and the concrete may freeze before gaining adequate strength, resulting in a weak, crumbly surface. In hot weather above 90 degrees, rapid evaporation causes surface cracking and reduces strength. Plan your pour for mild weather whenever possible, or use additives designed for extreme temperatures.
Frequently Asked Questions
How thick should concrete be for a driveway?
Residential driveways should be at least 4 inches thick for cars and light trucks, but 5 to 6 inches is recommended for longevity and heavy vehicle traffic. Commercial driveways or those regularly used by delivery trucks, RVs, or heavy equipment need 6 to 8 inches. The extra thickness costs relatively little compared to the total project but significantly extends the lifespan and load-bearing capacity.
How thick should concrete be for a patio?
A 4-inch thickness is standard and sufficient for most residential patios. This depth adequately supports foot traffic, patio furniture, and typical outdoor activities. If you plan to install heavy features like a hot tub, outdoor kitchen, or masonry fire pit, increase the thickness to 6 inches in those specific areas or throughout the entire patio for uniformity.
How thick should a sidewalk be?
Residential sidewalks require a minimum of 4 inches of concrete. This standard thickness handles pedestrian traffic and light equipment like wheelbarrows or lawn mowers. Public sidewalks often require 4 to 5 inches per local building codes. If vehicles will occasionally cross the sidewalk, such as at a driveway apron, increase the thickness to 6 inches in those transition areas.
When should I use bags versus truck delivery?
Use bagged concrete for projects requiring less than one cubic yard, such as fence posts, mailbox bases, and small repairs. For projects between one and two cubic yards, either option works depending on your budget and available help. For anything over two cubic yards, ready-mix delivery is strongly recommended. The cost difference diminishes at larger volumes, while the labor savings are substantial. A cubic yard requires mixing approximately 45 bags of 80-pound concrete by hand.
Should I add extra concrete for waste?
Yes, always order 10 percent more concrete than your calculated volume. This overage accounts for spillage during pouring, concrete that soaks into dry subgrade, slight overexcavation of forms, and inevitable waste. For projects with irregular shapes or uneven ground, increase the overage to 15 percent. Running short mid-pour creates a cold joint that weakens the slab, making extra material a worthwhile investment.
What is the difference between concrete and cement?
Cement is a fine powder that acts as a binding agent, while concrete is the finished product containing cement, sand, gravel, and water. The cement undergoes a chemical reaction with water called hydration, binding the aggregates into a solid mass. Bagged concrete premixes all dry ingredients in correct proportions. You only add water and mix. Never substitute cement alone for concrete, as it lacks the structural aggregates necessary for strength.
Do I need rebar in my concrete project?
Wire mesh suffices for most 4-inch residential slabs like patios and sidewalks. Driveways benefit from wire mesh or rebar, particularly in areas with clay soil that expands and contracts seasonally. Foundation slabs and any structural concrete require rebar per engineering specifications. When adding reinforcement, position it in the middle to lower third of the slab and support it on chairs or dobies to prevent it from sinking to the bottom during the pour.
How long does concrete need to cure before use?
Allow 24 to 48 hours before walking on fresh concrete. Light foot traffic is safe after 48 hours. Wait at least 7 days before driving vehicles on a driveway or applying heavy loads. Full cure takes 28 days, after which the concrete reaches its rated strength. During the first week, keep the surface moist and avoid dragging heavy items across it. Protect fresh concrete from rain, freezing temperatures, and direct sun exposure.
Can I pour concrete in cold weather?
Pouring in temperatures below 50 degrees Fahrenheit requires special precautions. Use hot water in the mix, add accelerating admixtures, and cover the fresh concrete with insulated blankets to retain heat. Never pour on frozen ground or when temperatures will drop below freezing within 24 hours. Cold-weather concrete takes longer to cure and requires extended protection periods. For best results, wait for mild weather or consult a professional.
How much does concrete cost per cubic yard?
Ready-mix concrete typically costs between $120 and $180 per cubic yard for standard residential mixes, with prices varying by location and current material costs. Add delivery fees of $50 to $100 per load. Bagged concrete costs approximately $4 to $6 per 80-pound bag, which yields about 0.6 cubic feet. At roughly $200 to $300 per cubic yard, bags cost more per volume but require no minimum order and allow flexible timing for small projects.
Concrete Project Examples
Patio Slab Example
A typical backyard patio measuring 12 feet by 14 feet at 4 inches thick requires 168 square feet of coverage. Volume calculation: 12 x 14 x (4/12) = 56 cubic feet, or 2.07 cubic yards. Adding 10 percent overage brings the total to 2.28 cubic yards. At this volume, ready-mix delivery is recommended over mixing 95 bags of 80-pound concrete by hand. Budget approximately $300 to $400 for concrete plus delivery fees.
Sidewalk Example
A 30-foot-long sidewalk at the standard 4-foot width and 4-inch depth requires 120 square feet. Volume: 30 x 4 x (4/12) = 40 cubic feet, or 1.48 cubic yards. With 10 percent overage, plan for 1.63 cubic yards. This project falls at the threshold where either bags or delivery works. Using approximately 74 bags allows weekend warriors to pour in sections, while a single ready-mix delivery enables completing the entire project in one day.
Driveway Section Example
Replacing a 10-foot by 20-foot section of driveway at 5 inches thick involves 200 square feet. Volume: 10 x 20 x (5/12) = 83.33 cubic feet, or 3.09 cubic yards. With overage, order 3.4 cubic yards of ready-mix. A driveway project of this size requires coordination with the concrete supplier for timing, as well as helpers for screeding and finishing. Rent a power trowel for professional-looking results on larger surfaces.
Concrete Tips for Success
Order 10 percent extra material. Running short mid-pour creates cold joints that weaken your slab and look unprofessional. Extra concrete can fill low spots or be used for small side projects. Suppliers charge for short loads but not for slight overages that remain in the truck.
Prepare your subbase properly. Remove all organic material, compact the soil, and add 4 to 6 inches of compacted gravel. A stable base prevents settling and cracking. Rent a plate compactor for large areas to ensure proper compaction. Loose or uneven subbase is the primary cause of concrete failure.
Time your pour strategically. Schedule delivery for early morning in hot weather to avoid the heat of the day. In cool weather, midday pours benefit from warmer temperatures. Ensure you have adequate help available before the truck arrives, as concrete waits for no one.
Master the basic finishing sequence. After pouring and screeding to level the surface, wait until surface water evaporates before floating. Use a bull float for large areas, then follow with a hand float or trowel for final smoothing. Edge along forms and cut control joints while the concrete is still workable. Rushing the finish when concrete is too wet causes surface defects and weakness.
Protect your fresh concrete. Cover with plastic sheeting if rain threatens. In hot, dry, or windy conditions, apply curing compound or wet-cure by spraying water several times daily for the first week. Concrete that dries out too quickly loses significant strength and develops surface cracking.
The Calculation Formulas
Slab or footing: Volume (cubic feet) = Length (ft) x Width (ft) x Depth (inches) / 12
Round column: Volume (cubic feet) = 3.14159 x (Radius in inches)^2 x Height (inches) / 1728
Convert to cubic yards: Cubic feet / 27 = Cubic yards
80 lb bags needed: Cubic feet / 0.6 (each bag yields 0.6 cubic feet)
These formulas provide theoretical volume. Always add 10% for waste, uneven ground, form leakage, and spillage. For sloped or uneven ground, calculate using the maximum depth.
Concrete Bag Coverage
- 40 lb bag: 0.011 cubic yards (0.30 cu ft)
- 60 lb bag: 0.017 cubic yards (0.45 cu ft)
- 80 lb bag: 0.022 cubic yards (0.60 cu ft)
Tip: Add 10% extra for waste and spillage.
Common Slab Thicknesses
- Sidewalks: 4 inches
- Patios: 4 inches
- Driveways: 4-6 inches
- Garage floors: 4-6 inches
- Foundation slabs: 6-8 inches
Did You Know?
- The Pantheon in Rome, built with concrete nearly 2,000 years ago, remains the world's largest unreinforced concrete dome.
- Concrete is the second most consumed material on Earth after water, with about 10 billion tons used annually worldwide.
- Concrete continues to strengthen over time. It is stronger at 50 years than at one year old, as the cement continues to hydrate slowly.
- The Romans made concrete using volcanic ash, which is why their structures have lasted millennia while modern concrete typically lasts 50 to 100 years.
- Concrete can withstand compression forces of over 3,000 PSI, but is weak in tension, which is why we add steel reinforcement.