Concrete Calculator: Free Online Concrete Volume Estimator
Use our free concrete calculator to determine exactly how much concrete you need for any project. Designed for contractors, estimators, and DIY homeowners, this tool calculates concrete volume in cubic yards, the number of bagged concrete needed (80 lb bags), and the total surface area from your project dimensions. Whether you are pouring a patio, sidewalk, driveway, or foundation, enter your length, width, and thickness to get instant, accurate results in both imperial and metric units.
How to use this calculator.
- Select your unit system. Choose imperial (feet, inches) or metric (meters, centimeters) using the toggle above. All inputs automatically convert to match your selection.
- Enter your project dimensions. Input the length and width of the area you are pouring. For irregular shapes, divide the area into rectangles and calculate each section separately, then sum the results.
- Set the concrete thickness. Enter the slab or pour thickness in inches. Standard residential slabs are 4 inches thick. Driveways should be at least 5-6 inches. Footings are typically 8-12 inches.
- Review your results. The calculator displays the total area (square feet), concrete volume (cubic yards), and the number of 80-pound bags needed if you are mixing by hand rather than ordering ready-mix.
- Order or purchase materials. For volumes over 1 cubic yard, ready-mix concrete delivered by truck is significantly cheaper and produces better results than bagged concrete. Use the bag count for small jobs like fence posts or small pads.
- Print or save. Use your browser's print function to generate a jobsite-ready summary of your concrete requirements.
How do you calculate concrete volume, bags, and cost?
To calculate concrete volume, multiply the area (length multiplied by width) by the thickness, convert from cubic feet to cubic yards, and then determine the number of bags if mixing by hand.
Area (sq ft) = Length (ft) × Width (ft)
Volume (cu yd) = Length (ft) × Width (ft) × Thickness (in) ÷ 324
80 lb Bags = Volume (cu yd) × 27 ÷ 0.60
Where:
- 324 is the conversion factor: 12 inches per foot × 27 cubic feet per cubic yard = 324. This converts inches-thickness × square-feet-area directly to cubic yards.
- 0.60 ft³ is the yield of one 80-pound bag of pre-mixed concrete. A 60-pound bag yields 0.45 ft³, and a 50-pound bag yields approximately 0.375 ft³.
- 27 is the number of cubic feet in one cubic yard of concrete.
Example calculation.
Let's calculate the concrete needed for a backyard patio.
Scenario: You are pouring a patio that is 15 feet long, 12 feet wide, and 4 inches thick using 80-pound bags of concrete mix.
- Calculate the area: 15 ft × 12 ft = 180 sq ft
- Calculate volume in cubic yards: 15 × 12 × 4 ÷ 324 = 2.22 cu yd
- Alternative method: convert thickness to feet: 4 inches ÷ 12 = 0.333 ft. Then: 15 × 12 × 0.333 = 60 cu ft ÷ 27 = 2.22 cu yd
- Calculate 80 lb bags needed: 2.22 × 27 ÷ 0.60 = 100 bags
- Add a 10% waste factor: 2.22 × 1.10 = 2.44 cu yd. Order 2.5 cubic yards of ready-mix.
Result: You need approximately 2.5 cubic yardsof concrete for this patio. If mixing by hand, that is 100 eighty-pound bags. For a job this size, ready-mix delivery is strongly recommended: it costs approximately $350-$450 delivered versus $500-$800 in bagged concrete.
Tips from the field.
- Ready-mix beats bags for anything over 1 cubic yard.Bagged concrete costs 2-3 times more per cubic yard than ready-mix and produces less consistent results. A 2 cubic yard pour from bags requires mixing roughly 90 eighty-pound bags: that is hours of hard labor.
- Always order 10-15% extra concrete. It is far cheaper to have a little extra for a birdbath or stepping stone than to call a short-load truck for one more yard. A short load (under 5 yards) incurs a surcharge of $50-$100.
- Prepare your subgrade properly. A well-compacted gravel base 4-6 inches deep prevents settlement and cracking. Wet and compact the base in 2-inch lifts using a plate compactor. Low spots in the subgrade consume extra concrete and create thin spots prone to cracking.
- Schedule your pour for early morning. If you run short, the batch plant is still open for a top-up load. Afternoon pours that run short risk a cold joint overnight if the plant closes before you can get more concrete.
- Do not add water to make concrete easier to work. Adding water on site weakens the concrete permanently. Every extra gallon of water per cubic yard reduces compressive strength by approximately 500-1,000 psi. Use a water reducer admixture instead for workability without sacrificing strength.
Disclaimer: This calculator provides estimates for planning purposes only. Actual material requirements vary based on site conditions, subgrade evenness, form accuracy, and spillage. For structural slabs or load-bearing applications, consult a licensed structural engineer. PavementCalculator.com assumes no liability for material overages or shortages.
Cost, price, and density figures on this page reflect approximate U.S. averages as of August 2026 and vary by region, supplier, and material availability.
Frequently asked questions.
How do I calculate how much concrete I need?
Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 to convert to cubic yards. For example, a 10×10 ft slab at 4 inches thick: 10 × 10 × (4÷12) = 33.3 cu ft ÷ 27 = 1.23 cu yd. Always add 5-10% for waste, spillage, and uneven subgrade. Use our concrete calculator above for instant, accurate results.
How many bags of concrete do I need?
An 80-lb bag of pre-mixed concrete yields approximately 0.6 cubic feet (0.022 cu yd). A 60-lb bag yields about 0.45 cu ft. A 50-lb bag yields about 0.375 cu ft. For a 10×10 ft slab at 4 inches thick (33.3 cu ft): 33.3 ÷ 0.6 = 56 bags (80-lb). For projects larger than 2 cubic yards, ready-mix concrete delivered by truck is typically more economical than bags.
How much does concrete cost per yard?
Ready-mix concrete costs $125-$175 per cubic yard for standard 3,000-4,000 psi mix, plus a delivery fee of $60-$100 per truck. Bags cost $4-$6 for 60-lb, $5-$8 for 80-lb. Per cubic yard equivalent, bagged concrete costs $250-$400: about 2-3× more than ready-mix. For projects over 1-2 cubic yards, ready-mix is more economical and produces a more consistent result.
What is the standard concrete mix ratio?
Standard concrete mix ratio is 1:2:3 (one part cement, two parts sand, three parts gravel) with approximately 0.5 parts water. This produces approximately 4,000 psi concrete when properly cured. For general construction: 1:2:4 mix (3,000-3,500 psi). For high-strength applications: 1:1.5:3 mix (4,500-5,000 psi). Pre-mixed bags already contain the correct proportions: just add water according to the bag instructions.
How thick should a concrete slab be?
Residential patios and walkways: 4 inches minimum. Driveways for passenger vehicles: 4 inches (reinforced with wire mesh or rebar). Driveways for heavy trucks: 5-6 inches reinforced. Garage floors: 4-6 inches. Basement floors: 4 inches over 4 inches of gravel base. For any slab supporting vehicles, use at least 3,500 psi concrete with reinforcement. Always check local building codes for minimum requirements.
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab at 4 inches thick requires 1.23 cubic yards of concrete. With a 10% waste factor, order 1.5 cubic yards. This equals approximately 56 eighty-pound bags if mixing by hand. Use our calculator above to compute your exact requirements based on your dimensions.
How many bags of concrete make a yard?
It takes 45 eighty-pound bags, 60 sixty-pound bags, or 72 fifty-pound bags to make one cubic yard of concrete. An 80 lb bag yields 0.60 cubic feet, a 60 lb bag yields 0.45 cubic feet, and a 50 lb bag yields 0.375 cubic feet. At approximately $5-$7 per 80 lb bag, a cubic yard of bagged concrete costs $225-$315: roughly double the cost of ready-mix delivered.
What PSI concrete do I need for a patio?
For a residential patio, 3,000-3,500 psi concrete is standard and sufficient for foot traffic, furniture, and grills. For driveways or areas supporting vehicle loads, use 3,500-4,000 psi concrete with reinforcement. High-strength 4,500-5,000 psi mixes are used for commercial applications and heavy traffic areas but are rarely necessary for residential work.
How thick should a concrete patio be?
A concrete patio should be a minimum of 4 inches thick. For patios that may support heavy features like hot tubs, outdoor kitchens, or large fire pits, increase thickness to 5-6 inches in those specific areas. Always pour over a 4-6 inch compacted gravel base and include wire mesh or rebar reinforcement for best long-term performance.
How long does concrete take to cure?
Concrete reaches approximately 50% of its design strength in 3-7 days, 75% in 7-14 days, and 90% in 28 days under proper curing conditions (moist, 50-75°F). Concrete continues to gain strength slowly for years. Keep the surface damp for the first 7 days by misting, covering with wet burlap, or applying a curing compound. Do not allow concrete to dry out too quickly: rapid drying causes surface cracking and reduces final strength significantly.