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Concrete Calculator — Volume & Materials Calculator

Calculate concrete volume and material requirements for your project

📦Bag Coverage

40 lb~0.011 yd³
60 lb~0.017 yd³
80 lb~0.022 yd³

📏Common Thickness

Sidewalk4 inches
Driveway4-6 inches
Patio4 inches
Garage6 inches

💡Construction Tips

• Order 5-10% extra material
• Consider delivery costs in budget
• Check local building codes
• Use proper reinforcement (rebar)

Concrete Calculator: Volume and Materials Calculator

Table of Contents - Concrete

How to Use This Calculator - Concrete

Select your Project Type from the buttons: Slab (flat horizontal surface like a patio or driveway), Footing (foundation support, typically rectangular), Column (vertical support, calculated as square cross-section), or Stairs (stepped structure).

Enter dimensions based on your selected project type:

  • Slab: Length (feet), Width (feet), Thickness (inches)
  • Footing: Length (feet), Width (feet), Depth (inches)
  • Column: Width (feet, assumes square), Height (feet)
  • Stairs: Width (feet), Total run length (feet), Number of steps

Select your Bag Size: 40-lb, 60-lb, or 80-lb bags. This determines how many bags you'll need for smaller projects.

Optionally, enter Bag Cost to calculate total material expense.

Click "Calculate" to see results. The output displays:

  • Volume in cubic feet
  • Volume in cubic yards (for ordering ready-mix)
  • Number of bags needed
  • Total cost (if bag price entered)

The Core Principle: Volume Calculation

Concrete is ordered and measured by volume. For ready-mix deliveries, the standard unit is cubic yards. For bagged concrete (small projects), you calculate how many bags fill your required volume.

The basic formula for rectangular shapes: Volume = Length × Width × Height (or Thickness/Depth)

Dimensions must be converted to consistent units. Thickness is often measured in inches but must convert to feet (divide by 12) to calculate cubic feet. Cubic feet convert to cubic yards by dividing by 27.

Concrete bags have rated yields—how much volume one bag fills when properly mixed. An 80-lb bag yields approximately 0.6 cubic feet; a 60-lb bag yields about 0.45 cubic feet; a 40-lb bag yields about 0.3 cubic feet.

Always order extra concrete. Uneven ground, spillage, and form irregularities consume more than calculated. Standard practice is adding 10% waste factor.

How to Calculate Concrete Volume Manually

Slab calculation:

Volume (cu ft) = Length (ft) × Width (ft) × Thickness (in) ÷ 12

Example: 20 ft × 12 ft patio, 4 inches thick Volume = 20 × 12 × (4 ÷ 12) = 20 × 12 × 0.333 = 80 cubic feet

Convert to cubic yards: 80 ÷ 27 = 2.96 cubic yards

Add 10% waste: 2.96 × 1.10 = 3.26 cubic yards Order 3.5 cubic yards to be safe.

Footing calculation:

Same formula as slab: Volume = Length × Width × Depth ÷ 12

Example: Perimeter footing 60 ft total length, 1 ft wide, 12 inches deep Volume = 60 × 1 × (12 ÷ 12) = 60 cubic feet = 2.22 cubic yards

Column calculation:

Volume = Width × Width × Height (for square columns)

Example: 2 ft × 2 ft column, 8 ft tall Volume = 2 × 2 × 8 = 32 cubic feet = 1.19 cubic yards

Bag calculation:

Number of bags = Volume (cu ft) ÷ Yield per bag

For 80-lb bags (0.6 cu ft yield): Bags = 80 cu ft ÷ 0.6 = 134 bags

Circular column:

Volume = π × radius² × height For 12" diameter (0.5 ft radius), 4 ft tall: Volume = 3.14159 × 0.5² × 4 = 3.14 cubic feet

Real-World Applications

Patio and driveway planning. Calculate how much concrete you need before calling suppliers. Know whether you're ordering 2 yards or 5 yards—it affects equipment, timing, and cost.

Cost estimation. Ready-mix costs $100-150 per cubic yard delivered (varies by location). Bagged concrete runs $4-6 per 80-lb bag. Calculate to compare options and budget accurately.

Project feasibility. A 20×20 foot patio at 4 inches thick needs about 5 cubic yards—a significant pour requiring proper preparation. Understanding volume helps you decide: DIY or hire professionals?

Material ordering. Order too little and you have a cold joint (where fresh concrete meets partially cured concrete—a weak point). Order too much and you waste money. Precise calculation with waste factor prevents both problems.

Form building. Knowing volume helps estimate lateral pressure for form bracing. A 4-foot-tall wall exerts significant pressure; forms must be built to withstand it.

Scenarios People Actually Run Into

The thickness shortfall. You calculated for 4 inches thick, but the ground is uneven. Low spots are 6 inches deep; high spots are 2 inches. Average thickness exceeds your plan, and you run short. Always survey the base before calculating.

The bag underestimate. You calculated 50 bags but forgot that mixing isn't 100% efficient—some sticks to the wheelbarrow, some to the mixer, some overflows. Buy 10-15% extra bags.

The ready-mix minimum. You need 1.5 cubic yards, but the ready-mix company has a 3-yard minimum. You're paying for 3 yards regardless. Consider expanding the project or using bags instead.

The timing crunch. You ordered 4 yards but didn't have enough help. Concrete sets in 1-2 hours. Working alone, you couldn't place and finish before it started hardening. Volume calculation should inform crew size.

The slump surprise. You ordered standard 4-inch slump concrete, but it's too stiff for your narrow forms. You add water on-site, weakening the mix. Specify appropriate slump when ordering based on your project.

Trade-Offs and Decisions People Underestimate

Bags versus ready-mix. Under about 1 cubic yard, bagged concrete is usually more economical. Above 1-2 yards, ready-mix saves labor (no mixing) and ensures consistent quality. The crossover depends on local prices.

Thickness requirements. Standard residential slabs are 4 inches. Driveways bearing vehicles need 5-6 inches. Heavy equipment areas need 6+ inches with reinforcement. Thicker uses more concrete but prevents cracking.

Reinforcement impact. Rebar or mesh doesn't significantly affect concrete volume (steel displaces minimal concrete), but it affects cost, labor, and structural capacity. Include it in project planning.

Waste factor selection. 10% is standard for typical rectangular pours on prepared bases. Irregular shapes, sloped ground, or complex forms may need 15-20% extra. Estimating waste factor appropriately is crucial.

Mixing time reality. An 80-lb bag takes 2-3 minutes to mix properly. Fifty bags is 2-3 hours of mixing alone, before placing. This labor time often tips the decision toward ready-mix.

Common Mistakes and How to Recover

Forgetting unit conversion. Thickness in inches, length in feet—you must convert inches to feet (÷ 12) before multiplying. This is the most common error, producing results off by a factor of 12.

Ignoring ground variation. Calculating from form tops assumes flat base. An uneven base that averages 2 inches deeper than planned uses 50% more concrete. Check base elevation before ordering.

Underestimating bags needed. Bag yields are theoretical maximums. Real yield is 10-15% less due to mixing losses. Always round up and buy extra.

No waste allowance. Even perfect pours have some waste—spillage, excess in the mixer, filling voids. 10% minimum waste factor prevents running short at the worst moment.

Wrong bag size assumptions. 80-lb bags yield about 0.6 cu ft, not 1 cubic foot. Check bag specifications—different brands and types vary slightly.

Related Topics

Concrete strength (PSI). Standard residential concrete is 3,000-4,000 PSI. Driveways and structural applications may need 4,000-5,000 PSI. Higher strength doesn't require more volume, but costs more.

Curing time. Concrete reaches working strength in 7 days, full strength in 28 days. Volume doesn't affect cure time, but thickness affects setting behavior.

Reinforcement calculation. Rebar is typically placed in a grid pattern, 18-24 inches on center. Calculate linear feet needed: slab area × 2 ÷ spacing gives approximate rebar count.

Form calculation. Forms for a slab perimeter: add lengths of all sides. Form boards are typically 2×4 or 2×6 lumber, stakes every 2-3 feet.

Aggregate and mix design. Standard concrete is cement, sand, gravel, and water. Mix proportions affect strength and workability. For special applications, specify the mix when ordering.

How This Calculator Works

Slab and footing volume: Volume (cu ft) = Length × Width × (Thickness or Depth ÷ 12)

Column volume: Volume (cu ft) = Width × Width × Height

Stairs volume (approximate): Volume (cu ft) = Width × Run length × 0.5 × Number of steps (This estimates average thickness of the stepped shape)

Cubic yard conversion: Cu yards = Cu feet ÷ 27

Bag calculation: Based on yield per bag:

  • 80-lb bag: 0.022 cubic yards (0.6 cu ft)
  • 60-lb bag: 0.017 cubic yards (0.45 cu ft)
  • 40-lb bag: 0.011 cubic yards (0.3 cu ft)

Bags needed = Volume (cu yards) ÷ Yield per bag

Cost calculation: Total cost = Number of bags × Cost per bag

Results are rounded up because you can't buy partial bags.

All calculations happen locally in your browser.

FAQs

How much concrete do I need for a 10x10 slab?

10 × 10 feet, 4 inches thick: Volume = 10 × 10 × (4÷12) = 33.3 cubic feet = 1.23 cubic yards With 10% waste: order 1.4 cubic yards or about 55 80-lb bags.

When should I use bags versus ready-mix?

Below 1 cubic yard, bags are usually cheaper and more practical. Above 2 cubic yards, ready-mix saves significant labor. Between 1-2 yards, compare local prices and consider your labor availability.

How many 80-lb bags are in a cubic yard?

Approximately 45 bags. Each 80-lb bag yields about 0.6 cubic feet; 27 cubic feet per yard ÷ 0.6 = 45 bags.

What thickness should my concrete be?

Sidewalks and patios: 4 inches. Driveways: 5-6 inches. Garage floors: 4-6 inches. Footings: per local code, typically 6-12 inches.

Should I add a waste factor?

Yes. 10% is standard for typical pours. Use 15-20% for complex shapes, uneven ground, or inexperienced crews. Running short mid-pour is costly and creates weak joints.

How long before I can walk on new concrete?

Light foot traffic: 24-48 hours. Normal use: 7 days. Full strength: 28 days. Don't drive on new concrete for at least 7 days, preferably 10-14.

Does concrete thickness affect how much I need?

Directly proportional. Doubling thickness doubles volume. A 4-inch slab uses half the concrete of an 8-inch slab of the same area.

What if my ground isn't level?

Measure depth at multiple points and use the average. Add extra waste factor (15-20%) to account for low spots filling more than expected.

How do I reinforce concrete?

Common reinforcement includes rebar (steel bars) in a grid pattern and wire mesh. For slabs, mesh is often sufficient. For footings, columns, and structural elements, rebar is typically required. Place reinforcement at mid-depth for slabs or per engineering specifications for structural elements.

What's the curing process?

Concrete hardens through hydration (chemical reaction with water), not drying. Keep fresh concrete moist for at least 7 days for proper curing—cover with plastic, spray with curing compound, or mist regularly. Premature drying weakens concrete.

Can I pour concrete in cold weather?

Concrete can be poured in cold weather but requires precautions. Below 40°F (4°C), hydration slows dramatically. Below freezing, water in the mix can freeze and damage the concrete. Use heated enclosures, accelerators, or wait for warmer conditions.

What causes concrete to crack?

Common causes include shrinkage during curing, thermal expansion/contraction, overloading, improper curing, and ground movement. Control joints (intentional weakened lines) encourage cracking in predetermined locations rather than randomly.

How do I calculate for circular shapes?

For circular slabs or footings: Volume = π × radius² × thickness. For tubes or round columns: calculate outer cylinder volume minus inner cylinder volume. Convert radius and thickness to consistent units before calculating.