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Concrete Calculator

Estimate concrete volume, bag counts, and material costs for slabs, footings, and other construction projects instantly for free.

Formula Verified by Dr. Arthur Pendelton, PhD

Quick actions: Save inputs, view saved runs, print, share, or export results

Saved Runs

Inputs

Lengthft

Widthft

Depthin

Waste allowance%

Results

Concrete needed

3.26 yd3

80 lb bags147

Total cubic feet88

Tip

Save this page to estimate concrete material orders on site or during project planning.

Formula

Concrete volume

Volume = length x width x depth; Cubic yards = cubic feet / 27

Slab dimensions are converted into cubic feet, adjusted for waste, then converted into cubic yards and estimated bag counts.

Step by step

How the calculation works

1Enter slab length and width in feet.
2Enter the pour depth in inches.
3Convert depth into feet and calculate cubic feet.
4Add waste and convert to cubic yards and bag counts.

Example

Example concrete estimate

A 20 x 12 ft slab at 4 inches thick with 10% waste needs about 3.26 cubic yards of concrete.

Related guides

Learn more about this calculation

Guide

Concrete Volume Guide: Slab Yardage, Waste Allowance, and Bag Estimates

Learn how to calculate concrete volume for slabs and footings. Convert length, width, and depth into cubic yards and 80 lb bags, and apply the correct waste allowance before ordering.

Guide

Unit Conversion Guide: Length, Weight, Temperature

Learn dimensional analysis and practical unit conversion for length, weight, and temperature with examples tied to the live DTC unit converter.

How to understand and use this calculator

Calculating Concrete Volume

Concrete is ordered and poured by volume, measured in Cubic Yards (or cubic meters in metric systems). Sizing a concrete pour requires calculating the geometric volume of the structure and converting it to the standard ordering unit.

To calculate the volume of a standard rectangular slab, multiply the length, width, and depth (thickness) in feet:

Cubic Feet = Length (ft) × Width (ft) × (Thickness (in))/12

Cubic Yards = (Cubic Feet)/27

We divide by 27 because 1 cubic yard is equal to 27 cubic feet (3 ft × 3 ft × 3 ft).

The Importance of a Waste Allowance

Never order the exact geometric volume calculated. In practice, ground subgrades are rarely perfectly level, concrete settles, and some material spills during the pour. A waste allowance of 10% is standard industry practice.

Total Ordered Volume = Cubic Yards × ( 1 + (Waste Allowance (%))/100 )

Converting Volume to Pre-Mixed Concrete Bags

For small projects (such as fence posts, small walkways, or patios), using pre-mixed bags is often more cost-effective than ordering a concrete delivery truck. Bags are sold by weight (usually 40, 60, or 80 pounds). The calculator converts volume to bag counts using standard densities:

* An 80-pound bag yields approximately 0.60 cubic feet of mixed concrete.

* A 60-pound bag yields approximately 0.45 cubic feet of mixed concrete.

* A 40-pound bag yields approximately 0.30 cubic feet of mixed concrete.

Concrete Strength: PSI and Mix Ratios

Concrete strength is measured in PSI (Pounds per Square Inch). A standard ratio for residential slabs (patios, driveways) is 3,000 to 4,000 PSI, requiring a balanced mix of cement, sand, gravel, and water. Excess water lowers PSI strength and increases cracking.

How to Use This Concrete Calculator

Enter the slab length and width in feet and the pour depth in inches. Add a waste allowance percentage — 10% is standard practice — to account for uneven subgrade, settling, and spillage during the pour.

The calculator returns the concrete volume in cubic yards, the total cubic feet including waste, and an estimated count of 80-pound pre-mixed bags if you plan to hand-mix rather than order a truck.

Worked Example: A 20x12 ft Patio Slab

A 20 ft by 12 ft slab poured 4 inches thick has a base volume of 80 cubic feet (20 × 12 × 4/12). With a 10% waste allowance, the total ordered volume is 88 cubic feet.

Converting to cubic yards (88 ÷ 27) gives approximately 3.26 cubic yards of ready-mix concrete, or about 147 bags of 80-pound pre-mixed concrete if hand-mixing (88 ÷ 0.6, rounded up).

Related Calculators

For other project sizing and area calculations, see the Square Footage Calculator to measure the footprint of your slab before pouring, or the Unit Converter for converting between feet, yards, and metric measurements.

Related concepts

Cubic yard

A unit of volume equal to a cube with one-yard sides, equal to 27 cubic feet; the standard unit for concrete ordering.

Waste allowance

An extra percentage added to a material estimate to cover waste, spills, or dimensions variations.

Curing Time

The hydration time required for concrete to reach its maximum design strength (PSI).

Frequently Asked Questions

Why is waste allowance important?+

Ground subgrades are rarely perfectly flat, and concrete settles or spills during placing. A 10% waste allowance is standard practice to ensure you do not run out of material mid-pour.

How many bags of concrete make a cubic yard?+

It takes approximately forty-five 80-pound bags of pre-mixed concrete to fill one cubic yard of volume.

How do I convert inches of thickness to feet?+

Divide the thickness in inches by 12. For example, a 4-inch thick slab is 4 / 12 = 0.33 feet thick.

What is the formula for concrete volume?+

Volume (Cubic Yards) = Length (ft) × Width (ft) × Thickness (ft) ÷ 27.

What does 3000 PSI mean?+

PSI stands for Pounds per Square Inch. 3000 PSI concrete can support up to 3,000 pounds of pressure per square inch after 28 days of curing, standard for driveways.

Can I mix different bag sizes?+

Yes, but it is easier to calculate and mix if you stick to one size. An 80lb bag yields 0.6 cu ft, while a 60lb bag yields 0.45 cu ft.

What is the role of aggregate (gravel and sand)?+

Aggregates make up the bulk of the concrete mix, providing strength and reducing the amount of cement paste needed, which lowers costs and cracking.

How long does concrete take to cure?+

Concrete cures through a chemical reaction (hydration). It reaches 70% of its strength in 7 days and is fully cured after 28 days.

Should I use rebar or wire mesh?+

Yes. Concrete has high compressive strength but low tensile strength. Adding steel rebar or wire mesh prevents cracking under heavy loads.

How much water do I add to a bag of concrete?+

Follow the manufacturer instructions carefully. Typically, an 80lb bag requires about 3 quarts of water. Adding too much water weakens the concrete.

How to understand and use this calculator

Calculating Concrete Volume

To calculate the volume of a standard rectangular slab, multiply the length, width, and depth (thickness) in feet:

Cubic Feet = Length (ft) × Width (ft) × (Thickness (in))/12

Cubic Yards = (Cubic Feet)/27

We divide by 27 because 1 cubic yard is equal to 27 cubic feet (3 ft × 3 ft × 3 ft).

The Importance of a Waste Allowance

Total Ordered Volume = Cubic Yards × ( 1 + (Waste Allowance (%))/100 )

Converting Volume to Pre-Mixed Concrete Bags

* An 80-pound bag yields approximately 0.60 cubic feet of mixed concrete.

* A 60-pound bag yields approximately 0.45 cubic feet of mixed concrete.

* A 40-pound bag yields approximately 0.30 cubic feet of mixed concrete.

Concrete Strength: PSI and Mix Ratios

How to Use This Concrete Calculator

The calculator returns the concrete volume in cubic yards, the total cubic feet including waste, and an estimated count of 80-pound pre-mixed bags if you plan to hand-mix rather than order a truck.

Worked Example: A 20x12 ft Patio Slab

A 20 ft by 12 ft slab poured 4 inches thick has a base volume of 80 cubic feet (20 × 12 × 4/12). With a 10% waste allowance, the total ordered volume is 88 cubic feet.

Converting to cubic yards (88 ÷ 27) gives approximately 3.26 cubic yards of ready-mix concrete, or about 147 bags of 80-pound pre-mixed concrete if hand-mixing (88 ÷ 0.6, rounded up).

Related Calculators

Related concepts

Cubic yard

A unit of volume equal to a cube with one-yard sides, equal to 27 cubic feet; the standard unit for concrete ordering.

Waste allowance

An extra percentage added to a material estimate to cover waste, spills, or dimensions variations.

Curing Time

The hydration time required for concrete to reach its maximum design strength (PSI).

Frequently Asked Questions

Why is waste allowance important?+

Ground subgrades are rarely perfectly flat, and concrete settles or spills during placing. A 10% waste allowance is standard practice to ensure you do not run out of material mid-pour.

How many bags of concrete make a cubic yard?+

It takes approximately forty-five 80-pound bags of pre-mixed concrete to fill one cubic yard of volume.

How do I convert inches of thickness to feet?+

Divide the thickness in inches by 12. For example, a 4-inch thick slab is 4 / 12 = 0.33 feet thick.

What is the formula for concrete volume?+

Volume (Cubic Yards) = Length (ft) × Width (ft) × Thickness (ft) ÷ 27.

What does 3000 PSI mean?+

PSI stands for Pounds per Square Inch. 3000 PSI concrete can support up to 3,000 pounds of pressure per square inch after 28 days of curing, standard for driveways.

Can I mix different bag sizes?+

Yes, but it is easier to calculate and mix if you stick to one size. An 80lb bag yields 0.6 cu ft, while a 60lb bag yields 0.45 cu ft.

What is the role of aggregate (gravel and sand)?+

Aggregates make up the bulk of the concrete mix, providing strength and reducing the amount of cement paste needed, which lowers costs and cracking.

How long does concrete take to cure?+

Concrete cures through a chemical reaction (hydration). It reaches 70% of its strength in 7 days and is fully cured after 28 days.

Should I use rebar or wire mesh?+

Yes. Concrete has high compressive strength but low tensile strength. Adding steel rebar or wire mesh prevents cracking under heavy loads.

How much water do I add to a bag of concrete?+

Follow the manufacturer instructions carefully. Typically, an 80lb bag requires about 3 quarts of water. Adding too much water weakens the concrete.

Concrete Calculator

Inputs

Results

Concrete volume

How the calculation works

Example concrete estimate

Learn more about this calculation

Concrete Volume Guide: Slab Yardage, Waste Allowance, and Bag Estimates

Unit Conversion Guide: Length, Weight, Temperature

Concrete Calculator

Inputs

Results

Key output metrics

Concrete Composition

Material Orders Estimation

How to understand and use this calculator

Calculating Concrete Volume

The Importance of a Waste Allowance

Converting Volume to Pre-Mixed Concrete Bags

Concrete Strength: PSI and Mix Ratios

How to Use This Concrete Calculator

Worked Example: A 20x12 ft Patio Slab

Related Calculators

Related concepts

Frequently Asked Questions

Concrete volume

How the calculation works

Example concrete estimate

Learn more about this calculation

Concrete Volume Guide: Slab Yardage, Waste Allowance, and Bag Estimates

Unit Conversion Guide: Length, Weight, Temperature

Key output metrics

Concrete Composition

Material Orders Estimation

How to understand and use this calculator

Calculating Concrete Volume

The Importance of a Waste Allowance

Converting Volume to Pre-Mixed Concrete Bags

Concrete Strength: PSI and Mix Ratios

How to Use This Concrete Calculator

Worked Example: A 20x12 ft Patio Slab

Related Calculators

Related concepts

Frequently Asked Questions