Concrete Volume Calculator - ft³ · yd³ · Bags · Cost

Concrete Calculator - Cubic Feet

Calculate Concrete Volume in ft³ for Slabs, Footings, Columns, Walls & Steps

Use our free concrete calculator in cubic feet to instantly calculate how many cubic feet of concrete you need for any project. Supports slabs, footings, round columns, walls, and steps. Get results in ft³ and cubic yards, plus bag count for 60 lb and 80 lb bags, and a cost estimate. Whether you're pouring a driveway, patio, foundation footing, or fence post, this concrete cubic feet calculator gives you everything you need to order the right amount of concrete.

🏠 Slabs & Patios🏗️ Footings & Foundations🔵 Columns & Posts🪜 Steps & Walls

Concrete Yard

1 yd³ = 27 ft³

80 lb Yield

80 lb bag = 0.60 ft³

60 lb Yield

60 lb bag = 0.45 ft³

Waste Rule

Always add 10% waste

Concrete cubic feet calculator

Slab Concrete Inputs

Enter project dimensions to find cubic feet of concrete, bag counts, cubic yards, and a practical order estimate with waste included.

Length

Width

Thickness

Waste

10% is the standard allowance for most slab pours.

Concrete Results

Slab Volume Results

Live update

Cubic Feet

33.33 ft³

Base volume before waste.

Cubic Yards

1.36 yd³

Order volume after waste, divided by 27.

With Waste

36.67 ft³

Recommended order volume with 10% waste.

Truck Order

1.50 yd³

Rounded up to the nearest 0.25 yd³.

10 ft x 10 ft x (4 in ÷ 12) = 33.33 ft³

Waste adds 3.33 ft³, bringing the recommended order to 36.67 ft³.

Bag Count (with waste)

Bag counts are rounded up so you do not run short.

Bags & cost

Bag Size	Bags Needed	Yield / Bag
80 lb bags	62 bags	0.60 ft³
60 lb bags	82 bags	0.45 ft³
40 lb bags	123 bags	0.30 ft³

Tip: For projects above 1 yd³, ready-mix usually beats bag mixing on labor and total time.

Cost Estimate

Price per yd³

Ready-mix cost: $203.70

Price per 80 lb bag

80 lb bags (62 x $6.50): $403.00

Exact order volume: 1.36 yd³

Suggested truck call-out: 1.50 yd³ to keep the order above the waste-adjusted requirement.

Key Concrete Facts

• 1 yd³ = 27 ft³
• 80 lb bag = 0.60 ft³
• 60 lb bag = 0.45 ft³
• 40 lb bag = 0.30 ft³
• Always add 10% for waste
• 1 yd³ is about 45 x 80 lb bags

Standard Concrete Thickness

• Sidewalk: 4 inches
• Patio: 4 inches
• Driveway: 4-6 inches
• Garage floor: 4-6 inches
• Structural slab: 6-8 inches
• Foundation wall: 8-12 inches
• Footing depth: 8-12 inches or below frost line

Quick Mental Math

• 4-inch slab: L x W x 0.333
• 6-inch slab: L x W x 0.5
• 1-inch slab: L x W x 0.0833
• Divide ft³ by 27 to get yd³
• 80 lb bags = ft³ ÷ 0.60
• 60 lb bags = ft³ ÷ 0.45

How Thick Should Your Concrete Be?

Recommended concrete thickness by project type, with the cubic feet impact clearly shown.

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Sidewalks & Walkways

Recommended: 4 inches (0.333 ft)
Minimum: 3.5 inches
High traffic: 6 inches
100 ft² at 3.5 in = 29.2 ft³
100 ft² at 4 in = 33.3 ft³
100 ft² at 6 in = 50.0 ft³
Use 3,000-3,500 PSI concrete with fiber or wire reinforcement for residential work.

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Driveways

Recommended: 4-6 inches
Passenger cars: 4 inches minimum
Heavy vehicles: 6 inches minimum
100 ft² at 4 in = 33.3 ft³
100 ft² at 5 in = 41.7 ft³
100 ft² at 6 in = 50.0 ft³
Use 3,500-4,000 PSI concrete and rebar for vehicle traffic.

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Patios & Garage Floors

Patio: 4 inches standard
Garage floor: 4-6 inches
100 ft² at 4 in = 33.3 ft³
100 ft² at 6 in = 50.0 ft³
Slope outdoor pours about 1/8 in per foot away from structures.
Use 3,000-3,500 PSI for patios and step up to 3,500+ PSI for garage use.

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Footings & Foundations

Continuous footing: 8-12 inches deep and 8-16 inches wide
Pier footing: 12-24 inches deep and 12-24 inches diameter
8 in W x 8 in D = 0.444 ft³ per linear foot
12 in W x 12 in D = 1.000 ft³ per linear foot
16 in W x 12 in D = 1.333 ft³ per linear foot
Depth should reach below the local frost line. Check local code before pouring.

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Fence Posts & Columns

Fence post holes often run 3x the post width and 1/3 to 1/2 of post height in depth.
6 in dia x 24 in deep = 0.393 ft³
8 in dia x 30 in deep = 0.873 ft³
10 in dia x 36 in deep = 1.636 ft³
12 in dia x 48 in deep = 3.142 ft³
Use 4,000+ PSI for structural columns and permanent post installations.

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Steps & Stairs

Standard rise: 7 inches
Standard run: 11 inches
Typical width: 36-48 inches
One 4 ft wide step module at 7 in rise and 11 in run = 2.14 ft³
A 3-step set at 4 ft wide = 12.83 ft³ base
Use rebar and a broom finish for traction and long-term durability.

Concrete Volume Formulas - All Shapes

The concrete calculator cubic feet formulas below cover every shape used in the main tool.

Concrete Volume Formulas (in cubic feet)

SLAB:
  ft³ = L(ft) x W(ft) x T(in) ÷ 12

FOOTING:
  ft³ = L(ft) x W(in)/12 x D(in)/12

COLUMN (round):
  ft³ = pi x (D(in)/24)² x H(ft) x qty

COLUMN (square):
  ft³ = (S(in)/12)² x H(ft) x qty

WALL:
  ft³ = L(ft) x H(ft) x T(in)/12 - openings

STEPS:
  ft³ = W(ft) x Rise(in)/12 x Run(in)/12
        x n(n + 1) / 2

UNIT CONVERSIONS:
  yd³ = ft³ ÷ 27
  bags (80 lb) = ft³ ÷ 0.60
  bags (60 lb) = ft³ ÷ 0.45
  bags (40 lb) = ft³ ÷ 0.30

WASTE FACTOR:
  order volume = calculated ft³ x 1.10

Why divide by 12?

Thickness is usually entered in inches, but concrete volume is ordered in cubic feet or cubic yards. Dividing the thickness by 12 converts inches into feet, so the final multiplication stays in feet all the way through.

Example: 4 inches = 4 ÷ 12 = 0.333 feet. A 10 x 10 slab that is 4 inches thick uses 10 x 10 x 0.333 = 33.33 cubic feet of concrete before waste.

Example 1: Concrete Patio Slab

A 12 ft x 16 ft patio, 4 inches thick.
Step 1: Convert thickness to feet = 4 in ÷ 12 = 0.333 ft
Step 2: Calculate volume = 12 x 16 x 0.333 = 64.00 ft³
Step 3: Add 10% waste = 64.00 x 1.10 = 70.40 ft³
Step 4: Convert to cubic yards = 70.40 ÷ 27 = 2.61 yd³
Step 5: 80 lb bags = 70.40 ÷ 0.60 = 118 bags
Result: Order about 2.75 yd³ ready-mix or 118 x 80 lb bags.

Example 2: Fence Post Holes

12 fence posts, 8-inch diameter holes, 30 inches deep.
Step 1: Radius = (8 ÷ 12) ÷ 2 = 0.333 ft and height = 30 ÷ 12 = 2.5 ft
Step 2: Volume per hole = pi x 0.333² x 2.5 = 0.873 ft³
Step 3: Total base volume = 0.873 x 12 = 10.47 ft³
Step 4: Add 10% waste = 11.52 ft³
Step 5: 80 lb bags = 11.52 ÷ 0.60 = 20 bags
Result: Buy 20 x 80 lb bags of concrete.

Example 3: Foundation Footing

A 40 ft x 30 ft rectangular house foundation footing, 12 inches wide and 12 inches deep.
Step 1: Total footing length = (40 + 30) x 2 = 140 linear feet
Step 2: Volume per linear foot = (12 ÷ 12) x (12 ÷ 12) = 1.0 ft³
Step 3: Base volume = 140 x 1.0 = 140 ft³
Step 4: Add 10% waste = 154 ft³
Step 5: Convert to cubic yards = 154 ÷ 27 = 5.70 yd³
Result: Order 6 yd³ of ready-mix concrete.

Concrete Volume Quick Reference by Project

Pre-calculated cubic feet of concrete for the projects people search most often.

Project	Dimensions	Thickness	ft³ (base)	ft³ (+10%)	80 lb Bags	yd³
Small sidewalk	3 ft x 20 ft	4"	20.0 ft³	22.0 ft³	37 bags	0.81 yd³
Standard sidewalk	4 ft x 20 ft	4"	26.7 ft³	29.3 ft³	49 bags	1.09 yd³
Small patio	10 ft x 10 ft	4"	33.3 ft³	36.7 ft³	62 bags	1.36 yd³
Medium patio	12 ft x 16 ft	4"	64.0 ft³	70.4 ft³	118 bags	2.61 yd³
Large patio	20 ft x 20 ft	4"	133.3 ft³	146.7 ft³	245 bags	5.43 yd³
1-car driveway	10 ft x 20 ft	4"	66.7 ft³	73.3 ft³	123 bags	2.72 yd³
2-car driveway	20 ft x 20 ft	6"	200.0 ft³	220.0 ft³	367 bags	8.15 yd³
Garage floor	20 ft x 24 ft	4"	160.0 ft³	176.0 ft³	294 bags	6.52 yd³
Garage floor	24 ft x 24 ft	6"	288.0 ft³	316.8 ft³	528 bags	11.73 yd³

Common Concrete Projects - Volume Planning Guide

Six real jobs where knowing the cubic feet of concrete makes ordering and budgeting easier.

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Residential Driveway

A standard 2-car driveway at 20 x 20 ft and 6 inches thick uses 200 ft³ of concrete before waste and about 220 ft³ after the standard 10% allowance.

• 8.15 yd³ of ready-mix
• About $1,222 at $150 per yd³
• Use 4,000 PSI concrete
• Cut control joints about every 10 ft

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House Foundation Footing

For a 40 x 30 ft house with 16 in wide by 12 in deep footing, the perimeter pour reaches 186.7 ft³ before waste and 205.3 ft³ after waste.

• 7.60 yd³ after waste
• Depth must reach below frost line
• Use 3,000-4,000 PSI concrete
• Confirm local code before ordering

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Backyard Patio Slab

Small patios can sometimes be bag-friendly, but once the volume climbs past about 1 yd³, ready-mix becomes much easier to place consistently.

• 10 x 10 ft at 4 in = 36.7 ft³ with waste
• 12 x 16 ft at 4 in = 70.4 ft³ with waste
• 20 x 24 ft at 4 in = 176.0 ft³ with waste
• Short-load fees often apply below 5-7 yd³

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Fence Post Installation

Concrete post holes are one of the most common bag-mix jobs. The calculator helps you move quickly from hole size and count to total bags of concrete.

• 10 in hole x 30 in deep = 1.36 ft³ per post hole
• 12 in hole x 36 in deep = 2.36 ft³ per post hole
• 20 small post holes can still add up to ready-mix territory
• Always account for frost depth in your region

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Concrete Entry Steps

Stair pours are visible finish work, so form accuracy and the waste allowance both matter. A 3-step set at 4 ft width needs 14.12 ft³ after waste.

• About 0.52 yd³
• 24 x 60 lb bags if mixing by hand
• Use rebar and a broom finish
• 4,000 PSI mix is a good default

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Retaining Wall

A 20 ft retaining wall that is 4 ft tall and 8 inches thick needs 58.67 ft³ after waste, plus footing volume if the wall sits on a poured base.

• Wall body: 2.17 yd³ after waste
• Typical footing can add another 1.09 yd³
• Drainage is essential behind the wall
• Engineering may be required above 4 ft

Common Concrete Slab Volumes at a Glance

Click any card to pre-fill the slab calculator above and instantly see cubic feet of concrete.

Frequently Asked Questions

Common questions about calculating concrete in cubic feet, cubic yards, and bags of concrete.

How do I calculate cubic feet of concrete for a slab?

Use this formula: ft³ = Length (ft) x Width (ft) x Thickness (in) ÷ 12 The ÷12 converts thickness from inches to feet. Examples: 10x10 ft, 4 in thick = 33.33 ft³ 20x20 ft, 6 in thick = 200.00 ft³ 12x16 ft, 4 in thick = 64.00 ft³ Always add 10% for waste and spillage. 33.33 x 1.10 = 36.67 ft³ to order.

How many 80 lb bags of concrete make 1 cubic foot?

One 80 lb bag yields approximately 0.60 ft³. That means: 1 ÷ 0.60 = 1.67 bags per ft³ Common bag yields: 80 lb bag = 0.60 ft³ 60 lb bag = 0.45 ft³ 40 lb bag = 0.30 ft³ For one cubic yard: 27 ÷ 0.60 = 45 bags of 80 lb mix.

How many cubic feet of concrete do I need for a 10x10 slab?

For a 10x10 ft slab: At 4 in thick: 10 x 10 x (4 ÷ 12) = 33.33 ft³ With 10% waste = 36.67 ft³ = 1.36 yd³ and 62 x 80 lb bags At 6 in thick: 10 x 10 x (6 ÷ 12) = 50.00 ft³ With 10% waste = 55.00 ft³ = 2.04 yd³ and 92 x 80 lb bags.

How do I convert cubic feet of concrete to cubic yards?

Divide cubic feet by 27: yd³ = ft³ ÷ 27 Examples: 33.33 ft³ ÷ 27 = 1.23 yd³ 100 ft³ ÷ 27 = 3.70 yd³ 200 ft³ ÷ 27 = 7.41 yd³ Ready-mix concrete is ordered in cubic yards. Round up after adding waste so the truck order stays above the required volume.

How many bags of concrete do I need for a 10x10 slab?

For a 10x10 ft slab at 4 inches thick: Base volume = 33.33 ft³ With 10% waste = 36.67 ft³ 80 lb bags: 36.67 ÷ 0.60 = 62 bags 60 lb bags: 36.67 ÷ 0.45 = 82 bags 40 lb bags: 36.67 ÷ 0.30 = 123 bags At $6.50 per 80 lb bag: 62 x $6.50 = $403.00

What is the 10% waste rule for concrete?

Always order 10% more concrete than the exact math says. The extra volume covers: Spillage during pouring Uneven subgrade Form imperfections Rounding and cleanup loss Formula: Order volume = calculated ft³ x 1.10 For structural pours, many contractors bump the allowance to 15%.

When should I use bags vs. ready-mix concrete?

Use bags when: Volume is below 0.5 yd³ The pour is small or remote You are doing fence posts, repairs, or small pads Use ready-mix when: Volume is above 1 yd³ You need a continuous pour The project is time-sensitive The labor of mixing dozens of bags becomes unrealistic A good rule of thumb: 1 yd³ = about 45 x 80 lb bags. Above 1.5 to 2 yd³, ready-mix usually wins on labor and consistency.