Home/Concrete Calculator

Free estimator · Updated 2026

Concrete Calculator: Cubic Yards, Bags & Cost for Any Pour Shape

Enter your dimensions and instantly get cubic yards, number of bags, and estimated material cost. Supports slabs, footings, columns, steps, and 4 more shapes.

Cubic yards

Ready-mix ordering volume with waste

Bag count

50, 60, and 80 lb bag equivalents

Cost compare

Ready-mix vs bagged material cost

Concrete volume calculator

Slab inputs

Rectangular slab

LengthftWidthftThicknessinQuantitypcs

PSI GradeWaste FactorRegionMaterial Source

Results

Slab estimate

3000 PSI · 10% waste

Volume

5.5 yd3

146.67 ft3 · 4.15 m3

Bags Needed

245 bags

326 x 60 lb · 392 x 50 lb

Material Cost

$989

Ready-Mix

Ready-mix: 5.5 yd3 x $180/yd3 = $989.

Bagged 80 lb: 245 bags x $7.00 = $1,715.

Ready-mix saves $726 vs. 80 lb bags.

At 4" thick, 1 yd3 covers about 81 sq ft. Your slab area is 400 sq ft.

How to calculate concrete

Formulas for All 8 Shapes

Measure dimensions in feet, calculate cubic feet using the matching shape formula, then divide by 27 to convert to cubic yards. For bags, divide cubic feet by the bag yield: 0.60 ft3 for 80 lb, 0.45 ft3 for 60 lb, and 0.375 ft3 for 50 lb.

Rectangular Slab

Volume (ft3) = Length x Width x (Thickness / 12)

20 ft x 20 ft x 4 in = 133.3 ft3 / 27 = 4.94 yd3. With 10% waste: 5.43 yd3.

Round Slab

Volume (ft3) = pi x (Diameter / 2)^2 x (Thickness / 12)

12 ft diameter x 4 in = pi x 6^2 x 0.333 = 37.70 ft3 / 27 = 1.40 yd3.

Round Column

Volume (ft3) = pi x (Diameter / 24)^2 x Height x Quantity

12 in diameter x 4 ft tall x 4 columns = 12.57 ft3 / 27 = 0.47 yd3.

Strip Footing

Volume (ft3) = Length x (Width / 12) x (Depth / 12)

40 ft x 12 in x 8 in = 26.67 ft3 / 27 = 0.99 yd3.

Steps

Total = sum of each step block + platform volume

3 steps, 4 ft wide, 7 in rise, 11 in run = about 0.48 yd3 before waste.

Wall

Volume (ft3) = Length x Height x (Thickness / 12)

20 ft x 8 ft x 8 in = 106.7 ft3 / 27 = 3.95 yd3.

Sonotube / Post Fill

Volume (ft3) = pi x (Diameter / 24)^2 x Depth x Quantity

10 in tubes x 4 ft deep x 4 posts = 8.73 ft3 / 27 = 0.32 yd3.

Irregular / Custom

Break the pour into simple shapes, sum volumes, then add 10-15% waste

For an L-shaped slab, calculate rectangle A + rectangle B and enter the total.

Concrete Volume Quick Reference

All values include a 10% waste factor. Ready-mix uses $179.89/yd3.

Project	Dimensions	Thickness	Volume	Bags (80 lb)	Ready-Mix Cost
Small patio	10x10 ft	4 in	1.23 yd3	56 bags	$221 + short load
Standard patio	20x20 ft	4 in	4.94 yd3	223 bags	$888
Sidewalk	4x20 ft	4 in	0.99 yd3	45 bags	$178 + short load
1-car garage	12x20 ft	4 in	2.96 yd3	134 bags	$532 + short load
2-car garage	24x24 ft	4 in	7.11 yd3	320 bags	$1,278
Shed base	12x16 ft	4 in	2.37 yd3	107 bags	$426 + short load
Driveway	16x40 ft	4 in	15.80 yd3	711 bags	$2,842
Strip footing	40 lin ft	12x8 in	0.99 yd3	45 bags	$178 + short load
Deck posts (4)	10 in x 4 ft	-	0.36 yd3	16 bags	$65 + short load
Concrete steps (3)	4 ft wide	7/11 in	0.53 yd3	24 bags	$95 + short load

Waste Factor Guide

5% - simple rectangular slab, experienced crew, clean subgrade.

10% - standard recommendation for most projects.

15% - complex shapes, irregular terrain, first-time DIY pour.

Running short mid-pour is more expensive than ordering slightly too much because a cold joint creates a permanent weak point.

Common Sonotube Volumes

Diameter	Depth	Per Tube	4 Tubes
8 in	4 ft	0.058 yd3	0.23 yd3
10 in	4 ft	0.091 yd3	0.36 yd3
12 in	4 ft	0.131 yd3	0.52 yd3
16 in	4 ft	0.233 yd3	0.93 yd3

Material decision

Ready-Mix vs Bagged Concrete

For projects over about 1.5 yd3, ready-mix is usually cheaper and much faster. For isolated post holes or tiny repairs, bags avoid short-load fees and delivery logistics.

Ready-mix

$160-$195/yd3

Best for slabs, driveways, garage floors, and time-sensitive pours.

Bagged mix

$315-$350/yd3 equivalent

Best for repairs, post holes, and projects under about 0.75 yd3.

Short load risk

$50-$150 extra

Most suppliers add fees when the order is below 3-4 yd3.

For current delivered pricing, see how much concrete costs per yard. For installed labor pricing, compare concrete slab cost including labor.

Concrete Calculator - Frequently Asked Questions

How do I calculate how much concrete I need?+

Multiply length x width x thickness in feet to get cubic feet, then divide by 27 to get cubic yards. Add 10% for waste. For a 20 x 20 ft slab at 4 inches thick: 20 x 20 x (4 / 12) = 133.3 ft3 / 27 = 4.94 yd3 x 1.10 = 5.43 yd3. Order 5.5 yd3.

How many cubic yards of concrete do I need?+

One cubic yard covers 81 square feet at 4 inches thick, or 54 square feet at 6 inches thick. A standard 2-car garage at 24 x 24 ft needs about 7.1 cubic yards with 10% waste. A 20 x 20 ft patio needs about 4.9 cubic yards.

How many bags of concrete do I need?+

One 80 lb bag yields about 0.60 cubic feet. One 60 lb bag yields 0.45 cubic feet. One 50 lb bag yields 0.375 cubic feet. To fill 1 cubic yard, you need about 45 bags of 80 lb mix, 60 bags of 60 lb mix, or 75 bags of 50 lb mix.

How do I calculate concrete for footings?+

For a strip footing, multiply total linear feet x footing width in feet x footing depth in feet to get cubic feet, then divide by 27. Example: 40 linear feet of 12-inch wide x 8-inch deep footing = 40 x 1.0 x 0.667 = 26.7 ft3 / 27 = 0.99 yd3.

How do I calculate concrete for steps?+

Use the stair-step method: calculate each step as a rectangular block from ground level to that step top, then add the top platform. Step 1 = width x run x 1 rise; Step 2 = width x run x 2 rises; continue through all steps.

How much does concrete cost per cubic yard?+

Ready-mix concrete costs $160-$195 per cubic yard in 2026, with a national average of $179.89/yd3 based on NRMCA 2024 data. Bagged concrete often costs the equivalent of $315-$350 per yd3 in materials.

What's the difference between cement and concrete?+

Cement is an ingredient: a fine powder that acts as the binding agent. Concrete is the finished product: cement, water, sand, and gravel. You pour concrete; cement is what helps it harden.