Concrete Calculator: How to Calculate Cubic Yards for Any Pour

Concrete is sold by the cubic yard, poured by feel, and estimated by math. Get the math wrong and you’re either scrambling to get more concrete mid-pour or watching your margin roll down the driveway. This guide walks through the formulas for slabs, footings, columns, and walls, explains the waste factor, and includes two worked examples so you can follow the exact steps I use on every job.

Use the concrete calculator to run your numbers once you understand what it’s doing. It handles all four pour types and adds the waste factor automatically.

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Quick Answer

The core formula for any concrete pour:

Length (ft) x Width (ft) x Depth (ft) / 27 = Cubic Yards

Divide by 27 because there are 27 cubic feet in one cubic yard. Add 10% for waste before you order.

The one thing that kills estimates: leaving thickness in inches. Always convert inches to feet first (divide by 12). Four inches is 0.333 feet. Six inches is 0.5 feet. Get that wrong and your order is off by a factor of 12.

The Math Behind Concrete Volume Calculations

Concrete is a three-dimensional material. You’re filling a space with length, width, and depth. The result is cubic feet, which you then convert to cubic yards (the unit ready-mix plants use to price and deliver).

Step by step:

Measure all dimensions in feet (convert inches to feet by dividing by 12)
Multiply length x width x depth to get cubic feet
Divide by 27 to get cubic yards
Multiply by 1.10 to add the 10% waste factor

That’s it. Everything below is the same formula applied to different shapes.

Formulas by Pour Type

Different pours use slightly different inputs, but the underlying math is identical. The table below shows the formula and typical dimensions for each common scenario.

Pour Type	Formula	Common Dimensions	Waste Factor
Slab (rectangular)	L x W x D / 27	4” thick residential	10%
Footing (continuous)	Linear ft x Width x Depth / 27	12” wide, 8-12” deep	12-15%
Column / post footing	3.14 x r^2 x Depth / 27	10-12” diameter Sonotube	10%
Wall (formed)	L x Thickness x Height / 27	6-10” thick, 4-8 ft tall	10%
Circular slab	3.14 x r^2 x Depth / 27	Any diameter	10%

I bump the waste factor on footings to 12-15% because trench walls are never perfectly clean. You end up filling more volume than the math shows. On formed walls, 10% is usually fine because concrete goes where the form tells it to go.

Slabs

Slabs are the most common residential pour: garage floors, patios, shed pads, utility areas. The formula is straightforward.

Formula:

L (ft) x W (ft) x Thickness (ft) / 27 = Cubic Yards

Standard thickness:

4 inches (0.333 ft) for residential patios and shed pads
5 inches (0.417 ft) for garage floors with standard vehicle traffic
6 inches (0.5 ft) for driveways, heavy vehicles, or poor soil conditions

The concrete calculator lets you enter dimensions and thickness in inches, so you do not have to do the decimal conversion by hand.

Slab Quick Reference (4” Thick)

Slab Size	Sq Ft	Yards (raw)	Yards + 10%
10 x 10	100	1.23	1.35
12 x 16	192	2.37	2.61
16 x 20	320	3.95	4.35
20 x 24	480	5.93	6.52
24 x 30	720	8.89	9.78

Round up to the nearest half yard when you order. The minimum surcharge from most plants kicks in below 1-2 yards, so if you’re at 1.35, just order 1.5.

Footings

Footings are the concrete base below frost line that prevents frost heave and transfers load to the ground. Residential footings are typically continuous (running around a foundation perimeter) or isolated (individual pads under columns and posts).

Continuous footing formula:

Linear Feet x Width (ft) x Depth (ft) / 27 = Cubic Yards

Calculate total linear feet by running the perimeter or adding up individual runs. Width and depth are your footing dimensions. Both need to be in feet.

One thing to watch: if you’re pouring footings on a sloped site or in a trench with loose soil, measure the actual trench volume at multiple points. The soil won’t hold a perfect form the way a built form does, and you’ll fill more concrete than the design calls for.

Columns and Round Post Footings

Sonotube forms (the cardboard tube forms) are common for deck posts, fence posts, and column footings. These are cylinders, so you use the circle area formula.

Cylinder formula:

3.14 x r^2 x Depth (ft) / 27 = Cubic Yards per column

Where r is the radius in feet (half the diameter).

For a 10-inch diameter Sonotube that is 3 feet deep:

Radius: 5 inches = 0.417 ft
Area: 3.14 x (0.417)^2 = 3.14 x 0.174 = 0.546 sq ft
Volume: 0.546 x 3 = 1.638 cubic feet
Per footing: 1.638 / 27 = 0.061 cubic yards

That is less than a tenth of a yard per footing. For a 4-post deck that is 0.24 yards total, fine for bagged concrete. But a pergola with 16 posts adds up to nearly 1 yard, at which point it is worth calling the plant or at least doing the math before you buy bags.

Walls

Formed concrete walls (basement walls, retaining walls, foundation walls) use the same formula as a slab but with height replacing width as one of the dimensions.

Wall formula:

Length (ft) x Thickness (ft) x Height (ft) / 27 = Cubic Yards

Watch the wall thickness. Most residential foundation walls are 8-10 inches thick. Convert to feet: 8 inches = 0.667 ft, 10 inches = 0.833 ft.

One practical note from job site experience: pour walls in lifts, not all at once. If you fill an 8-foot wall in one shot without stopping, form blowouts are a real risk. The hydrostatic pressure builds fast. Pour in 2-3 foot lifts, let the concrete stiffen slightly, then continue. Takes more time but you will not be chasing concrete across the yard.

Waste Factor: Why You Always Add 10%

Every contractor I know adds at least 10% to their concrete order. Here is what it covers:

Concrete left in the drum (trucks never fully empty)
Spillage during the pour
Sub-base variations (your gravel base is never perfectly level no matter how careful you are)
Slight form movement
Low spots you missed during prep

On footings, I push it to 12-15% because earth forms (trenches without built wood forms) absorb and irregularities add volume. On formed walls, 10% holds.

The cost of running short is not the extra yard of concrete. It is the cold joint. If your pour stops and restarts after the first batch has started to set, you have a structural weak point at that line. The ready-mix plant may or may not be able to get you another load the same day. In spring and summer, plants book up fast. Running short mid-pour is about the worst situation you can be in on a Saturday with a crew standing there.

Order a little extra. It is cheap insurance.

Worked Example 1: 14x18 Backyard Patio Slab

This is a straightforward residential job. Homeowner wants a 14-foot by 18-foot patio off the back door, 4 inches thick, standard residential use.

Step 1: Convert thickness to feet 4 inches / 12 = 0.333 ft

Step 2: Calculate cubic feet 14 ft x 18 ft x 0.333 ft = 83.97 cubic feet

Step 3: Convert to cubic yards 83.97 / 27 = 3.11 cubic yards

Step 4: Add 10% waste 3.11 x 1.10 = 3.42 cubic yards

Order: 3.5 yards

That half-yard buffer rounds you up nicely and covers a small amount of extra fill in a low spot or two. At $135/yard delivered, the difference between 3.42 and 3.5 is about $10. Not worth the risk of running short.

After pouring, verify your rough cost estimate with the concrete cost per yard calculator, which accounts for regional pricing variations and delivery minimums.

Worked Example 2: Perimeter Footing for a 12x16 Addition

This job involves a continuous footing running around the perimeter of a 12x16 addition. The footing is 12 inches wide and 10 inches deep, below frost line for the Pacific Northwest.

Step 1: Calculate total linear feet Perimeter: (12 + 16) x 2 = 56 linear feet

Step 2: Convert dimensions to feet Width: 12 inches = 1.0 ft Depth: 10 inches = 0.833 ft

Step 3: Calculate cubic feet 56 ft x 1.0 ft x 0.833 ft = 46.65 cubic feet

Step 4: Convert to cubic yards 46.65 / 27 = 1.73 cubic yards

Step 5: Add 12% waste (earth-formed footing) 1.73 x 1.12 = 1.93 cubic yards

Order: 2 yards

The extra at 2 yards (versus 1.93) gives you a small buffer and avoids the short-load surcharge that kicks in below the minimum delivery threshold at most plants.

Once your footing and slab estimates are complete, the construction cost estimator can help you roll up the full project cost including labor, form work, and finishing.

Bags vs. Ready-Mix: The Practical Breakdown

For small pours, bags are often the right call. For larger pours, the math strongly favors a ready-mix truck. Here is how to decide.

Scenario	Best Option	Why
Less than 0.5 yards	Bagged concrete	No minimum charge, easier logistics
0.5 to 1 yard	Either - depends on access	Bags work if no truck access
Over 1 yard	Ready-mix truck	Cheaper, faster, better consistency
No truck access	Bagged + portable mixer	Only viable option in tight sites

Cost comparison for a 2-yard job:

Ready-mix at $135/yard: $270 delivered, poured in under an hour
60-lb bags at $5 each (120 bags): $600 in materials, plus a full day of mixing

The bags option is nearly always more expensive and far more labor-intensive. I stopped mixing bags for anything over a half yard a long time ago. The truck saves money and saves your back.

The exception is tight access, which the PNW specializes in. Narrow side yards, gated properties, urban lots with no room to maneuver. In those cases, bags and a portable mixer or a wheelbarrow relay with helpers is the play. Factor the extra labor time into your job price.

If you are adding reinforcement, calculate your steel after you have your concrete volume dialed in. Our rebar spacing calculator helps you figure out bar spacing and quantity for slabs and walls.

FAQ

What is the formula for calculating concrete in cubic yards?

Length (ft) x Width (ft) x Depth (ft) / 27 = Cubic Yards. Always convert thickness from inches to feet before calculating (divide by 12). Add 10% for waste before ordering.

How many bags of concrete equal one cubic yard?

One 60-lb bag covers approximately 0.45 cubic feet. That means you need about 60 bags per cubic yard. One 80-lb bag covers about 0.60 cubic feet, so roughly 45 bags per yard.

Why do I divide by 27?

There are 27 cubic feet in one cubic yard (3 ft x 3 ft x 3 ft = 27 cubic feet). Since concrete is priced and delivered in cubic yards, you divide your cubic foot volume by 27 to get the number you need to order.

How much extra concrete should I order?

At minimum, 10% over your calculated volume. For earth-formed footings and trenches, bump it to 12-15%. Running short mid-pour creates a cold joint, which is a structural weak point. The extra concrete cost is always cheaper than the problem it prevents.

What thickness is standard for a residential slab?

Four inches is the standard for patios, shed pads, and light-use slabs with a properly compacted gravel base. Five to six inches for garage floors, driveways, or situations with poor soil or heavy loads.

Can I calculate concrete for an L-shaped or irregular area?

Yes. Break the shape into simple rectangles (or triangles and circles), calculate each section separately, and add the results together. For odd shapes, use the square footage calculator to get your area first, then multiply by depth and divide by 27.

How long in advance should I order ready-mix concrete?

Minimum 3-5 business days, longer in spring and summer when plants run at capacity. Confirm the morning of your pour. Last-minute orders often can’t be accommodated, and delays mean your forms are set and your crew is standing around costing you money.