How to Calculate Concrete Yardage (Without Running Short on the Job)

Jan 15
3 min read

Determining the quantity of concrete to order is one of the most important steps in any concrete project. Order too little and you risk cold joints, delays, and expensive fixes. If you order too much, you may waste material and money.

In this guide, we’ll break down simple, reliable ways to calculate concrete yardage, explain why consistent subgrade is critical, and show you an easy formula contractors rely on every day (no special apps or construction calculators required).

What Information Do You Need to Calculate Concrete?

You only need two pieces of information to calculate concrete yardage accurately:

Total square footage of the slab
Thickness of the concrete (in inches)

That’s it. Once you know those two numbers, the math is straightforward.

If you can measure length × width and know your slab thickness, you already have everything you need.

https://www.youtube.com/watch?v=WEbE5FCRvwg

The Simple Concrete Calculation Formula

Instead of converting square feet to cubic feet or memorizing multiple formulas, you can use one simple calculation method that works for any slab thickness.

Square Footage ÷ Divider Number = Cubic Yards of Concrete

The divider number is based on slab thickness and comes from this calculation:

324 ÷ slab thickness (in inches)

Example: 4" Thick Concrete Slab

Divide 324 ÷ 4 = 81
Divide your total square footage by 81

Example: 800 sq ft ÷ 81 = 9.87 cubic yards

That’s your base concrete yardage before accounting for site conditions and waste. This same method works whether you’re pouring:

Driveways
Patios
Sidewalks
Floors
Countertops
Thick structural pours

Just change the thickness.

Concrete Yardage Divider Chart

Use the table below as a quick reference when calculating yardage:

Slab Thickness	Divider Number	Calculation
2 inches	162	Sq Ft ÷ 162
3 inches	108	Sq Ft ÷ 108
4 inches	81	Sq Ft ÷ 81
5 inches	64.8	Sq Ft ÷ 64.8
6 inches	54	Sq Ft ÷ 54
8 inches	40.5	Sq Ft ÷ 40.5
12 inches	27	Sq Ft ÷ 27

This table provides quick estimates, but always verify measurements on-site.

Why Consistent Subgrade Is Critical

Concrete calculations assume consistent thickness across the entire slab. If your subgrade isn’t uniform, your concrete won’t be either. Even small variations can cause big problems:

Low spots increase concrete volume
Over-excavated areas use more material
Poor grading leads to running short mid-pour

If you calculate for a 4" slab but parts of the pour end up at 5", your concrete order will be insufficient.

Best Practice:

Aim to keep subgrade and forms within ¼ inch of your target thickness whenever possible. Consistency is far more important than being perfect.

Ordering Extra Concrete (And Why It Matters)

In the real world, concrete jobs rarely go exactly as planned. Minor grade changes, form movement, or site conditions can affect volume.

Most contractors choose to order a small buffer of extra concrete to protect the job, especially on:

Integral color pours
Decorative concrete
Stamped concrete
Time-sensitive placements

Running short on decorative concrete can create cold joints, color inconsistencies, and visible defects that can’t be fixed after the fact.

Many professionals allow up to around 10% extra when ordering to help account for real-world variables, but the exact amount depends on the job, layout, and conditions.

Other Ways to Calculate Concrete Yardage

If you prefer not to calculate yardage yourself, there are other options:

Call your local ready-mix plant and provide the square footage and thickness
Use an online concrete calculator
Use a construction calculator or mobile app

Even when using these tools, it’s still important to understand how the numbers are generated so you can catch mistakes before concrete arrives on site.

Concrete Calculator FAQs

How do I calculate the amount of concrete I need?

Measure the length, width, and thickness of the slab. Multiply the dimensions (in feet) and divide by 27 to convert to cubic yards.

Can I estimate concrete using square footage?

Yes. Multiply square footage by slab thickness (in feet) to get cubic feet, then divide by 27. You can also use divider numbers for faster estimates.

How much concrete is needed for a 10×10 slab?

For a 10×10 slab:

4 inches thick: approximately 1.23 cubic yards
6 inches thick: approximately 1.85 cubic yards

Should I order concrete by the yard or use bags?

For larger pours, such as driveways and patios, ordering ready-mix by the yard is far more efficient. Bags are better suited for small repairs or limited areas.