ConcreteFree

Free Concrete Calculator

Instantly estimate cubic yards, cubic feet, and bags of concrete needed for your slab. No signup required — just enter your dimensions.

Concrete Slab Calculator

Enter your slab dimensions below

Enter length, width, and thickness to see your estimate

How to Calculate Concrete for a Slab

Calculating how much concrete you need for a slab comes down to one simple formula: multiply the length by the width by the thickness of your slab. The result is the volume in cubic feet. Divide by 27 to convert to cubic yards, which is the standard unit for ordering ready-mix concrete.

Our free concrete calculator does all of this for you in real time. Just enter your slab dimensions and choose whether each measurement is in feet or inches. The tool instantly shows cubic yards, cubic feet, and the number of 40 lb, 60 lb, or 80 lb bags you would need.

Step-by-Step: Using the Concrete Calculator

  1. Measure the length of your slab in feet or inches.
  2. Measure the width of your slab in feet or inches.
  3. Decide on the thickness. Most residential slabs are 4 inches. Driveways are typically 4 to 6 inches. Structural slabs may be 6 inches or more.
  4. Enter all three values into the calculator above and toggle the ft/in buttons to match your measurements.
  5. Read your results: cubic yards for ordering ready-mix, bag counts for DIY bag projects.

Slab Thickness Recommendations

Choosing the right slab thickness is critical for structural performance and longevity. Here are common thickness guidelines:

  • Sidewalks and patios: 4 inches is standard for foot traffic.
  • Residential driveways: 4 to 6 inches depending on vehicle weight.
  • Garage floors: 4 to 6 inches, with 6 inches recommended if heavy vehicles are parked.
  • Basement floors: Typically 3.5 to 4 inches.
  • Heavy equipment pads: 6 to 8 inches or more depending on load.

Always Add a 10% Waste Factor

Experienced concrete contractors always order 10% more than the calculated volume. This buffer accounts for spillage during pouring, slight variations in form dimensions, uneven subgrade that requires extra fill, and the practical reality that it is nearly impossible to pour to the exact calculated volume. Running out of concrete mid-pour creates a cold joint — a structural weak point where old and new concrete meet after the first batch has begun to set.

Our calculator displays a “+10% Waste” column in the bag estimates table so you can see at a glance how many bags to purchase for a safe margin.

Cubic Yards vs. Cubic Feet: What You Need to Know

Ready-mix concrete suppliers sell concrete by the cubic yard. One cubic yard is a cube measuring 3 feet on each side, equal to 27 cubic feet. When you call a ready-mix plant, they will ask how many yards you need.

Bag concrete, on the other hand, is often described by how many cubic feet each bag yields. An 80 lb bag yields about 0.60 cubic feet (or 0.022 cubic yards). For large projects requiring more than one cubic yard, ready-mix is almost always more economical and practical. For small repairs or short pours, bags offer convenience and no minimum order.

When to Order Ready-Mix vs. Use Bags

The break-even point between bags and ready-mix is roughly one cubic yard. Below that volume, bags are cost-effective and manageable. Above one cubic yard — a 10x10 foot slab at 4 inches thick is about 1.23 cubic yards — ready-mix is the better option.

Ready-mix concrete arrives at the correct mix design, properly hydrated, and ready to pour. Mixing bags by hand for large pours is exhausting, time-consuming, and risks inconsistent mix ratios. For any project larger than a small pad or set of footings, call your local ready-mix plant and give them the cubic yard number from this calculator.

Frequently Asked Questions

How many bags of concrete do I need per cubic yard?
It depends on the bag size. A 40 lb bag covers about 0.011 cubic yards (0.30 cubic feet), a 60 lb bag covers about 0.017 cubic yards (0.45 cubic feet), and an 80 lb bag covers about 0.022 cubic yards (0.60 cubic feet). For one cubic yard you would need approximately 91 bags of 40 lb, 60 bags of 60 lb, or 45 bags of 80 lb concrete.
What is the standard thickness for a concrete slab?
Standard residential concrete slabs are typically 4 inches thick. Driveways that will support vehicle traffic are usually poured at 4 to 6 inches. Sidewalks and patios are commonly 4 inches. Structural floor slabs or slabs supporting heavy equipment may be 6 inches or more. Always check local building codes for your specific application.
Why should I add a 10% waste factor?
Concrete projects almost always require more material than the calculated volume due to several factors: uneven or sloped subgrade, spillage during mixing and pouring, slight variations in form dimensions, and the need to overfill forms slightly before screeding. Adding 10% to your order ensures you do not run short mid-pour, which can create cold joints and weaken the slab.
What is the difference between cubic yards and cubic feet?
Both are units of volume. One cubic yard equals 27 cubic feet (3 ft x 3 ft x 3 ft = 27). Ready-mix concrete is ordered and priced by the cubic yard, while bag concrete coverage is often listed in cubic feet. Our calculator shows both so you can easily compare and order the right amount regardless of which unit your supplier uses.
How do I measure for a concrete slab?
Measure the length and width of the area you plan to pour, then decide on your slab thickness. Use a tape measure for length and width; thickness is typically a design decision based on the intended use. Enter all three measurements into the calculator above, choosing feet or inches for each dimension. The calculator converts and computes everything automatically.
When should I order ready-mix concrete instead of bags?
Ready-mix concrete delivered by truck becomes cost-effective and practical for pours of one cubic yard (27 cubic feet) or more. Mixing bags by hand or with a small mixer for large pours is extremely labor-intensive, and batch inconsistency becomes a problem. For small projects like fence posts, stepping stones, or small repairs, bags are ideal. For patios, driveways, garage floors, and foundations, ready-mix is the better choice.

Related Calculators

→Concrete Slab Calculator

Calculate concrete for any slab size in yards and bags.

→Concrete Cost Calculator

Estimate total project cost including delivery and labor.

→Concrete Yard Calculator

Convert dimensions directly to cubic yards with a handy table.

→How Much Concrete Do I Need?

Step-by-step guide to measuring and calculating concrete.