Concrete Calculator

How to Use This Concrete Calculator

Select the shape: Slab / Rectangular, Circular, or Footing
Enter the dimensions and choose the correct units (feet, meters, inches, or cm)
Set the number of slabs/pads/footings if you're pouring more than one identical section
Select a waste factor — 10% is recommended for most projects
Choose bag size if you are buying bagged concrete (40 lb, 60 lb, or 80 lb)
Optionally enter a price — either per cubic yard (ready-mix) or per bag — to see estimated cost
Click Calculate Concrete to see volume, bags required, and cost. Press Enter in any field to calculate instantly.

Concrete Bag Yield Reference

Use this table to understand how many bags you need for a given volume. Yields follow Quikrete / Sakrete published values for standard pre-mix:

Bag Size	Approximate Yield	Bags per Cubic Yard
40 lb	0.30 cubic feet	~90 bags
60 lb	0.45 cubic feet	~60 bags
80 lb	0.60 cubic feet	~45 bags

Always round up to the next whole bag. Add 5–10% waste before ordering.

Common Project Examples (4-inch Slab)

Use this as a quick 4 inch concrete slab calculator reference for popular residential sizes. Volumes include a recommended 10% waste factor. Bag counts are rounded up. Results are also shown in cubic yards for ready-mix orders.

Slab Size	Area	Cubic Yards (with 10%)	80 lb Bags	60 lb Bags
4 × 4 ft	16 sq ft	0.22 yd³	10 bags	14 bags
8 × 8 ft	64 sq ft	0.87 yd³	40 bags	53 bags
10 × 10 ft	100 sq ft	1.36 yd³	62 bags	82 bags
12 × 12 ft	144 sq ft	1.96 yd³	89 bags	118 bags
10 × 20 ft	200 sq ft	2.72 yd³	123 bags	163 bags
20 × 20 ft	400 sq ft	5.43 yd³	245 bags	326 bags

Quick tip (4-inch slabs only): Divide the square footage by 81 to get approximate cubic yards before waste. Example: 100 sq ft ÷ 81 ≈ 1.23 yd³ (then add 10%). If you still need the floor area, use our square footage calculator first.

Formulas Used

Rectangular / Slab: Volume = Length × Width × Thickness × Number of Slabs

Circular: Volume = π × (Diameter ÷ 2)² × Thickness × Number of Pads

Footing: Volume = Length × Width × Depth × Number of Footings

Cubic Yards = Cubic Feet ÷ 27

A waste factor (usually 5–10%) is added so you do not run short on the job.

Note: These formulas follow standard volumetric methods referenced by the American Concrete Institute (ACI). For structural or load-bearing work, always confirm final dimensions with a licensed engineer or your local building department.

How Much Concrete Do I Need?

Knowing the right amount of concrete is critical. Ordering too little means delays and cold joints. Ordering too much wastes money. This calculator helps you get an accurate estimate for three of the most common concrete projects: slabs, circular pads, and footings.

Concrete Slab Calculator

Slabs are used for patios, garage floors, sidewalks, and house foundations. Most residential slabs are poured 4 inches thick. For driveways and garage floors that will carry vehicles, 5–6 inches is recommended. Many slabs sit on a compacted base — estimate that layer with our gravel calculator before you pour. Enter length, width, and thickness to get the exact volume. If you're pouring several identical slabs (stepping stones, for example), use the "Number of Slabs" field to total them in one calculation.

Circular Concrete Calculator

Useful for round patios, fire pit bases, or circular pads under tanks and equipment. Measure the full diameter across the circle and the desired thickness. The calculator uses the standard πr² formula to find the volume, then converts it to cubic yards and bags.

Concrete Footing Calculator

Footings support posts, decks, and foundations. Enter the length, width, and depth of one footing, then multiply by the number of footings. Typical deck footings are 12–48 inches deep depending on local frost lines and soil conditions. See our fence calculator if you're pricing out post footings for a fence line.

Looking for more material estimators? Browse all construction calculators on EntireCalc — paint, flooring, drywall, roofing and more.

Tips for Ordering Concrete

Always add waste: 5% for simple rectangular slabs with good forms, 10% for most projects, and up to 15% for uneven ground or many small sections.
Prepare the base: A level gravel or crushed-stone base improves drainage and slab performance. Use the gravel calculator to estimate cubic yards for the sub-base.
Small jobs vs large jobs: Under about 1 cubic yard → bagged concrete is usually practical. Over 1–2 yards → ready-mix delivery is almost always cheaper and more consistent.
Thickness matters: A 4-inch slab needs half the concrete of an 8-inch slab for the same area. Doubling thickness doubles the volume.
Check local codes: Frost depth, soil type, and load requirements affect footing size and minimum slab thickness.
Order a little extra: It is better to have leftover concrete than to stop a pour halfway through. Cold joints weaken the slab.

Frequently Asked Questions

How do I calculate how much concrete I need?

Measure the length, width, and thickness of the area. Multiply them together to get cubic feet, then divide by 27 to convert to cubic yards. Always add 5–10% extra for waste. This calculator does all of that automatically.

How many 80 lb bags of concrete make a cubic yard?

An 80 lb bag yields about 0.60 cubic feet. You need approximately 45 bags of 80 lb concrete to make one cubic yard (without waste). With a typical 10% waste factor the number rises to about 50 bags.

How many bags of concrete for a 10x10 slab?

For a 10×10 ft slab at 4 inches thick, the volume is about 1.23 cubic yards (33.3 ft³) without waste. With 10% waste you need roughly 1.36 yd³ — about 62 bags of 80 lb concrete, or about 82 bags of 60 lb. Enter your exact thickness in the calculator above if it differs from 4 inches.

What is the standard thickness for a concrete slab?

Most residential concrete slabs are 4 inches thick — the default many people use with a 4 inch concrete slab calculator for patios and shed pads. Driveways and garage floors are often 5–6 inches. Sidewalks are typically 4 inches. Always check local building codes.

Should I order extra concrete?

Yes. Always add a 5–10% waste factor. Uneven ground, formwork, spillage, and over-excavation mean you will use more concrete than the exact mathematical volume.

How much does a cubic yard of concrete cost?

Ready-mix concrete typically costs $120–$180 per cubic yard delivered in most U.S. markets (2025–2026 range), depending on location, strength (PSI), additives, and short-load fees. Bagged concrete costs more per yard but is convenient for small jobs under about 1 cubic yard.

Can I mix concrete myself for a large slab?

For projects larger than about 1 cubic yard, ready-mix delivered by truck is usually more practical, consistent, and cost-effective. Bagged concrete works well for small repairs, fence posts, and individual footings.

What is the difference between cubic feet and cubic yards?

1 cubic yard = 27 cubic feet. Concrete is almost always ordered and priced by the cubic yard. This calculator shows both so you can compare bag quantities (cubic feet) with ready-mix orders (cubic yards).

How deep should concrete footings be?

Footing depth depends on local frost lines and soil conditions. In many areas, deck and post footings are 12–48 inches deep. Always check your local building code or consult an engineer for load-bearing foundations.

Is a ready-mix concrete calculator different from a bag calculator?

No, the underlying volume math is the same. A ready-mix calculator reports the result in cubic yards for ordering a truck, while a bag calculator converts that same volume into the number of 40, 60, or 80 lb bags needed. This tool shows both at once.