concrete.work
United States

Concrete Slab Calculator

Enter the size and thickness of your slab to see the cubic yards, how many bags to buy and whether a ready-mix truck is worth it. Works for square, rectangular and round slabs.

What are you pouring?
Patio, driveway, shed base, path
Length Width Thickness

Tip: you can type feet and inches, like 10' 6".

Reinforcement for slabs (optional)
Gravel base under slabs (optional)

Gravel weight changes from quarry to quarry: ask your supplier for the compacted weight and we turn the volume into tons.

Use your own prices (optional)

You need

– cu yd

–

Poured, it weighs about – lb.

Premixed bags

Bag Bags To carry Est. cost

Mixing it yourself (1:2:3)

QUIKRETE Portland Cement data sheet: concrete mix 1 part cement, 2 parts All-Purpose Sand, 3 parts All-Purpose Gravel; 5 to 6 bags of 94 lb per yd³. A 94 lb bag is the customary 1 cu ft of cement.

Ready-mix truck

–

If a contractor pours it

How we price

Link to this calculation Download estimate (PDF)

Your materials at a glance

How to calculate concrete

  1. Measure length and width. In feet, inside the forms. For an L-shaped slab, split it into two rectangles.
  2. Pick the thickness. In inches: 4 in is typical for patios and walkways, 4–6 in for driveways.
  3. Multiply. Length × width × (thickness ÷ 12) gives cubic feet.
  4. Add 5–10% extra. Ground is never perfectly flat and forms flex.
  5. Convert. Divide by 27 for cubic yards, or by 0.60 for 80 lb bags, and round up.

Worked example: a 10 × 12 ft shed base, 4 inches thick

  1. Volume: 10 × 12 × (4 ÷ 12) = 40.0 cu ft.
  2. Plus 5%: 42.0 cu ft = 1.56 cubic yards.
  3. That is 70 bags of 80 lb or 94 of 60 lb: a pallet-sized job, so compare with a ready-mix order.

Concrete for common slab sizes

Slab Thickness Cubic yards 80 lb bags 60 lb bags
8 × 10 ft 4 in 1.04 47 63
8 × 10 ft 6 in 1.56 70 94
10 × 10 ft 4 in 1.30 59 78
10 × 10 ft 6 in 1.94 88 117
10 × 12 ft 4 in 1.56 70 94
10 × 12 ft 6 in 2.33 105 140
12 × 12 ft 4 in 1.87 84 112
12 × 12 ft 6 in 2.80 126 168
10 × 20 ft 4 in 2.59 117 156
10 × 20 ft 6 in 3.89 175 234
12 × 24 ft 4 in 3.73 168 224
12 × 24 ft 6 in 5.60 252 336
20 × 20 ft 4 in 5.19 234 312
20 × 20 ft 6 in 7.78 350 467
24 × 24 ft 4 in 7.47 336 448
24 × 24 ft 6 in 11.20 504 672

Includes 5% extra for waste.

Concrete formulas by shape

All measurements in feet; divide inches by 12 first. Results are in cubic feet.

Slab
length × width × thickness
Round slab
π × (diameter ÷ 2)² × thickness
Monolithic slab
length × width × thickness + perimeter band × (edge depth − thickness), where perimeter band = length × width − (length − 2 × edge width) × (width − 2 × edge width)

Cubic yards = cubic feet ÷ 27.

Frequently asked questions

How thick should a concrete slab be?

Most patios, walkways and shed bases are 4 inches thick. Driveways are usually 4 to 6 inches, with the thicker end for heavier vehicles. Your local building code has the final word.

How many bags of concrete do I need for a 10x10 slab?

At 4 inches thick with 5% extra, a 10 × 10 ft slab needs about 1.30 cubic yards: 59 bags of 80 lb or 78 bags of 60 lb.

How many square feet does a yard of concrete cover?

One cubic yard covers 81 sq ft at 4 inches thick, 65 sq ft at 5 inches and 54 sq ft at 6 inches.

How do I calculate concrete for a round slab?

Use π × radius² × thickness, with everything in feet. An 8 ft round pad 4 inches thick is about 16.8 cubic feet, or 0.62 cubic yards.

About the prices

Costs are estimated ranges with source and date, never a quote. Enter your local prices under "Use your own prices", or read how we estimate prices.