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Concrete Calculator: Bags or Yards for Any Slab

Enter your slab size. You get the number of bags to buy, or how much ready-mix to order, and what it costs at your store's price.

Slab size

Units
ft
ft
in

4 in is standard for patios and walkways. Driveways: 5 to 6 in.

Bag size
Extra for waste

10% covers uneven ground and spills. Use 15% on rough ground.

Shopping list

Updates as you type
62bags

of 80 lb (36 kg) concrete mix

or order 1.5 cu yd of ready-mix (1.36 cu yd needed)

  • 62 × 80 lb bags of concrete mix$310–$496
    36.7 cu ft with 10% extra ÷ 0.60 cu ft per bag = 61.1, rounded up.

    Typical price (estimate, checked Oct 2026): $5.00–$8.00 per 80 lb bag

    62 bags × $5.00–$8.00 = $310–$496Estimate
  • 5 × 8 ft 2×4 boards for forms$12.50–$35
    40 ft around the slab ÷ 8 ft boards, rounded up. Plus stakes.

    Typical price (estimate, checked Oct 2026): $2.50–$7.00 per 8 ft 2×4

    5 boards × $2.50–$7.00 = $12.50–$35Estimate
  • 1.5 cu yd of gravel for the base$45–$106.50
    1.36 cu yd (4 in base, +10% for compaction), rounded up to the half yard suppliers sell. Delivery not included.
    Buy gravel by the

    Typical price (estimate, checked Oct 2026): $30.00–$71.00 per cu yd

    1.5 cu yd × $30.00–$71.00 = $45–$106.50Estimate
  • Wire mesh or rebar for 100 sq ft · optional, not priced
Project total (estimate where no price is entered)$367.50–$637.50
Checked Oct 2026
Show the math
10 ft × 10 ft × 4 in ÷ 12 = 33.3 cu ft+ 10% = 36.7 cu ft = 1.36 cu yd36.7 cu ft ÷ 0.60 cu ft per bag = 61.1 → 62 bags

Your prices are saved on this device only.

Bagged vs bulk

Ready-mix is cheaper for this slab

With these prices, ready-mix starts to win at about 0.8 cu yd (about 60 sq ft at 4 in). Quarter-yard ordering makes the ready-mix cost rise in steps.

$0$1,000$2,00001 cu yd2 cu yd3 cu yd4 cu yd0.8 cu ydYour slab— Bags- - Ready-mix
Total cost against volume, at your prices or the middle of the typical range. Ready-mix includes the short-load fee.
62 bags, mix it yourself$310–$496
1.5 cu yd ready-mix, delivered$256–$375.50

Ready-mix · 1.5 cu yd

Typical price (estimate, checked Oct 2026): $130.00–$175.00 per cu yd

Typical price (estimate, checked Oct 2026): $61.00–$113.00 per order

1.5 cu yd × $130.00–$175.00 + $61.00–$113.00 fee = $256–$375.50

Bags also mean lifting: 62 bags of 80 lb is about 2.5 tons.

How it works

The formula

cu ft = L (ft) × W (ft) × T (in) ÷ 12
with waste = cu ft × 1.10
cu yd = cu ft ÷ 27
bags = cu ft ÷ bag yield, round up

Assumptions

  • 10% extra for waste by default. Use 5% for clean forms, 15% for rough ground.
  • Bag yields, from the volume printed on the bag: 80 lb makes 0.60 cu ft, 60 lb 0.45, 50 lb 0.375, 40 lb 0.30.
  • Ready-mix is ordered in quarter-yard steps, with a 1 yard minimum. Many plants add a short-load fee on small orders; ask yours.
  • Gravel base: 4 in, plus 10% for compaction; 1.4 tons per cu yd. Stone weight varies, so check with your supplier.

Worked example

A 10 × 10 ft patio, 4 in thick

  1. 10 × 10 × 4 ÷ 12 = 33.3 cu ft
  2. Add 10%: 33.3 × 1.10 = 36.7 cu ft
  3. In yards: 36.7 ÷ 27 = 1.36 cu yd
  4. In 80 lb bags: 36.7 ÷ 0.60 = 61.1, round up to 62 bags
  5. At $6.50 a bag (the middle of our typical range): 62 × $6.50 = $403

Common slab sizes at 4 in

Concrete needed for common slab sizes, 4 inches thick, 10% extra included
SlabCu yd80 lb bags60 lb bagsReady-mix order
10 × 10 ft1.3662821.5 cu yd
12 × 12 ft1.96881182 cu yd
10 × 20 ft2.721231632.75 cu yd
20 × 20 ft5.432453265.5 cu yd

Includes 10% extra. Bag counts are rounded up; ready-mix is rounded up to the quarter yard.

Questions people ask

How many 80 lb bags make a yard?

45 bags. A cubic yard is 27 cu ft, and each 80 lb bag makes about 0.60 cu ft. With 60 lb bags it takes 60.

How many bags for a 10x10 slab?

At 4 in thick, 62 bags of 80 lb mix with 10% extra, or 56 with no extra.

Is 4 inches thick enough?

For patios, walkways and shed floors, yes. Driveways usually need 5 to 6 in. Check your local code.

Why buy extra?

Ground is never perfectly flat and some mix gets spilled. Running short mid-pour is worse than one leftover bag.

When is ready-mix cheaper than bags?

Usually somewhere between 1.5 and 2.5 cubic yards, depending on your bag price and the plant's short-load fee. Enter your prices above and the chart shows where the lines cross for you.