Concrete Calculator

The full slab calculator: any rectangular pour, any thickness. Get exact cubic yards, bag counts for all four bag sizes, and a bags-vs-ready-mix cost comparison — updated as you type.

Your slab

4″ is standard for patios and walkways; 5–6″ for driveways.
Always order a little extra. You cannot send back a shortage mid-pour.
Prices — national-average defaults, edit to your local prices
National average; roughly $140–$170 in most areas.

Defaults are planning averages, not quotes. Your supplier's prices are the real ones — type them in.

Concrete to order (with your waste allowance)
0 cu yd
0
80 lb bags
0
60 lb bags
0
50 lb bags
0
40 lb bags
Mix it yourself (bags)
$0
Ready-mix truck
$0

Estimates only, for planning — not engineering advice. The math: volume = length × width × thickness. Bag counts use manufacturer-published yields and are always rounded up, because you cannot buy a partial bag or send back a shortage.

Worked example: a 10 × 10 ft slab at 4 inches thick: 10 × 10 × (4/12) = 33.33 cu ft = 1.23 cu yd. Exact bag count in 80 lb bags: 33.33 / 0.60 = 55.6, rounded up to 56. With 10% waste: 36.67 cu ft, so 62 bags or 1.36 cu yd.

How the math works

Concrete is sold by the cubic yard (27 cu ft) from a ready-mix truck, and by the bag for small jobs. Every calculator on this site does the same three steps:

  1. Volume in cubic feet — length × width × thickness (thickness converted from inches to feet).
  2. Cubic yards — divide by 27. This is the number a ready-mix plant will ask for.
  3. Bags — divide by the bag's yield: 80 lb = 0.60 cu ft, 60 lb = 0.45, 50 lb = 0.375, 40 lb = 0.30. Always rounded up, then the waste allowance is added and rounded up again.

Frequently asked questions

How do I measure an odd-shaped area?

Break it into rectangles, calculate each one, and add the volumes. For a shape that is almost a rectangle, measure the full rectangle and subtract the missing corners as their own small rectangles.

Should the thickness be the form depth or the slab depth?

Use the depth of concrete you will actually pour — the inside depth of your forms, above the compacted gravel base. If your forms are 2×6 boards (5.5 inches actual), a "6 inch" pour is really 5.5 inches of concrete.

How many bags do I need per cubic foot?

Roughly 1.7 bags of 80 lb, 2.2 bags of 60 lb, 2.7 bags of 50 lb, or 3.3 bags of 40 lb per cubic foot. For a full cubic yard: 45, 60, 72, or 90 bags respectively.

When should I skip bags and order a truck?

Around 1 cubic yard — 45+ eighty-pound bags — most people find a ready-mix truck is worth it even if the material costs a little more. Remember that plants typically have minimum orders and charge short-load fees on small loads, so call and ask.