Concrete Calculator

How much concrete do I need? Multiply length × width × (thickness in inches ÷ 12) for cubic feet, divide by 27 for cubic yards, then convert to bags using typical yields (40 lb ≈ 0.30, 60 lb ≈ 0.45, 80 lb ≈ 0.60 cu ft) and add about 10% waste before rounding up. A 10×10 ft slab at 4 inches with 60 lb bags is about 1.23 cubic yards — 82 bags.

Reviewed / updated August 2026

Full walkthrough: How to Estimate Materials for Any Home Project

Your inputs

ft
ft
in

Many sidewalks and pads use about 4 inches

Yields are typical bagged concrete planning figures — check your bag

Result

Concrete to order

1.23 cu yd

82 × 60 lb bags (incl. 10% waste)

Cubic yards
1.23 cu yd
Cubic feet
33.3 cu ft
60 lb bags
82

This slab is about 1.23 cubic yards. At 60 lb bags with a 10% waste buffer, buy 82 bags (rounded up).

4″ vs 6″ slab

Same footprint and bag size — only thickness changes.

ThicknessCu ydBags
4″1.2382
6″1.85123

Suggested next steps

  • Around a cubic yard or more, compare bag totals to a ready-mix delivery minimum from your local plant.
  • Compact the base and use reinforcement (mesh or fiber) as your project requires before pouring.
  • Have a plan for leftover mix — you cannot return opened bags once water is added.

How the concrete calculation works

Concrete volume starts as a rectangular prism. Thickness is usually given in inches, so convert to feet before multiplying:

  1. Cubic feet = length × width × (thickness inches ÷ 12)
  2. Cubic yards = cubic feet ÷ 27
  3. Bags = ceil((cubic feet ÷ bag yield) × 1.10)

| Bag size | Typical yield | | --- | --- | | 40 lb | 0.30 cu ft | | 60 lb | 0.45 cu ft | | 80 lb | 0.60 cu ft |

The 10% waste multiplier covers uneven subgrade, form bulge, and mix that sticks in the wheelbarrow. Bags always round up because you cannot buy a fraction of a sealed bag. If your project is close to a full cubic yard, price a ready-mix truck against the bag total—delivery minimums often win once labor and mixing time are counted.

Source note: Bag yields are common manufacturer planning figures for bagged concrete mix; always confirm the yield on your product. Updated August 2026.

Worked example

Slab 10 × 10 ft, thickness 4 inches, 60 lb bags:

  1. Cu ft = 10 × 10 × (4 ÷ 12) ≈ 33.3
  2. Cu yd ≈ 1.23
  3. Bags = ceil((33.3 ÷ 0.45) × 1.10) = 82

Those are the default inputs on this page. Compare 4″ vs 6″ in the what-if table for the same footprint—moving from four to six inches raises volume by 50% and jumps the bag count accordingly. Compact the base, set forms true, and have a place for leftover mix before you add water.

Tips and related tools

Frequently asked questions

How much concrete for a 10×10 slab at 4 inches?

Cubic feet = 10 × 10 × (4 ÷ 12) ≈ 33.3 cu ft. Cubic yards ≈ 33.3 ÷ 27 ≈ 1.23 cu yd. With 60 lb bags (~0.45 cu ft each) and 10% waste: ceil((33.3 ÷ 0.45) × 1.10) = 82 bags.

How many 60 lb bags of concrete per cubic yard?

At 0.45 cu ft per bag, one cubic yard (27 cu ft) needs 27 ÷ 0.45 = 60 bags before waste. With a 10% buffer, plan on about 66 bags per yard. Confirm the yield printed on your bag.

Should I use bags or ready-mix?

Bags suit small pads and post holes. Near a cubic yard or more, ready-mix delivery is often cheaper and more consistent — compare your plant’s minimum load to bag totals.

Why add 10% waste?

Uneven grade, form overrun, and mixing leftovers eat volume. The 10% factor is a DIY planning buffer before rounding up to whole bags you can buy.

Is 4 inches thick enough?

Many sidewalks and patio pads use about 4 inches. Driveways and garage floors often need more depth and reinforcement — check local code. Use the 4″ vs 6″ table to compare bag counts.

Related tools

Related calculators — live tools and upcoming pages for internal linking.