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Concrete Calculator

Estimate how much concrete a slab, footing, or round column needs — in cubic yards, cubic feet, and cubic metres — plus how many 40, 60, or 80 lb pre-mix bags.

FreeNo sign-upPrivate — runs in your browser, nothing is uploaded

Concrete needed
1.23 yd³

= 33.33 ft³ = 0.944 m³

Pre-mix bags: 56 × 80 lb · 75 × 60 lb · 112 × 40 lb

Order about 5–10% extra for spillage, uneven sub-grade, and waste.

Computed in your browser. Bag yields: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³.

Quick answer

To find concrete volume for a slab, multiply length × width × thickness in consistent units, then convert to cubic yards (divide cubic feet by 27). A 10 ft × 10 ft slab 4 inches thick is 10 × 10 × (4/12) = 33.3 ft³ ≈ 1.23 cubic yards, or about 56 eighty-pound bags. This calculator also handles round columns and reports cubic feet, cubic yards, cubic metres, and bag counts.

Formula & method

Volume for a rectangular slab or footing is length × width × thickness, with thickness in inches converted to feet (÷12). A round column is π × (diameter/2)² × height. Cubic feet are converted to cubic yards (÷27) and cubic metres (×0.0283168). Bag counts assume a yield of 0.60 ft³ per 80 lb bag, 0.45 ft³ per 60 lb bag, and 0.30 ft³ per 40 lb bag, rounded up.

Examples

Example 1: Standard slab
Input
10 ft × 10 ft × 4 in
Result
1.23 yd³ (≈ 56 × 80 lb bags)
Why
10 × 10 × (4/12) = 33.33 ft³; ÷27 = 1.23 yd³; 33.33 / 0.60 ≈ 56 bags.
Example 2: Thicker, larger slab
Input
20 ft × 20 ft × 6 in
Result
7.41 yd³
Why
400 × 0.5 = 200 ft³; 200 ÷ 27 = 7.41 cubic yards.
Example 3: Round column
Input
Column, 12 in diameter × 8 ft
Result
≈ 0.23 yd³
Why
π × (0.5 ft)² × 8 = 6.28 ft³; ÷27 ≈ 0.23 yd³.

When to use this tool

  • Pouring a slab, patio, sidewalk, footing, or fence-post column.
  • Deciding between bagged pre-mix and ready-mix delivery by the cubic yard.
  • Estimating material cost before a concrete project.

Common mistakes

  • Mixing thickness in inches with length in feet without converting — divide inches by 12 first.
  • Ordering the exact volume with no waste allowance; pros add roughly 5–10% for spillage and uneven sub-grade.
  • Using the radius where the formula needs the diameter (or vice-versa) for a round column.

Frequently asked questions

How many cubic yards is my slab?

Multiply length × width × thickness in feet to get cubic feet, then divide by 27. The tool does this and also converts thickness from inches automatically.

How many bags of concrete do I need?

It depends on bag size: an 80 lb bag makes about 0.60 ft³, a 60 lb bag 0.45 ft³, and a 40 lb bag 0.30 ft³. The calculator divides your volume by the yield and rounds up.

Should I order extra?

Yes — add about 5–10% for spillage, waste, and an uneven base. It's far cheaper than running short mid-pour.

When should I use ready-mix instead of bags?

Bagged mix is practical for small jobs, but past roughly 1 cubic yard (about 45–60 bags) ready-mix delivery is usually cheaper and faster.

Does it handle round columns?

Yes. Switch the pour type to Column and enter the diameter (inches) and height (feet); it uses the circle-area formula.

Is this an exact quote?

No — it's an estimate based on standard yields. Confirm with your supplier, especially for large or structural pours.

Sources & references

External references open in a new tab. We are independent and not affiliated with these organizations.

  • ✓ Free to use
  • ✓ No sign-up required
  • ✓ Runs entirely in your browser — nothing is uploaded.
  • ✓ Formula and method shown above
Last updated: How we calculateReport a correctionDisclaimer

Provided “as is” for general information only — results may be inaccurate, so verify before you rely on them. No warranty; use at your own risk.

Built and reviewed by HIFreeTools against the formula shown above and any authoritative references cited on this page. See our methodology and editorial standards.

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