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How many bags of concrete do I need?

Enter your project size and we'll estimate how many bags to buy, including extra for waste.

What are you pouring?
Units
Extra for waste

10% is a good default. Use more for uneven ground.

Bag size

Or start from an example:

10 ft4 in12 ft

Your estimate

98 bags

60 lb concrete mix

Includes 10% extra

Project volume
40 ft³
With extra
44 ft³
Approx. total weight
5,880 lb
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How did we calculate this?

Four simple steps — nothing hidden.

  1. 1

    We calculate the concrete volume

    Your length, width and thickness become a volume in cubic feet.

  2. 2

    We add your chosen extra

    Whatever waste percentage you pick — 0%, 5%, 10% or 15% — is added on top.

  3. 3

    We divide by what each bag covers

    Every bag size covers a slightly different amount, so we use the yield for the bag you chose.

  4. 4

    We round up to a whole bag

    You always get a full-bag number you can actually buy.

See the detailed formulas

  • Slab, path or patio

    length (ft) × width (ft) × thickness (in ÷ 12)

    4 inches is 4 ÷ 12 = 0.33 feet.

  • Fence post hole or round pad

    π × (width ÷ 24)² × (depth ÷ 12)

    Dividing the width by 24 turns inches into a radius in feet.

  • Steps

    width × tread (in ÷ 12) × riser (in ÷ 12) × n(n + 1) ÷ 2

    Each step is counted as a solid block rising from the base.

  • Small repair

    area (ft²) × thickness (in ÷ 12)

    Measure the patch in square feet and its depth in inches.

  • Your extra for waste

    volume × (1 + extra % ÷ 100)

  • Bags to buy

    round up (extra volume ÷ what one bag covers)

    Always up — never down.

Typical bag covers: 40 lb ≈ 0.3 ft³, 50 lb ≈ 0.375 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.6 ft³.

Worked example

10 × 12 ft patio, 4 in thick

Project volume
40 ft³
With 10% extra
44 ft³
60 lb bags required
98
10 ft4 in12 ft

The same numbers you'd get by typing them into the calculator above.

Common questions

How much extra concrete should I buy?

Ten percent extra is a safe bet for most patios, paths and floors. If your ground is uneven, slopes, or a hole has ragged edges, go with 15%. The calculator lets you pick the extra yourself and shows what it adds.

Which bag size should I choose?

For weekend jobs, 60 lb bags are the easiest to handle and mix. As a guide, a 40 lb bag covers about 0.30 cubic feet, a 50 lb bag about 0.375, a 60 lb bag about 0.45, and an 80 lb bag about 0.60. Check the yield printed on your bag before you buy — brands differ slightly.

Can I use this for fence posts?

Yes. Choose “Fence post holes”, then enter the width of the hole, how deep it is, and how many holes you're digging. Measure the widest part of the hole rather than the post itself, since the concrete fills the gap around it.

Why do you round up?

Concrete comes in whole bags and you mix one bag at a time, so we always round up to the next full bag instead of showing a decimal you can't buy. Volumes stay rounded to two decimal places so nothing is dressed up to look more precise than it is.

Is this number exact?

It's a close, honest estimate — good enough to shop with confidence. Real-world results vary a little with uneven ground, the brand of mix, and how much water you add. When in doubt, buy one extra bag rather than risk running out halfway through.

Where the numbers come from

Bag covers use the standard yields printed for common mix sizes: 40 lb ≈ 0.3 ft³, 50 lb ≈ 0.375 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.6 ft³. The formulas are standard geometry, and the waste allowance is yours to choose. Always check the yield printed on your bag — brands differ slightly.

Accuracy & assumptions

Estimates assume the measurements you enter, roughly level ground and standard bag yields. Real jobs vary a little — when in doubt, round up.

About Buy The Right Amount

Buy The Right Amount is a small, independent set of calculators for weekend projects. No signup, no accounts — every calculation runs right here in your browser.

Affiliate disclosure

If we ever link to products, some links may earn us a small commission at no extra cost to you. It never changes your results.

Privacy

Your measurements never leave your browser. We don't store or sell what you type.

A friendly disclaimer

These are honest estimates for planning a weekend project, not engineering advice. Check local codes before digging or pouring, and when in doubt grab one extra bag.