Concrete Calculator

Estimate concrete volume in cubic feet, cubic yards, and bag count for slabs or pours.

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Length (ft)

Width (ft)

Depth (in)

Bag size

Result

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Summary

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Detail

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Estimate how many cubic yards of concrete a slab requires, and how many pre-mixed bags to buy, from its length, width, and pour depth.

How It Works

How Concrete Calculator Works

The calculator multiplies length by width by depth to get volume — since depth is entered in inches while length and width are in feet, depth is first divided by 12 to convert it to feet, keeping all three dimensions consistent before multiplying.

That cubic-foot volume is divided by 27 (the number of cubic feet in a cubic yard, since ready-mix concrete is typically ordered by the yard) to give the cubic yardage, and also converted to cubic meters for reference.

For do-it-yourself pours using pre-mixed bags, the calculator divides total cubic feet by the yield of your chosen bag size (roughly 0.3 ft³ for a 40 lb bag, 0.45 ft³ for 60 lb, and 0.6 ft³ for 80 lb) and rounds up, since you can't buy a partial bag.

Worked Example

See It In Action

A 10 ft × 10 ft slab poured 4 inches deep needs 33.33 ft³ of concrete — about 1.23 cubic yards (0.94 m³) — or 56 bags of 80 lb pre-mixed concrete.

Real-World Use Cases

Who Uses Concrete Calculator and Why

Estimating ready-mix volume in cubic yards before calling a concrete supplier for a driveway or patio pour.
Working out how many 60 lb bags to buy at the hardware store for a small footing or fence post base.
Comparing bag size options (40 lb vs 60 lb vs 80 lb) to figure out which is more economical for a given job's total volume.

Common Mistakes

Mistakes to Avoid

Entering depth in feet instead of inches — the calculator expects slab depth in inches while length and width are in feet, and mixing that up produces a wildly wrong volume.
Ordering exactly the calculated cubic yardage with no buffer — the tool gives theoretical volume, but uneven subgrade, form leakage, and minor spillage routinely eat into that margin on an actual pour.
Switching bag sizes mid-project and assuming the bag count scales linearly without recalculating — an 80 lb bag doesn't yield double a 40 lb bag's volume, so always recompute rather than estimating by weight ratio.

Pro Tips

Tips for Best Results

For a pour over about 1 cubic yard, ready-mix delivery is usually more practical (and often cheaper per yard) than mixing dozens of bags by hand.
Round your slab dimensions up slightly before entering them if your forms aren't perfectly square — a little extra calculated volume is cheaper than a short pour.

Troubleshooting

Fixing Common Problems

My calculated bag count seems way too high for a small job. — Double check that depth was entered in inches, not feet — a 4-inch slab entered as "4" in a feet field would compute as 4 feet deep, inflating the volume roughly twelvefold.

I'm not sure whether to round the cubic yardage up or down when ordering ready-mix. — Always round up and add a 5-10% buffer — most ready-mix suppliers charge a short-load fee for small orders, so it's cheaper to slightly overorder than to call for a second truck.

Glossary

Terms Explained

Cubic yard: The standard unit ready-mix concrete is ordered in, equal to 27 cubic feet.

Yield: The volume of mixed concrete a single bag of dry mix produces once water is added — varies by bag weight and formulation.

FAQ

Frequently Asked Questions

Should I order extra concrete beyond the calculated amount?

Yes, most contractors add 5-10% for waste, uneven subgrade, and spillage, and ready-mix trucks often have minimum order quantities regardless of the exact volume needed — check with your supplier.

Which bag size should I choose?

80 lb bags cover the most volume per bag and are more cost-efficient for larger pours, while 40 lb or 60 lb bags are easier to lift and mix by hand for smaller DIY jobs.

Why is depth entered in inches while length and width are in feet?

This matches how concrete slabs are typically specified in practice — a 4-inch slab is a common residential standard, while length and width are more naturally measured in feet for a project area.

Does this account for rebar or wire mesh reinforcement?

No — this calculates concrete volume only. Reinforcement materials should be estimated and purchased separately based on your project's structural requirements.