How many bags of concrete do I need?

An 80-lb bag yields about 0.60 ft³; a 60-lb bag yields 0.45 ft³. Divide total cubic feet by the bag yield. Our calculator outputs bag count automatically once you enter your dimensions.

How many cubic yards of concrete do I need?

Calculate volume in cubic feet (L×W×D), then divide by 27 to get cubic yards. Most ready-mix trucks deliver in cubic yards, so use this figure when ordering.

How do I calculate concrete for a round footing or sonotube?

Use the cylinder formula: π × radius² × height. For a 10-inch diameter sonotube 4 ft tall: π × (0.417)² × 4 ≈ 2.18 ft³ or about 0.08 cubic yards. Our calculator handles round footings directly.

How much does a concrete slab cost?

Ready-mix concrete runs $125–$175 per cubic yard (2025 US average). A 10×10 ft, 4-inch slab needs ~1.23 yards, putting material cost at $155–$215 before labor.

What is the concrete mix ratio for a slab?

A standard 3,000 PSI slab mix uses roughly 1 part cement : 2 parts sand : 3 parts gravel with a water-to-cement ratio of ~0.5. Pre-mixed bags are pre-proportioned — just add water per the bag label.

How much does concrete weigh?

Normal-weight concrete weighs approximately 145 lb per cubic foot (3,900 lb per cubic yard). Multiply your volume in ft³ by 145 for total weight — useful for truck delivery limits.

Can I pour concrete in cold weather?

Per ACI 306, do not pour below 40°F without cold-weather admixtures and insulating blankets. Use our weather window to find the best pour days at your location.

How much concrete do I need?

Multiply length by width by thickness — all in feet — then divide by 27 to get cubic yards, which is the unit ready-mix is sold in. The step that catches most people is thickness: a 4-inch slab is 0.333 feet, not 4, and treating inches as feet overstates an order twelvefold. Once you have the raw volume, add 5 to 10 percent. ACI 301 recommends the overage because a sub-grade is never perfectly flat, forms bow slightly under the weight of wet concrete, and some material is always left in the chute and the wheelbarrow. Running short mid-pour is far worse than having a little left over: a cold joint forms where fresh concrete meets concrete that has begun to set, and that joint is a permanent weakness. Below about half a cubic yard, bagged concrete is usually more practical. Above it, ready-mix is cheaper per yard and dramatically less work — a single cubic yard is forty-five 80-pound bags to mix by hand.

Worked example

A 10 × 10 ft patio slab at 4 inches thick

1Convert the thickness: 4 in ÷ 12 = 0.333 ft
2Volume: 10 × 10 × 0.333 = 33.3 ft³
3Cubic yards: 33.3 ÷ 27 = 1.23 yd³
4Add 10% waste: 1.23 × 1.10 = 1.36 yd³

Order 1.5 cubic yards of ready-mix, or 62 eighty-pound bags.

Run your own numbers below — the Concrete Calculator handles the unit conversions and waste factor for you.

Concrete Calculator

Calculate cubic yards, cubic meters, and the number of 40, 60, and 80-lb concrete bags for slabs, footings, and columns. Optional price input gives a full project cost.

Formula and figures last reviewed August 6, 2026 — Cosyslabs · how we verify

Inputs

Project type

Length(ft)

Width(ft)

Thickness(in)

Diameter (column only)(in)

Height (column only)(ft)

Waste %

Price per yd³ (optional)($)

Result

Volume

1.36 yd³

Metric

1.04 m³

80-lb bags

60-lb bags: 82
40-lb bags: 123
Cubic feet: 36.7 ft³
Waste included: 10%

Waste default: 10% — ACI 301 recommends a 5–10% overage to account for sub-grade variation and spillage.

Diagram

Estimates only. Verify with a licensed contractor or supplier before purchasing. Rough Estimator! is not responsible for shortages, overages, or material variances. See our methodology for formulas and sources.

Formula

Volume (yd³) = L × W × D ÷ 27   ·   Bags (80 lb) = Volume (ft³) ÷ 0.6

References

Measure what the tool asks for
This calculator works from length in ft, width in ft and thickness in in. Measure the finished area you are covering, not the material — an estimate is only as good as the dimensions you feed it.
What it does with those numbers
Volume (yd³) = L × W × D ÷ 27 · Bags (80 lb) = Volume (ft³) ÷ 0.6 Every figure on the page comes out of that one expression, so you can check it by hand.
Why it adds 10% waste
ACI 301 recommends a 5–10% overage to account for sub-grade variation and spillage.
Choosing a project type
The project type changes the result, so pick the one that matches what you are actually buying rather than leaving the default.
Where the numbers come from
The constants are taken from ACI 301 — Specifications for Structural Concrete and Portland Cement Association — Concrete basics, linked in full below. If a figure here disagrees with your supplier's spec sheet, use the spec sheet — it describes the product you are buying.
What this will not tell you
This is a material takeoff, not a bid. It does not price labour, permits, delivery, equipment hire or site preparation, and it assumes a straightforward layout. Complex cuts, difficult access and awkward shapes all cost more than the geometry suggests.

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Frequently asked questions

Multiply the slab's length by width by thickness (all in feet), then divide by 27 to convert cubic feet to cubic yards. A 10×10 ft, 4-inch slab needs (10×10×0.333)÷27 ≈ 1.23 cubic yards.

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Cosyslabs. (2026). Concrete Calculator. Rough Estimator!. https://roughestimator.com/concrete-calculator

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