Toolivaro

Free Concrete Calculator

Estimate concrete volume in cubic yards or meters and the number of bags needed for a slab, footing, or pad, with cost.

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Units

Pour length.

Pour width.

Pour thickness (4 inches = 0.33 ft, 10 cm = 0.1 m).

Uneven excavation and spillage.

Price of one bag of the selected size.

How is the result calculated?

Patio slab: 10 ft × 4 ft, 4 inches deep

A small patio slab is 10 feet by 4 feet and 4 inches (1/3 foot) deep: the volume is 10 × 4 × 0.333 = 13.3 cubic feet. With 10% waste that becomes about 14.7 cubic feet. An 80 lb bag yields 0.6 cubic feet, so 14.7 ÷ 0.6 = 24.4 — you need 25 bags. At $6 per bag that is $150 of concrete, before form boards, rebar, and finishing tools.

Input Value
length 10
width 4
depth 0.33
wastePercent 10
bagSize 80lb
pricePerBag 6
Result 13.3 ft³ (0.49 yd³) · 14.7 ft³ with waste · 25 bags · ≈ $150

What is the formula and its assumptions?

Volume

volume = length × width × depth

Symbol Meaning
length pour length
width pour width
depth pour thickness

Bags needed

bags = ceil(volume × (1 + waste%) / bag yield)

Symbol Meaning
bag yield 0.45 ft³ (60 lb) · 0.6 ft³ (80 lb) · 0.0118 m³ (25 kg)
waste% allowance for uneven ground and spillage

What are the most common mistakes?

  • Ordering by bag count without checking the bag yield of the specific product.
  • Measuring in inches and entering inches as if they were feet (depth is the classic error — 4 inches is 0.33 ft).
  • Forgetting that a 4-inch slab needs form boards, rebar or mesh, and vapor barrier — concrete is not the whole project cost.

What are the assumptions and limitations?

  • Bag yields are manufacturer-style approximations; the label of your product governs.
  • The calculator models rectangular pours only — curved or stepped footings need a different method.
  • Cost covers bags only, not delivery, reinforcement, finishing, or labor.

Where do the numbers come from?

Last reviewed August 4, 2026 · Version 1.0.0 · Toolivaro does not guarantee external content.

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Frequently asked questions

Bags or ready-mix?

Bags make sense below about a cubic yard; above that, ready-mix is cheaper and easier to finish in time. The calculator’s volume result (in cubic yards) is what you order from the supplier — round up to the next quarter yard.

Why is my actual bag count different from the label math?

Bag yields assume a specific mix and water amount. Concrete that is wetter than recommended yields slightly less volume, and spillage happens. The waste allowance exists exactly for this.

Does depth include gravel base?

No — enter only the concrete thickness. A gravel base is separate material and volume; use the gravel calculator for that layer.

Found a mistake or have a correction? Report it — we review every correction.