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Calculators & guides

How Much Concrete Do I Need?

Measure, multiply, convert, add waste. Here is the full process, with the calculator ready when you are.

Units

Geometry dimensions — schematic, not to scaleL (length)HW
Schematic · not to scale · quantity repeats the entire solid

Concrete & bag assumptions

Density is the placed concrete density, not dry bag weight. Defaults are examples for normal-weight concrete. Bag weight does not determine yield; use the mixed yield printed on your product.

Estimate cost (optional · USD)

All cost fields are optional and user-entered. Ready-mix and bags are alternatives, never added together. No average or local prices are assumed — use a real quote. Labor per area is available only for shapes with a defined footprint.

Formula & solid model

V = L × W × H × Q

All lengths convert to metres before calculation. Blank waste = 0%. Zero dimensions or quantity produce zero volume. Display values are rounded; costs and bags use unrounded volumes.

Rectangular slab · incl. 10% waste

You need approximately 1.358 cubic yards

36.667 ft³ · 1.038 m³ · 1,038.28 litres

Volume

UnitExact+10% waste
Cubic yards1.2351.358
Cubic feet33.33336.667
Cubic metres0.9441.038
Litres943.8951,038.284

Placed weight · incl. waste

5,500 lb / 2,494.76 kg

Density: 150 lb/ft³ · weight = volume × density

Selected premix bags · incl. waste

62 × 80 lb bags

Yield: 0.6 ft³ / bag · total volume ÷ yield, rounded up.

Above about 1 yd³, ready-mix is often more practical than mixing bags.

Estimate inputs & formula

Length: 10 ft · Width: 10 ft · Thickness (H): 4 in · Quantity (Q): 1

V = L × W × H × Q

Estimate only, not engineering advice. Actual needs vary with subgrade, forms and spillage. Confirm final quantities and product yields with your supplier or contractor before ordering.

What this calculates

Measure every dimension of the form: length, width and depth for rectangles, or diameter and depth for round shapes. Measure in several places if the ground is uneven and use the average.

Convert all measurements to the same unit before multiplying. In the US, that usually means feet, then divide cubic feet by 27 to get cubic yards.

Add a waste allowance — commonly 5–10% for formed slabs and more for irregular trenches — and round up to how your supplier sells.

Decide on bags or ready-mix. Small pads and post holes suit bags; anything over roughly a yard usually suits a truck.

Formula

Cubic yards = (length ft × width ft × depth ft) ÷ 27

Order = cubic yards × (1 + waste % ÷ 100)

Worked example

Two non-overlapping patio sections, 4 in thick

  1. 1.Measure an 8 × 10 ft rectangle and an adjoining 4 × 6 ft rectangle; do not include a shared corner twice.
  2. 2.(8 × 10 + 4 × 6) × (4/12) = 34.6667 cu ft; divide by 27 = 1.2840 cu yd.
  3. 3.With 10% waste: 1.4123 cu yd. Ask your supplier about order increments and delivery minimums before choosing bags or ready-mix.

Common mistakes

  • Measuring once on uneven ground.
  • Ordering exactly the calculated amount with no waste.
  • Treating an online estimate as a structural design — it isn't.

Frequently asked questions

How much concrete do I need for a 10 × 10 slab?

At 4 inches thick, about 1.23 cubic yards, or roughly 1.36 cubic yards with 10% waste.

How much area does one 80 lb bag cover?

At about 0.60 cu ft per bag, one 80 lb bag covers roughly 1.8 square feet at 4 inches thick.

Can I trust online concrete calculators?

The math is straightforward, but the result is only as good as your measurements. Treat it as an estimate and confirm with your supplier or contractor.