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Topic · Concrete estimating

Concrete volume: formulas, waste and examples

By DIYCalcLab Editorial Team · Updated

This page is the method, not the tool list. Rectangular concrete volume is length × width × thickness, then cubic yards, cubic meters or bags. Worked examples below open in the slab calculator with the same inputs. Waste above the cited 5% or 10% overrun, and the point where a truck is worth pricing, are labeled as assumptions.

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Core calculators

Start with the tool that matches your job. Each page shows the formula, defaults, shopping list and sources.

Volume formulas

  • Rectangular slab: volume = length × width × thickness
  • Imperial: cu ft = length (ft) × width (ft) × thickness (in) ÷ 12; cu yd = cu ft ÷ 27
  • Metric: m³ = length (m) × width (m) × thickness (cm) ÷ 100
  • Bags = volume (with waste) ÷ yield per bag (from the bag label)

Variables

Thickness
Usually inches or centimeters while length/width are feet or meters — convert before multiplying
Waste
5% or 10% is a cited over-run for slight thickness changes (see Sources). Higher percentages on this page are editorial assumptions
Bag yield
Finished volume printed on the bag. Quikrete Concrete Mix 1101: 80 lb ≈ 0.60 cu ft. Metric 20/25/30 kg defaults on this site are assumptions
Unit conversions used on DIYCalcLab (NIST SP 811 factors where applicable)
FromTo
1 ft0.3048 m (exact)
1 in0.0254 m (exact)
1 cu yd27 cu ft exactly; 0.7645549 m³ (NIST 7.645 549 × 10⁻¹), shown as 0.7646 m³
1 lb0.45359237 kg (exact)

Worked examples

Each example uses the same inputs as the linked calculator. The link loads those inputs; the page URL without parameters stays the canonical.

10 × 10 ft patio slab, 4 in thick, 10% waste

Length 10 ft · Width 10 ft · Thickness 4 in · Waste 10% · 80 lb bags

  1. Area = 10 × 10 = 100 sq ft
  2. Volume = 100 × 4 ÷ 12 = 33.33 cu ft = 1.23 cu yd before waste
  3. With 10% waste = 1.36 cu yd (36.7 cu ft) required
  4. Bags at 0.60 cu ft each = 36.7 ÷ 0.60 → 62 × 80 lb bags purchased (62 × 0.60 = 37.2 cu ft). The extra 0.5 cu ft is rounding up, not more waste

Result: Required with 10% waste: 1.36 cu yd. Purchased: 62 × 80 lb bags (37.2 cu ft).

Open this example in the calculator →

12 × 16 ft shed base, 4 in thick, 10% waste

Length 12 ft · Width 16 ft · Thickness 4 in · Waste 10%

  1. Area = 12 × 16 = 192 sq ft
  2. Volume = 192 × 4 ÷ 12 = 64 cu ft = 2.37 cu yd before waste
  3. With 10% waste = 2.61 cu yd (70.4 cu ft) required
  4. 70.4 ÷ 0.60 = 117.3 → 118 × 80 lb bags purchased (70.8 cu ft, about 4.3 metric tonnes). Price a truck against that bag total; 118 bags is a labor problem, not proof the truck is cheaper

Result: Required with 10% waste: 2.61 cu yd. Purchased: 118 × 80 lb bags (~4.3 metric tonnes).

Open this example in the calculator →

Bags or ready-mix: a sequence you can follow

  1. Measure inside the forms. Keep thickness in inches or centimeters and length and width in feet or meters, then convert before multiplying (divide inches by 12, or centimeters by 100).
  2. Compute net volume, then multiply by 1.05 or 1.10. Use 1.10 unless the base is screeded and the forms are rigid. Only then consider the editorial 15% (or 10–20% for irregular holes) from the waste section.
  3. Divide by the yield on the bag you will buy and round up. At the Quikrete 1101 yield of 0.60 cu ft, 27 cu ft (1 cu yd) is exactly 45 × 80 lb bags, about 3,600 lb of mix. That spare from rounding is purchased volume, not a second waste factor.
  4. Under about 1 cu yd, bags are usually practical if you can place them before earlier batches stiffen. This is a labor checkpoint from the 45-bag arithmetic, not a manufacturer price rule.
  5. At about 1 cu yd and above, price both: bag count × shelf price, versus a delivered quote that includes the minimum order, any short-load fee and chute reach. Do not treat 1 cu yd as the point where the truck is automatically cheaper. Ask the plant which mix fits the thickness and use, and order in their increment (often ¼ yd).
  6. Have a legal place for leftover concrete. A second placement against concrete that has already stiffened can leave a cold joint — a practical risk, not a figure from the sources above.

What the waste percentages are — and are not

  • Cited: University of Nebraska–Lincoln Extension G82-630 calls a 5% or 10% increase (“over-run”) a common practice for slight deviations in slab thickness, thickened edges and similar small errors. DIYCalcLab defaults to 10%, the top of that range.
  • Cited limit, no percent: the QUIKRETE product guide says yields are approximate and do not allow for waste or an uneven sub-grade. It does not say 15%.
  • Editorial assumption, not in those sources: up to 15% when edges are hand-dug or the base is not screeded to a uniform thickness, and 10–20% for post holes whose sides are not true cylinders. Measure the low spots before using the higher band.

Limits

  • These pages estimate volume and purchase quantities only. Footings, structural slabs, frost depth and reinforcement must follow local code or an engineer’s design.
  • Bag yields vary by product (high-strength and fiber mixes often yield less). Advanced options on each calculator let you replace the default yield.
  • Metric defaults of 9.4 L (20 kg), 11.7 L (25 kg) and 14 L (30 kg) are site assumptions for generic premix. They are not on the Quikrete 1101 sheet, which covers US bag weights.
  • Default slab thickness (4 in / 10 cm) is a common non-structural starting point, not a code requirement.

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Guides

Quick questions

Which waste percent is cited, and which is an assumption?

University of Nebraska–Lincoln Extension G82-630 calls a 5% or 10% increase an over-run for slight thickness changes and thickened edges. This site defaults to 10%, the top of that range. The QUIKRETE product guide says yields do not include waste, and it does not give a percent. Rows above 10% — up to 15% for unscreeded edges, and 10–20% for irregular post holes — are editorial assumptions on this page.

Are the metric bag yields from Quikrete 1101?

No. Quikrete Concrete Mix 1101 is cited for US bags: an 80 lb bag yields about 0.60 cu ft. The 9.4 L, 11.7 L and 14 L figures for 20 kg, 25 kg and 30 kg bags are site assumptions. They are not on that sheet.

What does this page do, and what do the calculators do?

This page keeps the volume formulas, the cited over-run, and the assumptions that are not in those sources. A calculator applies one set of inputs and rounds the purchase. The concrete category hub is the tool list. The long walkthrough for slabs, footings and post holes is the concrete guide.

Sources

  1. Quikrete – Concrete Mix (No. 1101) product data sheet — accessed : “An 80 lb (36.2 kg) bag yields approximately 0.60 ft³ (17 L); 60 lb ≈ 0.45 ft³; 40 lb ≈ 0.30 ft³”
  2. NIST SP 811 – Appendix B.8 conversion factors (alphabetical) — accessed : “cubic yard (yd³) → cubic meter (m³): 7.645 549 E−01”
  3. University of Nebraska–Lincoln Extension – G82-630 Concrete Construction: Obtaining Quality Results — accessed : “A common practice is to increase the calculated volume by five or ten percent, called over-run, to help account for slight deviations in slab thickness, thickened edges, etc.”
  4. QUIKRETE product guide – yields exclude waste — accessed : “All yields are approximate and do not allow for waste or uneven sub-grade, etc.”

Prefer manufacturer data sheets and published factors. Coverage, yield and waste vary by product — confirm the label you buy.

Looking for every concrete tool in one list? See the Concrete calculators category hub · Methodology · Updated