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Concrete Calculator

Estimate concrete for a rectangular slab or footing. Add an allowance, compare volume units, and calculate whole bags from your product's mixed yield.

Estimate concrete volume

Each dimension can use a different unit. Changing an input unit keeps the typed number and interprets it in the newly selected unit.

A whole number from 1 to 1,000. Use 1 for a single slab.

Choose 0 to 100%; blank means 0. The initial 10% is an example, not a universal recommendation.

Use a period for decimals. Properly grouped numbers such as 1,000.50 are accepted. Enter positive dimensions; do not include unit symbols in the number fields.

Example: 5 m × 4 m × 0.1 m, 10% extra → 2.2 m³.

Concrete including allowance

Estimate

Enter your dimensions to estimate concrete.

≈ marks display rounding. Calculations run in your browser. Press Enter to calculate. Successful results appear in this tab's Recent calculations.

Concrete volume examples

Metric slab

5 m × 4 m × 100 mm

5 × 4 × 0.1 = 2 m³

A 10% allowance adds 0.2 m³ for a total of 2.2 m³. If the selected product yields 25 L per bag, the requirement is 88 bags. The 25 L yield is an illustrative input, not a standard bag size.

Mixed units

12 ft × 9 ft × 4 in

12 × 9 × (4 ÷ 12) = 36 ft³

Before allowance, this is 36 ft³, or 1⅓ yd³. With 10% extra, it becomes 39.6 ft³. At a chosen yield of 0.6 ft³ per bag, the arithmetic gives 66 bags.

Repeated sections

Four identical rectangular footings

2 × 0.4 × 0.5 × 4 = 1.6 m³

Each section measures 2 m × 0.4 m × 0.5 m. With a 5% allowance, the estimate is 1.68 m³. Calculate sections of different dimensions separately.

Formulas, units, and estimate limits

Convert each dimension to meters, then multiply length × width × depth × number of identical sections. This gives the total volume before allowance. Extra volume equals that total × allowance ÷ 100. Required volume is total volume plus extra volume.

For bags, divide required volume by the mixed concrete yield per bag and round upward to a whole number. Purchased mixed volume equals whole bags × yield per bag. Theoretical surplus equals purchased mixed volume minus required volume, including allowance. Bag count is calculated from the exact requirement before display rounding.

Conversions use the international foot and inch: 1 inch = 0.0254 m, 1 foot = 0.3048 m, and 1 yard = 0.9144 m, all exactly. Cubing these lengths gives 1 ft³ = 0.028316846592 m³ and 1 yd³ = 0.764554857984 m³. One litre is 0.001 m³. Input units can be mixed; selecting a new input unit keeps the typed number. Changing the main result unit converts the existing result.

Dimensions and active bag yield must be positive. Section count must be a whole number from 1 to 1,000. Allowance must be from 0 to 100%; blank means zero. Each numeric input accepts up to 64 characters, 15 integer digits, and 10 decimal places. Expressions, scientific notation, fractions, and embedded units are not accepted. Bag fields are ignored when bag calculation is off.

Calculations retain exact fractions. Only displayed decimals are rounded, with halfway values rounding away from zero and trailing zeros omitted. ≈ marks rounding. A displayed quantity of approximately 1 m³ might be slightly above 1 m³ and require an extra bag at a purchase boundary.

This calculator estimates rectangular, uniform-depth volumes. It does not choose structural dimensions, strength, reinforcement, mix proportions, water amounts, or costs. It does not subtract reinforcement or embedded objects, combine different bag sizes, or model settling and irregular excavation. Product yields are estimates, so actual quantities and leftover material can differ. The initial allowance is editable and is not a recommendation for every project.

References: QUIKRETE — concrete quantity calculator, QUIKRETE Concrete Mix — product-specific mixed yields (PDF), and NIST — exact inch and yard definitions. Use the current yield and instructions for the product you plan to use.

Common questions

Can I use feet for length and inches for depth?

Yes. Select the correct unit next to each measurement. For example, enter length 12 ft, width 9 ft, and depth 4 in. The calculator converts each dimension before multiplying.

What does yield per bag mean?

It is the volume of mixed concrete produced by one bag according to the product instructions. It is not dry bag weight, water added, or required mixer capacity. Different products and bag sizes can produce different volumes.

How much extra allowance should I use?

Choose an allowance for your measurements, site, placing method, and expected losses. The initial 10% illustrates the option and can be changed or set to zero. Your supplier or project specifications may give more suitable guidance.

Why did the bag count increase when the displayed volume stayed the same?

Visible decimals may be rounded, but whole bags use the unrounded requirement. If 40 bags each yield 25 L, any requirement strictly greater than 1 m³ needs a 41st bag in this model.

Can this determine whether my slab or footing is strong enough?

No. It calculates the volume of dimensions you enter. Structural dimensions, reinforcement, loading, and product selection require the appropriate project specifications.